大家看看，为什么不能用new Threelistener(),而必须用this

这个程序红色部分是有问题的，只要把它改成this就可以，为什么会这样？
import java.awt.*;
import java.awt.event.*;
public class Threelistener implements MouseMotionListener,MouseListener,WindowListener {
//实现了三个接口
private Frame f;
private TextField tf;
public static void main(String args[])
{
Threelistener two = new Threelistener();
two.go(); }
public void go() {
f = new Frame("Three listeners example");
f.add(new Label("Click and drag the mouse"),"North");
tf = new TextField(30);
f.add(tf,"South"); //使用缺省的布局管理器
f.addMouseMotionListener(new Threelistener()); //注册监听器MouseMotionListener
f.addMouseListener(new Threelistener()); //注册监听器MouseListener
f.addWindowListener(new Threelistener()); //注册监听器WindowListener
f.setSize(300,200);
f.setVisible(true);
}
public void mouseDragged (MouseEvent e) {
//实现mouseDragged方法
String s = "Mouse dragging : X="+e.getX()+"Y = "+e.getY();
tf.setText(s);
}
public void mouseMoved(MouseEvent e){}
//对其不感兴趣的方法可以方法体为空
public void mouseClicked(MouseEvent e){}
public void mouseEntered(MouseEvent e){
String s = "The mouse entered";
tf.setText(s);
}
public void mouseExited(MouseEvent e){
String s = "The mouse has left the building";
tf.setText(s);
}
public void mousePressed(MouseEvent e){}
public void mouseReleased(MouseEvent e){ }
public void windowClosing(WindowEvent e) {
//为了使窗口能正常关闭，程序正常退出，需要实现windowClosing方法
System.exit(0);
}
public void windowOpened(WindowEvent e) {}
//对其不感兴趣的方法可以方法体为空
public void windowIconified(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowClosed(WindowEvent e) {}
public void windowActivated(WindowEvent e) { }
public void windowDeactivated(WindowEvent e) {}
}

因为你new的ThreeListener里面的东西都是空的


private Frame f;
private TextField tf;

你能保证这些被初始化吗?
但是用this,就代表当前对象