General question about interface

1- its a design issue, require experience with OOP concept
.
2- *Adapter classes you will see in java.
if you implement an interface, you should implement all methods inside..
but imagine you need only 1 method of interface
and it has 10 methods.
so you must to do something like this for 9 methods:

public void method2(){/*nothing to do*/};

*Adapter classes does it for you, look inside of them.

Thank you both for the replies, I used those documents and it really help me but I still got this one question. I will write down two example applets from my study book, even though they are not doing the same thing, I would like to know why the first one is written in one file and the other applet has to be divided into two files.
here are the applets:

The first applet (tutorial29.java)
import java.awt.*;
import java.awt.event.*;
public class tutorial29 extends java.applet.Applet implements MouseListener {
TextField txt;
int i=0;
public void init() {
this.addMouseListener(this);
txt=new TextField("This is textField", 40);
add(txt);
}
public void mouseClicked(MouseEvent e) {/*some code*/}
public void mouseEntered(MouseEvent e) {/*some code*/}
public void mouseExited(MouseEvent e) {/*some code*/}
public void mousePressed(MouseEvent e) {/*some code*/}
public void mouseReleased(MouseEvent e) {/*some code*/}
}
The second applet: (mListener.java) /*The Listener*/

import java.awt.*;
import java.awt.event.*;
public class mListener extends MouseAdapter {
tutorial29b aplt;
mListener(tutorial29b aplt_temp) {
aplt=aplt_temp;
}
public void mouseClicked(MouseEvent e) {
aplt.x=e.getX();
aplt.y=e.getY();
aplt.repaint();
}
}
and his other file: (tutorial29b.java) /*the main Applet*/

import java.awt.*;
import java.awt.event.*;
public class tutorial29b extends java.applet.Applet {
mListener t;
int i=0,x=0,y=0;
public void init() {
t=new mListener(this);
this.addMouseListener(t);
}
public void paint(Graphics g) {
g.drawRect(x,y,100,100);
}
}

I do understand that if I implement an interface I must Implement all of his methods, even if I dont inted to use them, also I understand that the mListener which I created is inherited from the MouseAdapter object which already contains the empty methods. But still that doesnt gives me the answer why do I have to devide my applet into 2 files the other way.
thank you for your help, again!
I really appreciate that ;)
Itay.