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The Java programming language allows you to define a class within another class.

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Nested or Inner ?

• Nested classes that are declared static are called static nested classes.
• Non-static nested classes are called inner classes and requires an instance of the outer class to be referenced.

class OuterClass {
    ...
    static class StaticNestedClass {
        ...
    }
    class InnerClass {
        ...
    }
}

When to use ?

• 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.
• It increases encapsulation but increase dependency

Documentation / Reference

• Nested Classes