Overview > Guidelines > Implementation: JAVA Programming

Java Programming Guidelines

Copyright © 1999 Scott Ambler, Ambysoft, Inc.

The Java Coding Guidelines are provided under license from Scott Ambler, Ambysoft Inc., www.ambysoft.com. They have been reformatted for inclusion in the Rational Unified Process nad the Unified Process for EDUcation.

Contents

1      Introduction
        1.1    The first and last guideline

2    Coding Standards
      2.1    Naming conventions
       2.2    Documentation conventions
               2.2.1  Types of Java comments
               2.2.2  A quick overview of javadoc   

3    Standards for Member Functions
      3.1    Naming member functions
               3.1.1    Naming accessor member functions
                          3.1.1.1    Getters
                          3.1.1.2    Setters
       3.2    Naming constructors
       3.3    Member function visibility
      3.4    Documenting member functions
                3.4.1    The member function header
               3.4.2    Internal documentation
      3.5    Techniques for writing clean code
                3.5.1    Document your code
               3.5.2    Paragraph or indent your code
               3.5.3    Use whitespace in your code
               3.5.4    Follow the 30-second rule
               3.5.5    Write short, single command lines
               3.5.6    Specify the order of operations

4    Standards for Fields and Properties
       4.1    Naming fields
               4.1.1    Naming components (widgets)
                          4.1.1.1    Alternative for naming components: Hungarian notation
                         4.1.1.2    Alternative for naming components: postfix-Hungarian notation
                         4.1.1.3    Set component name standards
               4.1.2    Naming constants
               4.1.3    Naming collections
       4.2    Field visibility
                4.2.1    Do not "hide" names
      4.3    Documenting a field
       4.4    Using accessor member functions
              4.4.1    Why use accessors?               
                          4.4.1.1   When not to use accessors
              4.4.2    Naming accessors
               4.4.3    Advanced techniques for accessors
                         4.4.3.1    Lazy initialization
                         4.4.3.2    Accessors for constants
                         4.4.3.3    Accessors for collections
                          4.4.3.4   Accessing several fields simultaneously   
       4.5    Visibility of accessors
       4.6    Always initialize static fields

5    Standards for Local Variables
       5.1    Naming local variables
                   5.1.1    Naming streams
                    5.1.2    Naming loop counters
                    5.1.3    Naming exception objects
         5.2   Declaring and documenting local variables
                5.2.1     General comments about declaration

6    Standards for Parameters to Member Functions
      6.1    Naming parameters
       6.2    Documenting parameters
                6.2.1    Use interfaces for parameter types

7    Standards for Classes, Interfaces, Packages, and Compilation Units
      7.1    Standards for classes
                 7.1.1    Naming classes
                 7.1.2    Documenting a class
                 7.1.3    Class declarations
                 7.1.4    Minimize the public and protected interface
                             7.1.4.1    Define the public interface first
      7.2     Standards for interfaces
                 7.2.1    Naming interfaces
                             7.2.1.1    Alternative
                7.2.2    Documenting interfaces
       7.3     Standards for packages
                 7.3.1    Naming packages
                 7.3.2    Documenting packages
      7.4     Standards for compilation units
                 7.4.1    Naming compilations units
                 7.4.2    Documenting compilation units

8    Error Handling and Exceptions

9    Miscellaneous Standards and Issues
       9.1    Reusing
       9.2    Importing classes
       9.3    Optimizing Java code
       9.4    Writing Java test harnesses

10    Patterns of Success
        10.1    Using these standards effectively
       10.2    Other factors that lead to writing successful code

11    Summary
        11.1    Java naming conventions
         11.2    Java documentation conventions
                    11.2.1    Java comment types
                    11.2.2    What to document
         11.3    Java coding conventions (general)

12    References

13    Glossary

 

1    Introductionreturn to Contents

This document describes a collection of standards, conventions, and guidelines for writing solid Java code. They are based on sound, proven software engineering principles that lead to code that is easy to understand, to maintain, and to enhance. Furthermore, by following these coding standards your productivity as a Java developer should increase remarkably. Experience shows that by taking the time to write high-quality code right from the start, you will have a much easier time modifying it during the development process. Finally, following a common set of coding standards leads to greater consistency, making teams of developers significantly more productive.

1.1    The first and last guideline  return to Contents

Use common sense. When you cannot find a rule or guideline, when the rule obviously does not apply, when everything else fails: use common sense, and check the fundamental principles. This rule overrides all of the others. Common sense is required.

2    Coding Standards return to Contents

Coding standards for Java are important because they lead to greater consistency within your code and the code of your teammates. Greater consistency leads to code that is easier to understand, which means it is easier to develop and to maintain. This reduces the overall cost of the applications that you create.

You have to remember that your Java code will exist for a long time; long after you have moved on to other projects. An important goal during development is to ensure that you can transition your work to another developer, or to another team of developers so they can continue to maintain and enhance your work without having to invest an unreasonable effort to understand your code. Code that is difficult to understand runs the risk of being scrapped and rewritten.

2.1    Naming conventions return to Contents

We will be discussing naming conventions throughout the standards, so let's set the stage with a few basics:

  1. Use full English descriptors that accurately describe the variable, field, and class; for example, use names like firstName, grandTotal, or CorporateCustomer. Although names like x1, y1, or fn are easy to type because they're short, they do not provide any indication of what they represent and result in code that is difficult to understand, maintain, and enhance.
  2. Use terminology applicable to the domain. If your users refer to their clients as customers, then use the term Customer for the class, not Client. Many developers make the mistake of creating generic terms for concepts when perfectly good terms already exist in the industry or domain.
  3. Use mixed case to make names readable. Use lowercase letters in general, but capitalize the first letter of class names and interface names, as well as the first letter of any non-initial word. [KAN97]
  4. Use abbreviations sparingly, but if you do so then use them intelligently. This means you should maintain a list of standard short forms (abbreviations), you should choose them wisely, and you should use them consistently. For example, if you want to use a short form for the word "number", then choose one of nbr, no, or num, document which one you chose (it does not really matter which one), and use only that one.
  5. Avoid long names (< 15 characters is a good idea). Although the class name PhysicalOrVirtualProductOrService might seem to be a good class name at the time, this name is simply too long and you should consider renaming it to something shorter, perhaps something like Offering.
  6. Avoid names that are similar or differ only in case. For example, the variable names persistentObject and persistentObjects should not be used together, nor should anSqlDatabase and anSQLDatabase.
  7. Avoid leading or trailing underscores. Names with leading or trailing underscores are usually reserved for system purposes and may not be used for any user-created names except for preprocessor defines. More importantly, underscores are annoying and difficult to type so try to avoid their use whenever possible.

