1

嘿伙计们，我正在尝试制作一个 GUI，当按下 Next 或 Previous 按钮时，它可以在 JTextAreas 中导航。我正在使用 CardLayout 在 JTextArea 之间切换。我可以编译程序，但每次我按下下一个或上一个按钮时，我总是得到一个 NullPointerException。我不知道为什么 CardLayout 对象在下一个和上一个方法中为空，这让我很困惑。

错误来自这段代码

cl.show(cardPanel, "" + (currentCard));

感谢您的帮助！

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;


public class AsciiCanvas extends JTextArea
{

private JPanel jp1;
private JPanel jp2;
private JPanel jp3;
private JPanel jp4;
private JPanel jp5;
private JPanel jp6;
private JPanel jp7;
private JPanel jp8;
private JPanel jp9;
private JPanel jp10;
private JTextArea text1;
private JTextArea text2;
private JTextArea text3;
private JTextArea text4;
private JTextArea text5;
private JTextArea text6;
private JTextArea text7;
private JTextArea text8;
private JTextArea text9;
private JTextArea text10;

private JPanel cardPanel;
private CardLayout cl;
private int currentCard = 1;

private JFileChooser fc;


public AsciiCanvas()
{
   Font mono = new Font("Monospaced", Font.PLAIN, 12);

    cardPanel = new JPanel();
    CardLayout cl = new CardLayout();
    cardPanel.setLayout(cl);


    JTextArea text1 = new JTextArea();
    JTextArea text2 = new JTextArea();
    JTextArea text3 = new JTextArea();
    JTextArea text4 = new JTextArea();
    JTextArea text5 = new JTextArea();
    JTextArea text6 = new JTextArea();
    JTextArea text7 = new JTextArea();
    JTextArea text8 = new JTextArea();
    JTextArea text9 = new JTextArea();
    JTextArea text10 = new JTextArea();



   JPanel jp1 = new JPanel();
   JPanel jp2 = new JPanel();
   JPanel jp3 = new JPanel();
   JPanel jp4 = new JPanel();
   JPanel jp5 = new JPanel();
   JPanel jp6 = new JPanel();
   JPanel jp7 = new JPanel();
   JPanel jp8 = new JPanel();
   JPanel jp9 = new JPanel();
   JPanel jp10 = new JPanel();

   jp1.add(text1);
   jp2.add(text2);
   jp3.add(text3);
   jp4.add(text4);
   jp5.add(text5);
   jp6.add(text6);
   jp7.add(text7);
   jp8.add(text8);
   jp9.add(text9);
   jp10.add(text10);

   cardPanel.add(jp1, "1");
   cardPanel.add(jp2, "2");
   cardPanel.add(jp3, "3");
   cardPanel.add(jp4, "4");
   cardPanel.add(jp5, "5");
   cardPanel.add(jp6, "6");
   cardPanel.add(jp7, "7");
   cardPanel.add(jp8, "8");
   cardPanel.add(jp9, "9");
   cardPanel.add(jp10, "10");

   setBorder(BorderFactory.createTitledBorder("Animat ion here"));
   setFont(mono);

   fc = new JFileChooser();
}


public void Next()
{
  if(currentCard < 10)
  {

     currentCard +=1;
     cl.show(cardPanel, "" + (currentCard));
  }


}

public void Previous()
{
  if(currentCard > 1)
  {
     currentCard -= 1;
     cl.show(cardPanel, "" + (currentCard));

  }
}

错误信息* **

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AsciiCanvas.Next(AsciiCanvas.java:108)
4

2 回答 2

2

问题是它cl是空的，原因如下：

private CardLayout cl;

...

public AsciiCanvas()
{
    ...
    CardLayout cl = new CardLayout();

在该构造函数中，您声明了一个新的局部变量，而不是为实例变量赋值。局部变量隐藏或隐藏实例变量。您的构造函数代码应为：

public AsciiCanvas()
{
    ...
    cl = new CardLayout();

（顺便说一句，我强烈建议使用String.valueOf(...)而不"" + ...是以空安全的方式将值转换为字符串。您对字符串连接并不真正感兴趣，是吗？）

于 2012-03-30T20:39:52.573 回答
1

这是构造函数中的局部变量，它隐藏了成员cl：

CardLayout cl = new CardLayout();

表示该成员cl在null方法Next()和Previous()中。

改成：

cl = new CardLayout();
于 2012-03-30T20:39:10.857 回答