Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

5.11. Nested Classes

All the classes you have defined so far have been separate from each other—each stored away in its own source file. Not all classes have to be defined like this. You can put the definition of one class inside the definition of another class. The inside class is called a nested class. A nested class can itself have another class nested inside it, if need be.

When you define a nested class, it is a member of the enclosing class in much the same way as the other class members. A nested class can have an access attribute just like other class members, and the accessibility from outside the enclosing class is determined by the attributes in the same way:

public class Outside {

  // Nested class
  public class Inside {
    // Details of Inside class...
  }

  // More members of Outside class...
}

Here the class Inside is nested inside the class Outside. The Inside class is declared as a public member of Outside, so it is accessible from outside Outside. Obviously, a nested class should have some specific association with the enclosing class. Arbitrarily nesting one class inside another would not be sensible. The enclosing class here is referred to as a top-level class. A top-level class is a class that contains a nested class but is not itself a nested class.

The nested class here has meaning only in the context of an object of type Outside. This is because the Inside class is not declared as a static member of the class Outside. Until an object of type Outside has been created, ...