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Java Static Member Classes

In this chapter you will learn:

  1. How to declare and use Static Member Classes
  2. Example - Java Static Member Classes
  3. Example - implementation for different data types

Description

A static member class is a static member of an enclosing class. A static member class cannot access the enclosing class's instance fields and invoke its instance methods.

A static member can access the enclosing class's static fields and invoke its static methods including private fields and methods.

Example

The following code has a static member class declaration.


class Demo {/*ww  w  . ja v a 2 s. co m*/
  public static void main(String[] args) {
    Main.EnclosedClass.accessEnclosingClass(); 
    Main.EnclosedClass ec = new Main.EnclosedClass();
    ec.accessEnclosingClass2(); 
  }
}

class Main {
  private static int outerVariable;

  private static void privateStaticOuterMethod() {
    System.out.println(outerVariable);
  }

  static void staticOuterMethod() {
    EnclosedClass.accessEnclosingClass();
  }

  static class EnclosedClass {
    static void accessEnclosingClass() {
      outerVariable = 1;
      privateStaticOuterMethod();
    }

    void accessEnclosingClass2() {
      staticOuterMethod();
    }
  }
}

The static member classes can declare multiple implementations of their enclosing class.

Example 2

The following code declares a Rectangle class and it uses static member class to provide Rectangle implementation for different data types, one is for double type and another is for float type.


abstract class Rectangle {
  abstract double getX();
/*  w w w . j a v  a 2 s. c  o m*/
  abstract double getY();

  abstract double getWidth();

  abstract double getHeight();

  static class Double extends Rectangle {
    private double x, y, width, height;

    Double(double x, double y, double width, double height) {
      this.x = x;
      this.y = y;
      this.width = width;
      this.height = height;
    }

    double getX() {
      return x;
    }

    double getY() {
      return y;
    }

    double getWidth() {
      return width;
    }

    double getHeight() {
      return height;
    }
  }

  static class Float extends Rectangle {
    private float x, y, width, height;

    Float(float x, float y, float width, float height) {
      this.x = x;
      this.y = y;
      this.width = width;
      this.height = height;
    }

    double getX() {
      return x;
    }

    double getY() {
      return y;
    }

    double getWidth() {
      return width;
    }

    double getHeight() {
      return height;
    }
  }

  private Rectangle() {
  }

  boolean contains(double x, double y) {
    return (x >= getX() && x < getX() + getWidth()) && (y >= getY() && y < getY() + getHeight());
  }
}

public class Main {
  public static void main(String[] args) {
    Rectangle r = new Rectangle.Double(10.0, 10.0, 20.0, 30.0);
    r = new Rectangle.Float(10.0f, 10.0f, 20.0f, 30.0f);
  }
}

Next chapter...

What you will learn in the next chapter:

  1. What are the three types of class variables
  2. How Java class member variables got initialized
  3. Example - Member variable
  4. How does automatic variable(method local variables) got initialized
  5. How does Class variable (static variable) got initialized
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