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Some definitions
Before getting started with a discussion of design guidelines for object initialization, I'd like to clarify a few terms.

Designer vs. client programmers
If you're like most Java programmers, you alternate between two hats, which you wear at different times. Sometimes you wear your "designer" hat and build libraries of classes for others to use; other times you wear your "client" hat and make use of a library of classes created by someone else. Some Java programmers -- completely oblivious to the rules of fashion -- are known to wear both hats at the same time.

One aspect of the flexibility of a body of code is the ease with which a client programmer can understand the code. Whether a client programmer is planning to change code or just use it as is, that programmer often has to figure out how to change or use the code by reading it.

The guidelines discussed in the remainder of this article, and in subsequent articles of this column, will talk about flexibility in terms of client programmers. Designs and implementations that are flexible are those that are easy for client programmers to understand, use, and change.

Java jargon related to initialization
This article uses several terms related to initialization that are defined in precise ways by the Java Language Specification (JLS).

• Default values are the values given to instance (and class) variables when they are first allocated on the heap -- before any initialization code is executed

• A no-arg constructor is a constructor that takes no arguments

• A default constructor is a no-arg constructor generated implicitly by the compiler for classes that don't have any constructors explicitly declared in the source file

• An instance variable initializer is an equals sign (=) and expression sitting between an instance variable declaration and its terminating semicolon

• An instance initializer is a block of code executed during object initialization in textual order along with instance variable initializers

For more information on these terms, see the companion article to this month's column, "Object initialization in Java."