The Java programming language allows you to define a class within another class. Such a class is called a nested class and is illustrated here:A nested class is a member of its enclosing class and, as such, has access to other members of the enclosing class, even if they are declared private. As a member ofclass OuterClass { ... class NestedClass { ... } }OuterClass
, a nested class can be declaredprivate
,public
,protected
, or package private. (Recall that outer classes can only be declaredpublic
or package private.)
Terminology: Nested classes are divided into two categories: static and non-static. Nested classes that are declaredstatic
are simply called static nested classes. Non-static nested classes are called inner classes.class OuterClass { ... static class StaticNestedClass { ... } class InnerClass { ... } }Why Use Nested Classes?
There are several compelling reasons for using nested classes, among them:
- It is a way of logically grouping classes that are only used in one place.
- It increases encapsulation.
- Nested classes can lead to more readable and maintainable code.
Logical grouping of classes—If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.
Increased encapsulation—Consider two top-level classes, A and B, where B needs access to members of A that would otherwise be declared
private
. By hiding class B within class A, A's members can be declared private and B can access them. In addition, B itself can be hidden from the outside world.More readable, maintainable code—Nesting small classes within top-level classes places the code closer to where it is used.
Static Nested Classes
As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class it can use them only through an object reference.
Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.Static nested classes are accessed using the enclosing class name:
For example, to create an object for the static nested class, use this syntax:OuterClass.StaticNestedClassOuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();Inner Classes
As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:
An instance ofclass OuterClass { ... class InnerClass { ... } }InnerClass
can exist only within an instance ofOuterClass
and has direct access to the methods and fields of its enclosing instance. The next figure illustrates this idea.An InnerClass Exists Within an Instance of OuterClass
To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:
Additionally, there are two special kinds of inner classes: local classes and anonymous classes (also called anonymous inner classes). Both of these will be discussed briefly in the next section.OuterClass.InnerClass innerObject = outerObject.new InnerClass();