7.2 Java1636 java oop member classes  (Page 2/15)

As you can learn at (External Link) , the list also includes:

• Static initializer blocks
• Instance initializers

Can also contain other class definitions

In this and the next two modules, you will learn that a class definition can also contain the following three kinds of inner classes :

• Member classes
• Local classes
• Anonymous classes

A class can also contain Nested top-level classes and interfaces , which you can learn about at (External Link) .

(Note that it is questionable whether a nested top-level class or interface should be referred to as an inner class, because an object of anested top-level class can exist in the absence of an object of the enclosing class. Regardless of whether the term inner class applies, anested top-level class is defined within the definition of another class, so its definition is internal to the definitionof another class.)

This module will be dedicated to an explanation of member classes . Subsequent modules will explain the other two types of inner classes in the above list .

What is a member class ?

A member class is a class that is defined inside the definition of another class, (without the use of the static modifier as is the case with a nested top-level class).

An object of the member class must be internally linked to an object of the enclosing class, (which is not the case with a nested top-level class).

Thus, a member class is truly an inner class. (An object of the member class cannot exist in the absence of an object of the enclosing class.)

What about a member 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 member classes ?

Probably the most important benefit of member classes has to do with accessing the other members of enclosing classes. The methods of a member classhave direct access to all the members of the enclosing classes, including private members. Thus the use of member classes can eliminate the requirement toconnect objects together via constructor parameters.

This is particularly useful in those cases where there is no reason for an object of a member class to exist in the absence of an object of the enclosingclass, and where the methods of the object of the member class need access to members of the object of the enclosing class.

Data structures and iterators

For example, there is usually no reason for an Iterator object to exist in the absence of the data-structure object for which it is designed toprovide iterator services. Also, the iterator object usually needs to have ready access to the members of the data-structure object, some or all of which may beprivate. Thus, a class from which an Iterator object can be constructed is a good candidate for inclusion as a member class in the classfrom which the associated data-structure object is instantiated.

Listener objects

Another common use for inner classes is in the definition of classes from which listener objects (which listen for events fired by other objects) are instantiated. (However, it may be more common to use anonymous classes than member classes for this purpose.)