java - java中的剪贴板监听器

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我正在尝试编写一个简单的程序来监听剪贴板副本并保存它们。我编写了以下代码:

package CopyPaste;

import javax.swing.JFrame;

public class Main {

    public static void main(String[] args){
        JFrame frame = new JFrame("Copy Paste");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(600,600);
        frame.setLocationRelativeTo(null);

        MyPanel pane = new MyPanel();
        frame.add(pane);

        frame.setVisible(true);
    }
}

<小时/>

package CopyPaste;

import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.TextArea;
import java.awt.Toolkit;
import java.awt.datatransfer.Clipboard;
import java.awt.datatransfer.ClipboardOwner;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.FlavorEvent;
import java.awt.datatransfer.FlavorListener;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.IOException;
import java.lang.Object;

import javax.swing.JButton;
import javax.swing.JPanel;

public class MyPanel extends JPanel implements ClipboardOwner{

    private final static Font f = new Font("david", Font.BOLD,22);
    private static TextArea text;
    private JButton btnGet;
    private Clipboard c;

    public MyPanel(){
        this.setLayout(new BorderLayout());

        text = new TextArea();
        text.setFont(f);
        add(BorderLayout.CENTER,text);
        add(BorderLayout.SOUTH,this.getSouthButton());

        c.addFlavorListener(new FlavorListener() {

            @Override
            public void flavorsChanged(FlavorEvent e) {
                proccessClipboard(c);

            }
        });
    }

    private static void proccessClipboard(Clipboard c){
        String s = null;
        c = Toolkit.getDefaultToolkit().getSystemClipboard();
        try {
            s = (String) c.getContents(null).getTransferData(DataFlavor.stringFlavor);
        } catch (UnsupportedFlavorException | IOException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }

        text.setText(s);
    }

    //return the south panel button
    private JPanel getSouthButton(){
        JPanel south = new JPanel();
        btnGet = new JButton("Get");
        ButtonLis lis = new ButtonLis();
        btnGet.addActionListener(lis);
        south.add(btnGet);
        return south;
    }

    private class ButtonLis implements ActionListener{

        public void actionPerformed(ActionEvent e) {
            String s = null;
            if (e.getSource()==btnGet){
                c = Toolkit.getDefaultToolkit().getSystemClipboard();
                try {
                    s = (String) c.getContents(null).getTransferData(DataFlavor.stringFlavor);
                } catch (UnsupportedFlavorException | IOException e1) {
                    // TODO Auto-generated catch block
                    e1.printStackTrace();
                }

                text.setText(s);
            }


        }

    }

    @Override
    public void lostOwnership(Clipboard clipboard, Transferable contents) {
        // TODO Auto-generated method stub

    }
}

<小时/>

error:
Exception in thread "main" java.lang.NullPointerException
    at CopyPaste.MyPanel.<init>(MyPanel.java:39)
    at CopyPaste.Main.main(Main.java:13)

出于某种原因，Eclipse 没有显示任何错误，但是当我点击“运行”按钮时，我收到一些编译错误。有人知道是什么原因造成的吗？

Answer 1

这是一篇旧帖子，但我将为搜索相同内容的其他人回答它。正确的方法是在定义 FlavorListener 之前初始化剪贴板。因此“MyPanel”构造函数必须正确更新:

public MyPanel(){
        this.setLayout(new BorderLayout());

        text = new TextArea();
        text.setFont(f);
        add(BorderLayout.CENTER,text);
        add(BorderLayout.SOUTH,this.getSouthButton());

        c = Toolkit.getDefaultToolkit().getSystemClipboard();
        c.addFlavorListener(new FlavorListener() {

            @Override
            public void flavorsChanged(FlavorEvent e) {
                proccessClipboard(c);

            }
        });
    }