2.2    Documentation conventions return to Contents

We will also be discussing documentation conventions, so let's discuss some of the basics first:

  1. Comments should add to the clarity of your code. The reason you document your code is to make it more understandable to you, your coworkers, and to any other developer who comes after you.
  2. If your program is not worth documenting, it is probably not worth running. [NAG95]
  3. Avoid decoration; that is, do not use banner-like comments. In the 1960s and 1970s, COBOL programmers got into the habit of drawing boxes, typically with asterisks, around their internal comments. Sure, it gave them an outlet for their artistic urges, but frankly it was a major waste of time that added little value to the end product. You want to write clean code, not pretty code. Furthermore, because many of the fonts used to display and print your code are proportional, and many are not, you can't line up your boxes properly anyway.
  4. Keep comments simple. Some of the best comments are simple, point-form notes. You do not have to write a book; you just have to provide enough information so that others can understand your code.
  5. Write the documentation before you write the code. The best way to document code is to write the comments before you write the code. This gives you an opportunity to think about how the code will work before you write it and will ensure that the documentation gets written. Alternatively, you should at least document your code as you write it. Because documentation makes your code easier to understand, you are able to take advantage of this fact while you are developing it. If you are going to invest the time writing documentation, you should at least get something out of it. [AMB98]
  6. Document why something is being done, not just what. For example, the code in Example 1 below shows that a 5% discount is being given on orders of $1,000 dollars or more. Why is this being done? Is there a business rule that says that large orders get a discount? Is there a limited-time special on large orders or is it a permanent program? Was the original programmer just being generous? You do not know unless it is documented somewhere, either in the source code itself or in an external document.

Example 1:

if ( grandTotal >= 1000.00)

{

grandTotal = grandTotal * 0.95;

}

2.2.1    Types of Java commentsreturn to Contents

Java has three styles of comments: 

  • documentation comments that start with /** and end with */
  • C-style comments that start with /* and end with */
  • single-line comments that start with // and go until the end of the source-code line

The following chart is a summary of a suggested use for each type of comment, as well as several examples.

Comment Type Usage Example
Documentation Use documentation comments immediately before declarations of interfaces, classes, member functions, and fields to document them. Documentation comments are processed by javadoc, see below, to create external documentation for a class. /**
Customer: A customer is any person or organization that we sell services and products to.
@author S.W. Ambler
*/
C style Use C-style comments to document out lines of code that are no longer applicable, but that you want to keep just in case your users change their minds, or because you want to temporarily turn it off while debugging. /*
This code was commented out by B. Gustafsson on June 4, 1999 because it was replaced by the preceding code. Delete it after two years if it is still not applicable.
. . . (the source code )
*/
Single line Use single line comments internally within member functions to document business logic, sections of code, and declarations of temporary variables. // Apply a 5% discount to all
// invoices over $1000 due to
// generosity campaign started in
// Feb. of 1995.

The important thing is that your organization should set a standard as to how C-style comments and single-line comments are to be used, and then to follow that standard consistently. Use one type to document business logic and use the other to document out old code. Use single-line comments for business logic because you can put the documentation on the same line as the code (this is called inlining). Use C-style comments for documenting out old code because that allows you to comment out several lines at once. Because C-style looks very similar to documentation comments, to avoid confusion don't use them elsewhere.

Beware Endline Comments—[MCO93] strongly argues against the use of inline comments, also known as endline comments or end of line comments. McConnell points out that the comments have to be aligned to the right of the code so they don't interfere with the visual structure of the code. As a result, they tend to be hard to format and "if you use many of them, it takes time to align them. Such time is not spent learning more about the code; it is dedicated solely to the tedious task of pressing the spacebar or the tab key." He also points out that endline comments are also hard to maintain because when the code on the line grows, it bumps the endline comment out and if you are aligning them, you have to do the same for the rest of them.

2.2.2        A quick overview of javadoc return to Contents

Included in Sun's Java Development Kit (JDK) is a program called javadoc that processes Java code files and produces external documentation, in the form of HTML files, for your Java programs. Javadoc supports a limited number of tags; reserved words that mark the beginning of a documentation section. Please refer to the JDK javadoc documentation for further details.

Tag Used for Purpose
@author name Classes, Interfaces Indicates the author(s) of a given piece of code. One tag per author should be used.
@deprecated Classes, Member Functions Indicates that the API for the class has been deprecated and, therefore, should not be used any more.
@exception name description Member Functions Describes the exceptions that a member function throws. You should use one tag per exception and give the full class name for the exception.
@param name description Member Functions Used to describe a parameter passed to a member function, including its type or class and its usage. Use one tag per parameter.
@return description Member Functions Describes the return value, if any, of a member function. You should indicate the type or class and the potential use(s) of the return value.
@since Classes, Member Functions Indicates how long the item has existed; that is, since JDK 1.1.
@see ClassName Classes, Interfaces, Member Functions, Fields Generates a hypertext link in the documentation to the specified class. You can, and probably should, use a fully qualified class name.
@see ClassName#member functionName Classes, Interfaces, Member Functions, Fields Generates a hypertext link in the documentation to the specified member function. You can, and probably should, use a fully qualified class name.
@version text Classes, Interfaces Indicates the version information for a given piece of code.

The way that you document your code has a huge impact, both on your own productivity and on the productivity of everyone else who later maintains and enhances it. By documenting your code early in the development process, you become more productive because it forces you to think through your logic before you commit it to code. Furthermore, when you revisit code you wrote days or weeks earlier, you can easily determine what you were thinking when you wrote it because it's already documented for you.

3    Standards for Member Functions return to Contents

Never forget that the code you write today may still be in use many years from now, and will likely be maintained and enhanced by somebody other than you. You must strive to make your code as "clean" and understandable as possible, because these factors make it easier to maintain and to enhance.

3.1    Naming member functions return to Contents

Member functions should be named using a full English description, using mixed case with the first letter of any non-initial word capitalized. It is also common practice for the first word of a member function name to be a strong, active verb.

Examples:

openAccount()

printMailingLabel()

save()

delete()

This convention results in member functions whose purpose can often be determined just by looking at their names. Although this convention results in a little extra typing by the developer because it often results in longer names, this is more than made up for by the increased understandability of your code.

3.1.1    Naming accessor member functions return to Contents

We will discuss accessors, member functions that get and set the values of fields (fields or properties) in greater detail in a next chapter. The naming conventions for accessors, however, are summarized below.

