What are the purposes of inner classes

Question

I am reviewing the concept of inner classes in java. so far from what I\'ve understood and applied java inner classes has a link or access to the methods and fields of its o

Answer 1

1. See the Java tutorial for the main reasons.

2. If by "helper class" you mean something for internal use only, then no, not necessarily. You might want to do something like

   class Outer {
       private static class Inner implements InterestingInterface {
           // whatever
       }
       public InterestingInterface make_something_interesting() {
           return new Inner();
       }
   }
   

   Here, Inner is not a "helper class" in the sense that the outside world does get to see instances of it, but its implementation is entirely hidden -- the outside world only knows it gets some object that implements InterestingInterface.

Answer 2

The inner class used for grouping classes logic, for example, if you have class B and this class used only at class A, So it is better to put class B as an inner class at class A, as this will give readability and reusability for your code.

Happy code :)

Answer 3

One purpose of inner classes is to attach listeners. For example, suppose you have a JMenuItem. You can make it quit your app as shown in this code:

JMenuItem quitItem = new JMenuItem("Quit");
quitItem.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent e)
    {
        //cleanup code before exiting
        System.exit(0);
    }
});

You may also want a class to have access to outer class state variables which is entirely subservient to that class. For example, consider writing a simple color calculator. It might have a text area into which you type a hex code. When you hit enter, you want a JPanel to display the color. Here is a crude outline of what you might do.

public class ColorCalc extends JPanel implements Runnable
{
    Color displayedColor;
    JTextArea colorEnterArea;
    public ColorCalc()
    {
        displayedColor = Color.white
        colorEnterArea = new JTextArea();
    }
    public void run()
    {
        //build GUI here
    }
    public static void main(String[] args)
    {
         ColorCalc cc = new ColorCalc();
         javax.swing.SwingUtilities.invokeLater(cc);
    }
    //subservient inner class with access to outer class state variable.
    class ColorPanel extends JPanel
    {
         public void paintComponent(Graphics g)
         {
             g.setColor(displayedColor);
             g.fillRect(0,0,getWidth(), getHeight());
         } 
    }

}

Answer 4

This is a style question. Anything that can be done with an inner class can also be done as a as series of external classes. Inner classes are especially useful for classes that are lightweight or tightly bound to the enclosing class. For example, a comparator is frequently both these things. It needs intimate knowledge of the implementation of the class, and may only be a few lines long. It may be an ideal candidate as an internal class.