Nonstatic Member Classes

A nonstatic member class is a non-static member of an enclosing class. Each instance of the nonstatic member class associates with an instance of the enclosing class.

The nonstatic member class's instance methods can call instance methods in the enclosing class and access the enclosing class instance's nonstatic fields.


class EnclosingClass {
  private int outerVariable;

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

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

public class Main {
  public static void main(String[] args) {
    EnclosingClass ec = new EnclosingClass();
    ec.new EnclosedClass().accessEnclosingClass(); // Output: 1
  }
}

The following code uses inner class ItemList to store the items.

 
class Item {
  private String name;
  private String desc;

  Item(String name, String desc) {
    this.name = name;
    this.desc = desc;
  }

  String getName() {
    return name;
  }

  String getDesc() {
    return desc;
  }

  @Override
  public String toString() {
    return "Name = " + getName() + ", Desc = " + getDesc();
  }
}

class ItemManager {
  private ItemList itemList;
  private int index = 0;

  ItemManager() {
    itemList = new ItemList(2);
  }

  boolean hasMoreElements() {
    return index < itemList.size();
  }

  Item nextElement() {
    return itemList.get(index++);
  }

  void add(Item item) {
    itemList.add(item);
  }

  private class ItemList {
    private Item[] itemArray;
    private int index = 0;

    ItemList(int initSize) {
      itemArray = new Item[initSize];
    }

    void add(Item item) {
      if (index >= itemArray.length) {
        Item[] temp = new Item[itemArray.length * 2];
        for (int i = 0; i < itemArray.length; i++)
          temp[i] = itemArray[i];
        itemArray = temp;
      }
      itemArray[index++] = item;
    }

    Item get(int i) {
      return itemArray[i];
    }

    int size() {
      return index;
    }
  }
}

public class Main {
  public static void main(String[] args) {
    ItemManager itemManager = new ItemManager();
    itemManager.add(new Item("1", "A"));
    itemManager.add(new Item("2", "B"));
    itemManager.add(new Item("3", "C"));
    while (itemManager.hasMoreElements())
      System.out.println(itemManager.nextElement());
  }
}

The following program illustrates how to define and use an inner class.

 
class Outer {
  int outer_x = 100;
  void test() {
    Inner inner = new Inner();
    inner.display();
  }
  class Inner {
    void display() {
      System.out.println("display: outer_x = " + outer_x);
    }
  }
}
public class Main {
  public static void main(String args[]) {
    Outer outer = new Outer();
    outer.test();
  }
}

Output from this application is shown here:


display: outer_x = 100

The inner class members are accessible only within the inner class and may not be used by the outer class. If you try to compile the following code, you will get error message.

 
public class Main {
  int outer_x = 100;
  // this is an inner class
  class Inner {
    int y = 10; // y is local to Inner

    void display() {
      System.out.println("display: outer_x = " + outer_x);
    }
  }

  void showy() {
    System.out.println(y); 
  }
}

When compiling the code above:


D:\>javac Main.java
Main.java:13: cannot find symbol
symbol  : variable y
location: class Main
    System.out.println(y);
                       ^
1 error

You can define a nested class within the block defined by a method.

The following program defines a class inside a for loop.

  

public class Main {
  int outer_x = 100;
  void test() {
    for (int i = 0; i < 10; i++) {
      class Inner {
        void display() {
          System.out.println("display: outer_x = " + outer_x);
        }
      }
      Inner inner = new Inner();
      inner.display();
    }
  }
  public static void main(String args[]) {
    Main outer = new Main();
    outer.test();

  }
}

The output from this version of the program is shown here.


display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100

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