Java Object Oriented Design - Java static Inner Classes

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A static Member Class Is Not an Inner Class

A member class defined within the body of another class may be declared static.

Example

The following code declares a top-level class A and a static member class B:

class A {
  // Static member class
  public static class B {
    // Body for class B goes here
  }
}

Note

A static member class is not an inner class. It is considered a top-level class.

A static member class is also called a nested top-level class.

An instance of class A and an instance of class B can exist independently because both are top-level classes.

A static member class can be declared public, protected, package-level, or private to restrict its accessibility outside its enclosing class.

There are two advantages of using a static member class:

• A static member class can access the static members of its enclosing class including the private static members.
• A package acts like a container for top-level classes by providing a namespace. Top-level classes having static member classes provide an additional layer of namespaces.

A static member class is the direct member of its enclosing top-level class, not a member of the package.

An object of a static member class is created the same way as you create an object of a top-level class using the new operator. To create an object of class B, you write

A.B bReference = new A.B();

Since the simple name of class B is in the scope inside class A, we can use its simple name to create its object inside class A as

B  bReference2 = new B(); // This  statement appears inside class  A  code

We can also use the simple name B outside class A by importing the com.java2s.innerclasses.A.B class.

Example 2

The following code shows how to use static inner class.

public class Main {
  public static void main(String[] args) {
    Car.Tire m = new Car.Tire(17);
    Car.Tire m2 = new Car.Tire(19);
//w  w w.  j  a  va 2  s  .  c o m
    Car.Keyboard k = new Car.Keyboard(122);
    Car.Keyboard k1 = new Car.Keyboard(142);

    System.out.println(m);
    System.out.println(m2);
    System.out.println(k);
    System.out.println(k1);
  }
}
class Car {
  // Static member class - Monitor
  public static class Tire {
    private int size;

    public Tire(int size) {
      this.size = size;
    }

    public String toString() {
      return "Monitor   - Size:" + this.size + "  inch";
    }
  }

  // Static member class - Keyboard
  public static class Keyboard {
    private int keys;

    public Keyboard(int keys) {
      this.keys = keys;
    }

    public String toString() {
      return "Keyboard  - Keys:" + this.keys;
    }
  }
}

The code above generates the following result.

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