3.1.1.1    Getters  return to Contents

Getters are member functions that return the value of a field. You should prefix the word "get" to the name of the field, unless it is a boolean field and then you prefix "is" to the name of the field instead of "get".

Examples:

getFirstName()

getAccountNumber()

isPersistent()

isAtEnd()

By following this naming convention, you make it obvious that a member function returns a field of an object, and for boolean getters you make it obvious that it returns true or false. Another advantage of this standard is that it follows the naming conventions used by the Beans Development Kit (BDK) for getter member functions. [DES97] The main disadvantage is that "get" is superfluous and requires extra typing.

Alternative naming convention for Getters: has and can

A viable alternative, based on proper English conventions, is to use the prefix "has" or "can" instead of "is" for boolean getters. For example, getter names such as hasDependents() and canPrint() make a lot of sense when you are reading the code. The problem with this approach is that the BDK will not pick up on this naming strategy (yet). You could rename these member functions isBurdenedWithDependents() and isPrintable().

3.1.1.2    Setters return to Contents

Setters, also known as mutators, are member functions that modify the values of a field. You should prefix the word "set" to the name of the field, regardless of the field type.

Examples:

setFirstName(String aName)

setAccountNumber(int anAccountNumber)

setReasonableGoals(Vector newGoals)

setPersistent(boolean isPersistent)

setAtEnd(boolean isAtEnd)

By following this naming convention, you make it obvious that a member function sets the value of a field of an object. Another advantage of this standard is that it follows the naming conventions used by the BDK for setter member functions. [DES97] The main disadvantage is that "set" is superfluous and requires extra typing.

3.2    Naming constructors return to Contents

Constructors are member functions that perform any necessary initialization when an object is first created. Constructors are always given the same name as their class. For example, a constructor for the class Customer would be Customer(). Note that the same case is used.

Examples:

Customer()

SavingsAccount()

PersistenceBroker()

This naming convention is set by Sun Microsystems and must be strictly adhered to.

3.3    Member function visibility return to Contents

For a good design where you minimize the coupling between classes, the general rule of thumb is to be as restrictive as possible when setting the visibility of a member function. If a member function does not have to be public, then make it protected, and if it does not have to be protected, then make it private.

Visibility Description Proper Usage
public A public member function can be invoked by any other member function in any other object or class. When the member function must be accessible by objects and classes outside of the class hierarchy in which the member function is defined.
protected A protected member function can be invoked by any member function in the class in which it is defined or any subclasses of that class. When the member function provides behavior that is needed internally within the class hierarchy but not externally.
private A private member function can only be invoked by other member functions in the class in which it is defined, but not in the subclasses. When the member function provides behavior that is specific to the class. Private member functions are often the result of refactoring, also known as reorganizing, the behavior of other member functions within the class to encapsulate one specific behavior.

3.4    Documenting member functions return to Contents

The manner in which you document a member function will often be the deciding factor as to whether or not it is understandable and, therefore, maintainable and extensible.

3.4.1    The member function header return to Contents

Every Java member function should include some sort of header, called member function documentation, at the top of the source code that documents all of the information that is critical to understanding it. This information includes, but is not limited to, the following:

  1. What and why the member function does what it does. By documenting what a member function does, you make it easier for others to determine if they can reuse your code. Documenting why it does something makes it easier for others to put your code into context. You also make it easier for others to determine whether or not a new change should actually be made to a piece of code (perhaps the reason for the new change conflicts with the reason why the code was written in the first place).
  2. What member function must be passed as parameters. You also need to indicate what parameters, if any, must be passed to a member function and how they will be used. This information is needed so that other programmers know what information to pass to a member function. javadoc @param tag, discussed in section 2.2.2, A quick overview of javadoc, is used for this.
  3. What a member function returns. You need to document what, if anything, a member function returns so that other programmers can use the return value or object appropriately. The javadoc @return tag, discussed in section 2.2.2, A quick overview of javadoc, is used for this.
  4. Known bugs. Any outstanding problems with a member function should be documented so that other developers understand the weaknesses and difficulties with the member function. If a given bug is applicable to more than one member function within a class, then it should be documented for the class instead.
  5. Any exceptions that a member function throws. You should document any and all exceptions that a member function throws so that other programmers know what their code will need to catch. The javadoc @exception tag, discussed in section 2.2.2, A quick overview of javadoc, is used for this.
  6. Visibility decisions. If you feel that your choice of visibility for a member function will be questioned by other developers—perhaps you have made a member function public even though no other objects invoke the member function yet—then you should document your decision. This will help to make your thinking clear to other developers so that they do not waste time worrying about why you did something questionable.
  7. How a member function changes the object. If a member function changes an object, for example the withdraw() member function of a bank account modifies the account balance, then this needs to be indicated. This information is needed so that other Java programmers know exactly how a member function invocation will affect the target object.
  8. Avoid the use of headers containing information such as author, phone numbers, dates of creation and modification, and location of unit (or file name), because this information rapidly becomes obsolete. Place ownership copyright notices at the end of the unit. For instance, readers do not want to have to scroll through two or three pages of text that is not useful to the understanding of the program, nor do they want to scroll through text that does not carry any program information at all, such as a copyright notice. Avoid the use of vertical bars or closed frames or boxes, which just add visual noise and are difficult to keep consistent. Use a configuration management tool to keep unit history.
  9. Examples of how to invoke the member function if appropriate. One of the easiest ways to determine how a piece of code works is to look at an example. Consider including an example or two about how to invoke a member function.
  10. Applicable preconditions and postconditions. A precondition is a constraint under which a member function will function properly, and a postcondition is a property or assertion that will be true after a member function is finished running. [MEY88] In many ways preconditions and postconditions describe the assumptions that you have made when writing a member function [AMB98], defining exactly the boundaries of how a member function is used.
  11. All concurrency issues. Concurrency is a new and complex concept for many developers and, at best, it is an old and complex topic for experienced concurrent programmers. The end result is that if you use the concurrent programming features of Java, then you need to document it thoroughly. [LEA97] suggests that when a class includes both synchronized and unsynchronized member functions, you must document the execution context that a member function relies on, especially when it requires unrestricted access so that other developers can use your member functions safely. When a setter, a member function that updates a field, of a class that implements the Runnable interface is not synchronized, then you should document your reason(s) why. Finally, if you override or overload a member function and change its synchronization, you should also document why.
  12. You should document something only when it adds to the clarity of your code. You would not document all of the factors described above for each and every member function because not all factors are applicable to every member function. You would, however, document several of them for each member function that you write.

