Inner classes in Java

There are four types of classes in Java, those are loosely called as inner classes. Used correctly, inner classes are an elegant and powerful feature of the Java language.

The inner classes are

  1. Static member classes
  2. Member classes
  3. Local classes
  4. Anonymous classes.

Static member classes

A static member class is a class (or interface) defined as a static member of another class. A static method is called a class method, so, by analogy, we could call this type of inner class a “class class,” but this terminology would obviously be confusing. A static member class behaves much like an ordinary top-level class, except that it can access the static members of the class that contains it. Interfaces can be defined as static members of classes.

Member classes

A member class is also defined as a member of an enclosing class, but is not declared with the static modifier. This type of inner class is analogous to an instance method or field. An instance of a member class is always associated with an instance of the enclosing class, and the code of a member class has access to all the fields and methods (both static and non-static) of its enclosing class. There are several features of Java syntax that exist specifically to work with the enclosing instance of a member class. Interfaces can only be defined as static members of a class, not as non-static members.

Local classes:

A local class is a class defined within a block of Java code. Like a local variable, a local class is visible only within that block. Although local classes are not member classes, they are still defined within an enclosing class, so they share many of the features of member classes. Additionally, however, a local class can access any final local variables or parameters that are accessible in the scope of the block that defines the class. Interfaces cannot be defined locally.

Anonymous classes:

An anonymous class is a kind of local class that has no name; it combines the syntax for class definition with the syntax for object instantiation. While a local class definition is a Java statement, an anonymous class definition (and instantiation) is a Java expression, so it can appear as part of a larger expression, such as method invocation. Interfaces cannot be defined anonymously.

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