始终无法实现窗体的放大缩小！求请教~~~

package testyy001;


import java.awt.*;


public class test001 {

    public static void main(String[] args) {
         
                Frame j = new Frame("呼叫系统");/
               
                j.setSize(500,500);
                j.setLocation(300,200);
                j.setLayout(new FlowLayout());
               
                j.addWindowListener();
               
                Button hjan = new Button("呼叫");
                j.add(hjan);
               
                        
                j.addWindowStateListener(); // 这句无法实现功能  求请教。里面赋值NULL 不报错。但无法实现！
               
                        
                    j.setVisible(true);