简单

Question

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class Copy {
    JFrame f=new JFrame();
    JButton copy=new JButton("Copy");
    JButton clear=new JButton("Clear");
    JTextField t1=new JTextField();
    JTextField t2=new JTextField();
    JPanel p_top=new JPanel();
    JPanel p_b=new JPanel();
    JPanel p=new JPanel();
    public Copy() {
        Container c=f.getContentPane();
        p_top.setLayout(new GridLayout(2,1));
        p_top.add(t1);
        p_top.add(t2);
        p_b.setLayout(new FlowLayout());
        p_b.add(copy);
        p_b.add(clear);
        p.setLayout(new BorderLayout());
        p.add(p_top,BorderLayout.CENTER);
        p.add(p_b,BorderLayout.SOUTH);
        c.add(p);
        copy.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                         t2.setText(t1.getText());
                    }
                }
        );
        clear.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        t1.setText("");
                        t2.setText("");
                    }
                }
        );
        f.setSize(400,400);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
    public static void main(String[] args) {
        new Copy();
    }
}先编的，GUI可能不好看。

Answer 1

有空指针错误，你的程序运行的时候试图对一个NULL值进行操作，所以抛出了java.lang.NullPointerException，无法得到你要的结果. try {
if(e.getActionCommand().equals("Copy"))
t2.setText(t1.getText());
         }

catch(NullPointerException nullPointerException) {
System.exit(0);
}

if (e.getActionCommand().equals("Clear"))
{
                   ...
                   }
换成这一段就可以看出来是这个错误，试着找找错误看吧。

Answer 2

你定义了两个全局变量,t1,t2,但在First方法里面又定义一次t1,t2,但在
copy.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                         t2.setText(t1.getText());
                    }
                }
        );
这个时候访问的t1,t2是全局变量t1,t2,他们仅仅是被申明,却没有被初始化,要改的话,很简单,只需要将First里的
JTextField t1=new JTextField();
    JTextField t2=new JTextField();
改成:
t1=new JTextField();
t2=new JTextField();就行了