3.4.2    Internal documentation return to Contents

In addition to the member function documentation, you also need to include comments within your member functions to describe your work. The goal is to make your member function easier to understand, maintain, and enhance.

There are two types of comments that you should use to document the internals of your code C-style comments ( /* and */ ) and single-line comments ( // ). As previously discussed, you should seriously consider choosing one style of comments for documenting the business logic of your code and one for commenting out unneeded code. It is suggested that you use single-line comments for your business logic, because you can use this style of comments both for full comment lines and for inline comments that follow at the end of a line of code. Use C-style comments to document out lines of unneeded code because it makes it easier to take out several lines with only one comment. Furthermore, because C-style comments look so much like documentation comments, their use can be confusing, which takes away from the understandability of your code. Therefore, use them sparingly.

Internally, you should always document the following:

  1. Control structures. Describe each control structure, such as comparison statements and loops. You should not have to read all the code in a control structure to determine what it does; instead you should just have to look at a one or two line comment immediately preceding it.
  2. Why, as well as what, the code does. You can always look at a piece of code and figure out what it does, but for code that is not obvious, you can rarely determine why it was done that way. For example, you can look at a line of code and easily determine that a 5% discount is being applied to the total of an order. That is easy. What is not easy is figuring out WHY that discount is being applied. Obviously there is some sort of business rule that says to apply the discount, so that business rule should at least be referred to in your code so that other developers can understand why your code does what it does.
  3. Local variables. Although we will discuss this in greater detail in Chapter 5, each local variable defined in a member function should be declared on its own line of code and should usually have an inline comment describing its use.
  4. Difficult or complex code. If you find that you either can't rewrite it, or do not have the time, then you must thoroughly document any complex code in a member function. A general rule of thumb is that if your code is not obvious, then you need to document it.
  5. The processing order. If there are statements in your code that must be executed in a defined order, then you should ensure that this fact gets documented [AMB98]. There's nothing worse than making a simple modification to a piece of code only to find that it no longer works, then spending hours looking for the problem only to find that you have gotten things out of order.
  6. Document your closing braces. Every so often, you will find that you have control structures within control structures within control structures. Although you should avoid writing code like this, sometimes you find that it is better to write it this way. The problem is that it becomes confusing as to which ending brace the } character belongs to, which control structure. The good news is that some code editors support a feature that, when you select a open brace, will automatically highlight the corresponding closing one; the bad news is that not every editor supports this. I have found that marking the ending braces with an inline comment, such as //end if, //end for, //end switch, makes your code easier to understand.

3.5    Techniques for writing clean code return to Contents

This section covers several techniques that help separate the professional developers from the hack coders. These techniques are:

  • Document your code.
  • Paragraph or indent your code.
  • Use whitespace.
  • Follow the 30-second rule.
  • Write short, single command lines.
  • Specify the order of operations.

3.5.1    Document Your Code  return to Contents

Remember—if your code is not worth documenting, then it is not worth keeping.[NAG95] When you apply the documentation standards and guidelines proposed in this document appropriately, you can greatly enhance the quality of your code.

3.5.2   Paragraph or indent your code  return to Contents

One way to improve the readability of a member function is to paragraph it or, in other words, indent your code within the scope of a code block. Any code within braces, the { and } characters, forms a block. The basic idea is that the code within a block should be uniformly indented one unit.

The Java convention appears to be that the open brace is to be put on the line following the owner of the block and that the closing brace should be indented one level. The important thing, pointed out by [LAF97], is that your organization chooses an indentation style and sticks to it. Use the same indentation style that your Java development environment uses for the code that it generates.

3.5.3    Use whitespace in your code  return to Contents

A few blank lines, called whitespace, added to your Java code can help make it much more readable by dividing it into small, easy-to-digest sections. [VIS96] suggests using a single blank line to separate logical groups of code, such as control structures, with two blank lines to separate member function definitions. Without whitespace, it is very difficult to read and to understand.

3.5.4   Follow the 30-second rule  return to Contents

Other programmers should be able to look at your member function and fully understand what it does, why it does it, and how it does it in less than 30 seconds. If this is not possible, then your code is too difficult to maintain and should be improved. Thirty seconds; that's it. A good rule of thumb is that if a member function is more than a screen, then it's probably too long.

3.5.5    Write short, single command lines  return to Contents

Your code should do one thing per line. Back in the days of punch cards, it made sense to try to get as much functionality as possible on a single line of code. Whenever you attempt to do more than one thing on a single line of code, you make it harder to understand. Why do this? We want to make our code easier to understand so that it is easier to maintain and enhance. Just like a member function should do one thing and one thing only, you should only do one thing on a single line of code.

Furthermore, you should write code that remains visible on the screen [VIS96]. You should not have to scroll your editing window to the right to read the entire line of code, including code that uses inline comments.

3.5.6    Specify the order of operations  return to Contents

A really easy way to improve the understandability of your code is to use parentheses, also called "round brackets", to specify the exact order of operations in your Java code [NAG95] and [AMB98]. If you have to know the order of operations for a language to understand your source code, then something is seriously wrong. This is mostly an issue for logical comparisons where you AND and OR several other comparisons together. Note that if you use short, single-command lines, as previously suggested, then this really should not become an issue.

4    Standards for Fields and Properties return to Contents

The term field used here refers to a field that the BDK calls a property [DES97]. A field is a piece of data that describes an object or class. Fields may be base data type, like a string or a float, or may be an object, such as a customer or a bank account.

4.1    Naming fields return to Contents

You should use a full English descriptor to name your fields, [GOS96] and [AMB98], thereby making it obvious what the field represents. Fields that are collections, such as arrays or vectors, should be given names that are plural to indicate that they represent multiple values.

Examples:

firstName

zipCode

unitPrice

discountRate

orderItems

4.1.1    Naming components (widgets) return to Contents

For names of components (interface widgets), you should use a full English descriptor postfixed by the widget type. This makes it easy for you to identify the purpose of the component, as well as its type, which makes it easier to find each component in a list. Many visual programming environments provide lists of all components in an applet or application and it can be confusing when everything is named button1, button2, and so on.

Examples:

okButton

customerList

fileMenu

newFileMenuItem

4.1.1.1    Alternative for naming components: Hungarian notation  return to Contents

The "Hungarian Notation" [MCO93] is based on the principle that a field should be named using the following approach: xEeeeeeEeeeee where x indicates the component type and EeeeeEeeeee is the full English descriptor.

