Versions

[{“Name”:“Java SE 1.1”,“GroupName”:null},{“Name”:“Java SE 1.2”,“GroupName”:null},{“Name”:“Java SE 1.3”,“GroupName”:null},{“Name”:“Java SE 1.4”,“GroupName”:null},{“Name”:“Java SE 5”,“GroupName”:null},{“Name”:“Java SE 6”,“GroupName”:null},{“Name”:“Java SE 7”,“GroupName”:null},{“Name”:“Java SE 8”,“GroupName”:null},{“Name”:“Java SE 9 (Early Access)”,“GroupName”:null}]

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