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Nested and Inner Classes

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Introduction

Using Java, developers have the ability to define a class within another class. Such a class is called a Nested Class. Nested Classes are called Inner Classes if they were declared as non-static, if not, they are simply called Static Nested Classes. This page is to document and provide details with examples on how to use Java Nested and Inner Classes.

Syntax

  • public class OuterClass { public class InnerClass { } } // Inner classes can also be private
  • public class OuterClass { public static class StaticNestedClass { } } // Static nested classes can also be private
  • public void method() { private class LocalClass { } } // Local classes are always private
  • SomeClass anonymousClassInstance = new SomeClass() { }; // Anonymous inner classes cannot be named, hence access is moot. If 'SomeClass()' is abstract, the body must implement all abstract methods.
  • SomeInterface anonymousClassInstance = new SomeInterface() { }; // The body must implement all interface methods.

Remarks

Terminology and classification

The Java Language Specification (JLS) classifies the different kinds of Java class as follows:

A top level class is a class that is not a nested class.

A nested class is any class whose declaration occurs within the body of another class or interface.

An inner class is a nested class that is not explicitly or implicitly declared static.

An inner class may be a non-static member class, a local class, or an anonymous class. A member class of an interface is implicitly static so is never considered to be an inner class.

In practice programmers refer to a top level class that contains an inner class as the "outer class". Also, there is a tendency to use "nested class" to refer to only to (explicitly or implicitly) static nested classes.

Note that there is a close relationship between anonymous inner classes and the lambdas, but lambdas are classes.

Semantic differences

  • Top level classes are the "base case". They are visible to other parts of a program subject to normal visibility rules based on access modifier semantics. If non-abstract, they can be instantiated by any code that where the relevant constructors are visible based on the access modifiers.

  • Static nested classes follow the same access and instantiation rules as top level classes, with two exceptions:

    • A nested class may be declared as private, which makes it inaccessible outside of its enclosing top level class.
    • A nested class has access to the private members of the enclosing top-level class and all of its tested class.

    This makes static nested classes useful when you need to represent multiple "entity types" within a tight abstraction boundary; e.g. when the nested classes are used to hide "implementation details".

  • Inner classes add the ability to access non-static variables declared in enclosing scopes:

    • A non-static member class can refer to instance variables.
    • A local class (declared within a method) can also refer to the local variables of the method, provided that they are final. (For Java 8 and later, they can be effectively final.)
    • An anonymous inner class can be declared within either a class or a method, and can access variables according to the same rules.

    The fact that an inner class instance can refer to variables in a enclosing class instance has implications for instantiation. Specifically, an enclosing instance must be provided, either implicitly or explicitly, when an instance of an inner class is created.

Related Examples