Examples:

pbOk

lbCustomer

mFile

miNewFile

The main advantage is that this is an industry standard common for C++ code so many people already follow it. From the name of the variable, developers can quickly judge its type and how it's used. The main disadvantages are that the prefix notation becomes cumbersome when you have a lot of the same type of widgets and you break from the full English descriptor naming convention.

4.1.1.2    Alternative for naming components: postfix-Hungarian notation  return to Contents

Basically this is a combination of the other two alternatives and it results in names such as okPb, customerLb, fileM, and newFileMi. The main advantage is that the name of the component indicates the widget type and that widgets of the same type are not grouped together in an alphabetical list. The main disadvantage is that you still are not using a full English description, making the standard harder to remember because it deviates from the norm.

4.1.1.3    Set component name standards  return to Contents

Whatever convention you choose, you'll want to create a list of "official" widget names. For example, when naming buttons, do you use Button or PushButton, b or pb? Create a list and make it available to every Java developer in your organization

4.1.2    Naming constants return to Contents

In Java, constants—values that do not change—are typically implemented as static final fields of classes. The recognized convention is to use full English words, all in uppercase, with underscores between the words [GOS96].

Examples:

MINIMUM_BALANCE

MAX_VALUE

DEFAULT_START_DATE

The main advantage to this convention is that it helps you distinguish constants from variables. We will see later in this document that you can greatly increase the flexibility and maintainability of your code by not defining constants; instead you should define getter member functions that return the value of constants.

4.1.3    Naming collections return to Contents

A collection, such as an array or a vector, should be given a pluralized name representing the types of objects stored by the array. The name should be a full English descriptor, with the first letter of all non-initial words capitalized.

Examples:

customers

orderItems

aliases

The main advantage of this convention is that it helps to distinguish fields that represent multiple values (collections) from those that represent single values (non-collections).

4.2    Field visibility return to Contents

When fields are declared protected, there is the possibility of member functions in subclasses to directly access them, effectively increasing the coupling within a class hierarchy. This makes your classes more difficult to maintain and to enhance, therefore, it should be avoided. Fields should never be accessed directly; instead accessor member functions (see below) should be used.

Visibility Description Proper Usage
public A public field can be accessed by any other member function in any other object or class. Do not make fields public.
protected A protected field can be accessed by any member function in the class in which it is declared or by any member functions defined in subclasses of that class. Do not make fields protected.
private A private field can only be accessed by member functions in the class in which it is declared, but not in the subclasses. All fields should be private, and should be accessed by getter and setter member functions (accessors).

For fields that are not persistent (they will not be saved to permanent storage), you should mark them as either static or transient [DES97]. This makes them conform to the conventions of the BDK.

4.2.1    Do not "hide" names  return to Contents

Name hiding refers to the practice of naming a local variable, argument, or field the same (or similar) as that of another one of greater scope. For example, if you have a field called firstName do not create a local variable or parameter called firstName, or anything close to it like firstNames or fistName. This makes your code difficult to understand and prone to bugs because other developers, or you, will misread your code while they are modifying it and will make difficult-to-detect errors.

4.3    Documenting a field return to Contents

Every field should be documented well enough so that other developers can understand it. To be effective, you need to document:

  1. Its description. You need to describe a field so that people know how to use it.
  2. All applicable invariants. Invariants of a field are the conditions that are always true about it. For example, an invariant about the field dayOfMonth might be that its value is between 1 and 31 (obviously you could get far more complex with this invariant, restricting the value of the field based on the month and the year). By documenting the restrictions on the value of a field, you help to define important business rules that make it easier to understand how your code works.
  3. Examples. For fields that have complex business rules associated with them, you should provide several example values to make them easier to understand. An example is often like a good picture: it's worth a thousand words.
  4. Concurrency issues. Concurrency is a new and complex concept for many developers; actually, at best, it is an old and complex topic for experienced concurrent programmers. The end result is that if you use the concurrent programming features of Java, then you need to document them thoroughly.
  5. Visibility decisions. If you have declared a field to be anything but private, then you should document why you have done so. Field visibility was previously discussed in section 4.2, Field visibility and the use of accessor member functions to support encapsulation is covered next in section 4.4, Using of accessor member functions. The bottom line is that you better have a really good reason for not declaring a variable as private.

4.4    Using accessor member functions return to Contents

In addition to naming conventions, the maintainability of fields is achieved by the appropriate use of accessor member functions—member functions that provide the functionality to either update a field or to access its value. Accessor member functions come in two flavors: setters (also called mutators) and getters. A setter modifies the value of a variable, whereas a getter obtains it for you.

Although accessor member functions used to add overhead to your code, Java compilers are now optimized for their use so this is no longer true. Accessors help to hide the implementation details of your class. By having two control points at most from which a variable is accessed, one setter and one getter, you are able to increase the maintainability of your classes by minimizing the points at which changes need to be made. Optimization of Java code is discussed in section 9.3, Optimizing Java Code.

One of the most important standards that your organization can enforce is the use of accessors. Some developers do not want to use accessor member functions because they do not want to type the few extra keystrokes required; for example, for a getter you need to type in "get" and "()" above and beyond the name of the field. The bottom line is that the increased maintainability and extensibility from using accessors more than justifies their use.

Accessors are the only place to access fields. A key concept with the appropriate use of accessor member functions is that the ONLY member functions that are allowed to directly work with a field are the accessor member functions themselves. Yes, it's possible to directly access a private field within the member functions of the class in which the field is defined, but you do not want to do so because you would increase the coupling within your class.

4.4.1    Why use accessors?  return to Contents

"Good program design seeks to isolate parts of a program from unnecessary, unintended, or otherwise unwanted outside influences. Access modifiers (accessors) provide an explicit and checkable means for the language to control such contacts." [KAN97]

Accessor member functions improve the maintainability of your classes in the following ways:

  1. Updating fields. You have single points of update for each field, making it easier to modify and to test. In other words your fields are encapsulated.
  2. Obtaining the values of fields. You have complete control over how fields are accessed and by whom.
  3. Obtaining the values of constants and the names of classes. By encapsulating the value of constants and of class names in getter member functions when those values/names change you only need to update the value in the getter and not every line of code where the constant/name is used.
  4. Initializing fields. The use of lazy initialization ensures that fields are always initialized and that they are initialized only if they are needed.
  5. Reducing the coupling between a subclass and its superclass(es). When subclasses access inherited fields only through their corresponding accessor member functions, it makes it possible to change the implementation of fields in the superclass without affecting any of its subclasses, effectively reducing coupling between them. Accessors reduce the risk of the "fragile base class" where changes in a superclass ripple throughout its subclasses.
  6. Encapsulating changes to fields. If the business rules pertaining to one or more fields change you can potentially modify your accessors to provide the same ability as before the change, making it easier for you to respond to the new business rules.
  7. Simplifying concurrency issues. [LEA97] points out that setter member functions provide a single place to include a notifyAll if you have waits based on the value of that field. This makes moving to a concurrent solution much easier.
  8. Name-hiding becomes less of an issue. Although you should avoid name hiding, giving local variables the same names as fields, the use of accessors to always access fields means that you can give local variables any name you want. You do not have to worry about hiding field names because you never access them directly anyway.
4.4.1.1    When not to use accessors  return to Contents

The only time that you might want to not use accessors is when execution time is of the utmost importance. However, it's a very rare case when the increased coupling within your application justifies this action.

