7.3 Java1638 java oop local classes  (Page 2/11)

What is a local class ?

A local class is a class that is defined within a block of Java code. While local classes are probably most frequently defined within methods andconstructors, they can also be defined inside static initializer blocks and instance initializers.

As is the case for an object of a member class (discussed in the previous module ), an object of a local class must be internally linked to an object of the enclosing class. I will often refer to that object as the containingobject, and make comments about the containment hierarchy.

A local class is truly an inner class, because an object of the local class cannot exist in the absence of an object of the enclosing class.

What about a local interface?

Interfaces defined within classes are implicitly static. This means that they are always top-level. There is no such thing as a member interface, a local interface, or an anonymous interface.

Why use local classes ?

Objects instantiated from local classes share many of the characteristics of objects instantiated from member classes. However, in some cases, a local classcan be defined closer to its point of use than would be possible with a memberclass, leading to improved code readability.

Probably the most important benefit of local classes has to do with accessing the members of enclosing classes. Just like with member classes, methods of alocal class have direct access to all the members of the enclosing classes, including private members. Thus the use of local classes can sometimes eliminatethe requirement to connect objects together via constructor parameters.

(We will also see in the example program in this module that methods of a local class have direct access to protected members of the superclassof the enclosing class.)

Can be particularly useful when ...

A local class can be particularly useful in those cases where

• There is no reason for an object of the local class to exist in the absence of an object of the enclosing class
• There is no reason for an object of the local class to exist outside a method of the enclosing class
• Methods of the object of the local class need access to members of the object of the enclosing class
• Methods of the object of the local class need access to final local variables and method parameters belonging to the method in which the localclass is defined

This module approaches local classes from a somewhat theoretical viewpoint. The next module will approach local classes from a more practical viewpoint,including a comparison between local classes and anonymous classes.

Local classes versus member classes

A local class has approximately the same relationship to a member class that a local variable in a method has to an instance variable of the class containingthe method.

The scope of a local class

As is the case with local variables, the name of a local class is visible and usable only within the block of code in which it is defined (and blocks nested within that block). Further, the name of the local class is visible and usable only to code following the class definition within that block.