Question

In below code I have some doubt in following line :

btn.addActionListener(this);
th= new Thread(this);

code :

public class Foo  extends Applet implements  Runnable,ActionListener
{
    Button btn;
    Thread th;
    public void init()
    {
        btn=new Button("Click on me");
        add(btn);
        btn.addActionListener(this);  // pass reference as this
        th=null;
    }
    public void run()
    {
        int i=0;
        while(i++<10)
        {
            try{
                th.sleep(500);
                showStatus(new Integer(i).toString());
            }
            catch(Exception e){}
        }
    }
    public void actionPerformed(ActionEvent e)
    {
        if(th==null)
        {
            th= new Thread(this);  // pass reference as this
            th.start();
        }       
    }
}

in Thread class constructor Thread(Runnable target) Allocates a new Thread object.
We can pass Runnable Target but I have passed this as parameter. I have implemented Runnable interface though it is possible
But again I have passed this as parameter In this case we can pass ActionListener target.
If we pass this as parameter in both the cases how It can get resolved.
I think this reference is targeting to reference of
1. Foo
2. Runnable
3. ActionListener
so how suitable reference is selected for method or constructor?.

Answer 1

You have a IS-A relationship :

• Foo IS-A Applet
• Foo IS-A Runnable
• Foo IS-A ActionListener

When new object of type Foo is created , Foo will be “selected reference” type. You have to cast to others if you want to have different type (from the selected list).

As long as Foo IS-A Applet, Runnable and ActionListener, the contract of the methods are fullfiled : btn.addActionListener(this); takes Foo which IS-A ActionListener, th= new Thread(this); takes Foo which IS-A Runnable.