4.4.2    Naming accessors return to Contents

Getter member functions should be given the name "get" + field name, unless the field represents a boolean (true or false), and then the getter is given the name "is" + field name. Setter member functions should be given the name "set" + field name, regardless of the field type ([GOS96] and [DES97]). Note that the field name is always in mixed case with the first letter of all words capitalized. This naming convention is used consistently within the JDK and is required for beans development.

Examples:
Field Type Getter name Setter name 
firstName
 
string
 
getFirstName()
 
setFirstName()
 
address
 
Address 
object
 
getAddress()
 
setAddress()
 
persistent
 
boolean
 
isPersistent()
 
setPersistent()
 
customerNo
 
int
 
getCustomerNo()
 
setCustomerNo()
 
orderItems
 
Array of 
OrderItem 
objects
 
getOrderItems()
 
setOrderItems()

 

4.4.3    Advanced techniques for accessors return to Contents

Accessors can be used for more than just getting and setting the values of instance fields. This section discusses how to increase the flexibility of your code by using accessors to:

  • initialize the values of fields
  • access constant values
  • access collections
  • access several fields simultaneously
4.4.3.1    Lazy initialization  return to Contents

Variables need to be initialized before they are accessed. There are two schools of thought to initialization: Initialize all variables at the time the object is created (the traditional approach) or initialize at the time it's first used. 

The first approach uses special member functions that are invoked when the object is first created, called constructors. Although this works, it often proves to be error-prone. When adding a new variable, you can easily forget to update the constructors.

An alternative approach is called lazy initialization where fields are initialized by their getter member functions, as shown below. Note how a setter member function is used within the getter member function. Notice that the member function checks to see if the branch number is zero; if it is, then it sets it to the appropriate default value.

/** Answers the branch number, which is the leftmost

four digits of the full account number.

Account numbers are in the format BBBBAAAAAA.

*/

protected int getBranchNumber()

{

if( branchNumber == 0)

{

// The default branch number is 1000, which

// is the main branch in downtown Bedrock.

setBranchNumber(1000);

}

return branchNumber;

}

It is quite common to use lazy initialization for fields that are actually other objects stored in the database. For example, when you create a new inventory item, you do not need to fetch whatever inventory item type from the database that you have set as a default. Instead, use lazy initialization to set this value the first time it is accessed, so that you only have to read the inventory item type object from the database when and if you need it. 

This approach is advantageous for objects that have fields that are not regularly accessed. Why incur the overhead of retrieving something from persistent storage if you are not going to use it?

Whenever lazy initialization is used in a getter member function, you should document why the default value is what it is, as we saw in the example above. When you do this, you take the mystery out of how fields are used in your code, which improves both its maintainability and its extensibility.
4.4.3.2    Accessors for constants return to Contents

The common Java idiom is to implement constant values as static final fields. This approach makes sense for "constants" that are guaranteed to be stable. For example, the class Boolean implements two static final fields called TRUE and FALSE, which represents the two instances of that class. It would also make sense for a DAYS_IN_A_WEEK constant whose value probably is never going to change.

However, many so-called business "constants" change over time because the business rules change. Consider the following example: The Archon Bank of Cardassia (ABC) has always insisted that an account has a minimum balance of $500 if it is to earn interest. To implement this, we could add a static field named MINIMUM_BALANCE to the class Account that would be used in the member functions that calculate interest. Although this would work, it is not flexible. What happens if the business rules change and different kinds of accounts have different minimum balances, perhaps $500 for savings accounts but only $200 for checking accounts? What would happen if the business rules were to change to a $500 minimum balance in the first year, $400 in the second, $300 in the third, and so on? Perhaps the rule will be changed to $500 in the summer, but to only $250 in the winter? Perhaps a combination of all of these rules will need to be implemented in the future.

The point being made is that implementing constants as fields is not flexible. A much better solution is to implement constants as getter member functions. In our example above, a static (class) member function called getMinimumBalance() is far more flexible than a static field called MINIMUM_BALANCE because we can implement the various business rules in this member function and subclass it appropriately for various kinds of accounts. 

/** Get the value of the account number. Account numbers are in the following 
    format: BBBBAAAAAA, where BBBB is the branch number and 
    AAAAAA is the branch account number.
*/
public long getAccountNumber()
{
	return ( ( getBranchNumber() * 100000 ) + getBranchAccountNumber() );
}
/**
	Set the account number. Account numbers are in the following 
	format: BBBBAAAAAA where BBBB is the branch number and
	AAAAAA is the branch account number.
*/
public void setAccountNumber(int newNumber)
{
	setBranchAccountNumber( newNumber % 1000000 );
	setBranchNumber( newNumber / 1000000 );
}

Another advantage of constant getters is that they help to increase the consistency of your code. Consider the code shown above—it doesn't work properly. An account number is the concatenation of the branch number and the branch account number. Testing our code, we find that the setter member function, setAccountNumber() does not update branch account numbers properly; it takes the three leftmost digits, not four. That is because we used 1,000,000 instead of 100,000 to extract the field branchAccountNumber. Had we used a single source for this value, the constant getter getAccountNumberDivisor() as we see below, our code would have been more consistent and would have worked. 

/**
	Returns the divisor needed to separate the branch account number from the 
	branch number within the full account number. 
	Full account numbers are in the format BBBBAAAAAA.
*/
public int getAccountNumberDivisor()
{
	return ( (long) 1000000);
}
/**
	Get the value of the account number. Account numbers are in the following 
	format: BBBBAAAAAA, where BBBB is the branch number and 
	AAAAAA is the branch account number.
*/
public long getAccountNumber()
{
	return ( ( getBranchNumber() * getAccountNumberDivisor() ) + getBranchAccountNumber() );
}
/**
	Set the account number. Account numbers are in the following 
	format: BBBBAAAAAA where BBBB is the branch number and
	AAAAAA is the branch account number.
*/
public void setAccountNumber(int newNumber)
{
	setBranchAccountNumber( newNumber % getAccountNumberDivisor() );
	setBranchNumber( newNumber / getAccountNumberDivisor() );
}

By using accessors for constants, we decrease the chance of bugs and, at the same time, increase the maintainability of our system. When the layout of an account number changes, and we know that it eventually will, chances are that our code will be easier to change because we have both hidden and centralized the information needed to build or divide account numbers.

4.4.3.3    Accessors for collections return to Contents

The main purpose of accessors is to encapsulate the access to fields to reduce the coupling within your code. Collections, such as arrays and vectors, being more complex than single value fields naturally need to have more than just the standard getter and setter member function implemented for them. In particular, because you can add and remove to and from collections, accessor member functions need to be included to do so. Add the following accessor member functions where appropriate for a field that is a collection:

Member Function type Naming convention Example
Getter for the collection 
getCollection()
 
getOrderItems()
Setter for the collection 
setCollection() 
setOrderItems()
Insert an object into the collection 
insertObject()
 
insertOrderItem()
Delete an object from the collection 
deleteObject()
 
deleteOrderItem()
Create and add a new object into the collection 
newObject()
 
newOrderItem()

The advantage of this approach is that the collection is fully encapsulated, allowing you to later replace it with another structure, perhaps a linked list or a B-tree.

4.4.3.4    Accessing several fields simultaneously return to Contents

One of the strengths of accessor member functions is that they enable you to enforce business rules effectively. Consider, for example, a class hierarchy of shapes. Each subclass of Shape knows its position via the use of two fields "xPosition and yPosition" and can be moved on the screen on a two-dimensional plane by invoking the member function move(Float xMovement, Float yMovement). For our purposes, it does not make sense to move a shape along one axis at a time; instead we'll move along both the x and the y axes simultaneously (it is acceptable to pass a value of 0.0 as for either parameter of the move() member function). The implication is that the move() member function should be public, but the member functions setXPosition() and setYPosition() should both be private, being invoked by the move() member function appropriately.

An alternative implementation would be to introduce a setter member function that updates both fields at once, as shown below. The member functions setXPosition() and setYPosition() would still be private so that they may not be invoked directly by external classes or subclasses (you would want to add some documentation, shown below, indicating that they should not be directly invoked). 

/** Set the position of the shape */
protected void setPosition(Float x, Float y)
{
	setXPosition(x);
	setYPosition(y);
}
/** Set the x position.  Important: Invoke setPosition(), not this member function. */
private void setXPosition(Float x)
{
	xPosition = x;
}
/** Set the y position of the shape
    Important: Invoke setPosition(), not this member function.
*/
private void setYPosition(Float y)
{
	yPosition = y;
}

4.5    Visibility of accessors return to Contents

Always strive to make accessors protected, so that only subclasses can access the fields. Only when an "outside class" needs to access a field should you make the corresponding getter or setter public. Note that it's common that the getter member function be public and the setter protected.

Sometimes you need to make setters private to ensure certain invariants hold. For example, an Order class may have a field representing a collection of OrderItem instances, and a second field called orderTotal, which is the total of the entire order. The orderTotal is a convenience field that is the sum or all subtotals of the ordered items. The only member functions that should update the value of orderTotal are those that manipulate the collection of order items. Assuming that those member functions are all implemented in Order, you should make setOrderTotal() private, even though getOrderTotal() is more than likely public.

4.6    Always initialize static fields return to Contents

Static fields, also known as class fields, should be given valid values because you cannot assume that instances of a class will be created before a static field is accessed.

 

5    Standards for Local Variables return to Contents

A local variable is an object or data item that is defined within the scope of a block, often a member function. The scope of a local variable is the block in which it is defined. The important coding standards for local variables focus on:

  • Naming conventions
  • Declarations and documentation conventions

5.1    Naming local variables return to Contents

In general, local variables are named following the same conventions used for fields; in other words, use full English descriptors with the first letter of any non-initial word in uppercase.

For the sake of convenience, however, this naming convention is relaxed for several specific types of local variable:

  • Streams
  • Loop counters
  • Exception objects

5.1.1    Naming streams return to Contents

When there is a single input and/or output stream being opened, used, and then closed within a member function, the common convention is to use in and out for the names of these streams, respectively [GOS96]. For a stream used for both input and output, the implication is to use the name inOut.

A common alternative to this naming convention, although conflicting with Sun's recommendations, is to use the names inputStream, outputStream, and ioStream instead of in, out, and inOut respectively.

5.1.2    Naming loop counters return to Contents

Because loop counters are a very common use for local variables, and because it was acceptable in C/C++, in Java programming the use of i, j, or k, is acceptable for loop counters [GOS96]. If you use these names for loop counters, use them consistently.

A common alternative is to use names like loopCounter or simply counter, but the problem with this approach is that you often find names like counter1 and counter2 in member functions that require more than one counter. The bottom line is that i, j, k work as counters; they're quick to type in and they're generally accepted.

5.1.3    Naming exception objects return to Contents

Because exception handling is also very common in Java coding, the use of the letter e for a generic exception is considered acceptable [GOS96].

5.2    Declaring and documenting local variables return to Contents

There are several conventions regarding the declaration and documentation of local variables in Java. These conventions are:

  1. Declare one local variable per line of code. This is consistent with one statement per line of code and makes it possible to document each variable with an inline comment.
  2. Document local variables with an inline comment. Inline commenting is a style, where a single line comment, denoted by //, immediately follows a command on the same line of code (this is called an endline comment). You should document what a local variable is used for, where its use is appropriate, and why it is used. This makes your code easier to understand.
  3. Use local variables for one thing only. Whenever you use a local variable for more than one reason, you effectively decrease its cohesion and make it difficult to understand. You also increase the chances of introducing bugs into your code from the unexpected side effects of previous values of a localvariable from earlier in the code. Yes, reusing local variables is more efficient because less memory needs to be allocated, but reusing local variables decreases the maintainability of your code and makes it more fragile. This usually is not worth the small savings gained from not having to allocate more memory.

5.2.1    General comments about declaration return to Contents

Local variables that are declared between lines of code, for example, within the scope of an if statement, can be difficult to find by people who are not familiar with your code.

One alternative to declaring local variables immediately before their first use is to instead declare them at the top of the code. Because your member functions should be short anyway, see section 3.5.5, Write short, single command lines, it should not be all that bad having to go to the top of your code to determine what the local variable is all about.

6    Standards For Parameters to Member Functions return to Contents

The standards that are important for parameters and arguments to member functions focus on how they are named and how they are documented. The term parameter is used to refer to a member function argument.

6.1    Naming parameters return to Contents

Parameters should be named following the same conventions used for local variables. As with local variables, name hiding is an issue.

Examples:

customer

inventoryItem

photonTorpedo

in

e

A viable alternative, taken from Smalltalk, is to use the naming conventions for local variables, with the addition of "a" or "an" on the front of the name. Adding "a" or "an" helps make the parameter stand out from local variables and fields, and avoids the name hiding problem. This is the preferred approach.

Examples:

aCustomer

anInventoryItem

aPhotonTorpedo

anInputStream

anException

6.2    Documenting parameters return to Contents

Parameters to a member function are documented in the header documentation for the member function using the javadoc @param tag. You should describe the following:

  1. What it should be used for. You need to document what a parameter is used for, so that other developers understand the full context of how the parameter will be used.
  2. Any restrictions or preconditions. If the full range of values for a parameter is not acceptable to a member function, then the invoker of that member function needs to know. Perhaps a member function only accepts positive numbers or strings of less than five characters.
  3. Examples. If it isn't completely obvious what a parameter should be, then you should provide one or more examples in the documentation.

6.2.1    Use interfaces for parameter types return to Contents

Instead of specifying a class, such as Object, for the type of a parameter, if appropriate specify an interface, such as Runnable. The advantage is that this approach, depending on the situation, can be more specific (Runnable is more specific than Object), or potentially may be a better way to support polymorphism. Instead of insisting on a parameter being an instance of a class in a specific class hierarchy, specify that it supports a specific interface implying that it only needs to be polymorphically compliant to what you need.

7    Standards for Classes, Interfaces, Packages, and Compilation Units return to Contents

This chapter concentrates on standards and guidelines for classes, interfaces, packages, and compilation units. A class is a template from which objects are instantiated (created). Classes contain the declaration of fields and member functions. Interfaces are the definition of a common signature, including both member functions and fields, which a class that implements an interface must support. A package is a collection of related classes. Finally, a compilation unit is a source code file in which classes and interfaces are declared. Because Java allows compilation units to be stored in a database, an individual compilation unit may not directly relate to a physical source code file.

7.1    Standards for classes return to Contents

The standards that are important for classes are based on:

  • naming conventions
  • documentation conventions
  • declaration conventions
  • the public and protected interface

7.1.1    Naming classes return to Contents

The standard Java convention uses a full English descriptor, starting with the first letter capitalized and using mixed case for the rest of the name. ([GOS96] and [AMB98])

Class names must be in the singular form.

Examples:

Customer

Employee

Order

OrderItem

FileStream

String

7.1.2    Documenting a class return to Contents

The following information should appear in the documentation comments immediately preceding the definition of a class:

  1. The purpose of the class. Developers need to know the general purpose of a class so they can determine whether or not it meets their needs. Make it a habit to document any good things to know about a class; for example, is it part of a pattern or are there any interesting limitations to using it [AMB98]?
  2. Known bugs. If there are any outstanding problems with a class, they should be documented so that other developers understand the weaknesses and difficulties with the class. Furthermore, the reason for not fixing the bug also needs to be documented. Note that if a bug is specific to a single member function, then it should be directly associated with the member function instead.
  3. The development or maintenance history of the class. It's common practice to include a history table listing dates, authors, and summaries of changes made to a class. This provides maintenance programmers insight into any modifications made to a class in the past and documents who has done what to a class.
  4. Document applicable invariants. An invariant is a set of assertions about an instance or class that must be true at all "stable" times, where a stable time is defined as the period before a member function is invoked on the object or class and immediately after a member function is invoked [MEY88]. By documenting the invariants of a class, you provide valuable insight to other developers about how a class can be used.
  5. The concurrency strategy. Any class that implements the interface Runnable should have its concurrency strategy fully described. Concurrent programming is a complex topic that's new for many programmers, therefore you need to invest the extra time to ensure that people can understand your work. It's important to document your concurrency strategy and why you chose that strategy over others. Common concurrency strategies [LEA97] include the following:
  • synchronized objects
  • balking objects
  • guarded objects
  • versioned objects
  • concurrency policy controllers
  • acceptors

7.1.3    Class declarations return to Contents

One way to make your classes easier to understand is to declare them in a consistent manner. The common approach in Java is to declare a class in the following order:

  • public member functions
  • public fields
  • protected member functions
  • protected fields
  • private member functions
  • private fields

[LAF97] points out constructors and finalize() should be listed first, presumably because these are the first member functions that another developer will look at to understand how to use the class. Furthermore, because we have a standard to declare all fields as private, the declaration order really comes down to:

constructors 

finalize()

public member functions

protected member functions

private member functions

private fields

Within each grouping of member functions, it's common to list them in alphabetical order. Many developers choose to list the static member functions within each grouping first, followed by instance member functions; and then within each of these two sub-groupings, list the member functions alphabetically. Both of these approaches are valid; you just need to choose one and stick to it.

7.1.4    Minimize the public and protected interface return to Contents

One of the fundamentals of object-oriented design is to minimize the public interface of a class. There are several reasons for this:

  1. Ease of learning. To learn how to use a class, you should only have to understand its public interface. The smaller the public interface, the easier a class is to learn.
  2. Reduced coupling. Whenever the instance of one class sends a message to an instance of another class, or directly to the class itself, the two classes become coupled. Minimizing the public interface implies that you are minimizing the opportunities for coupling.
  3. Greater flexibility. This is directly related to coupling. Whenever you want to change the way that a member function in your public interface is implemented—perhaps you want to modify what the member function returns—then you potentially have to modify any code that invokes the member function. The smaller the public interface; the greater the encapsulation; and, therefore, the greater your flexibility.

It's clear that it's worth your while to minimize the public interface, but often what is not so clear is that you also want to minimize the protected interface as well. The basic idea is that, from the point of view of a subclass, the protected interfaces of all of its superclasses are effectively public. Any member function in the protected interface can be invoked by a subclass. Therefore, you want to minimize the protected interface of a class for the same reasons that you want to minimize the public interface.

7.1.4.1    Define the public interface first return to Contents

Most experienced developers define the public interface of a class before they begin coding it. 

  • First, if you don't know what services or behaviors a class will perform, then you still have some design work to do.
     
  • Second, it enables you to stub out the class quickly so that other developers who rely on it can at least work with the stub until the "real" class has been developed. 
  • Third, this approach provides you with an initial framework around which to build your class.

7.2    Standards for i