Question

我只是在学习单选按钮，并且认为自从它编译好以后我终于得到了它。但是当我尝试运行程序时，会发生以下错误：

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1090)
at java.awt.Container.add(Container.java:410)
at ActionFrame.makeComponents(ActionFrame.java:43)
at ActionFrame.<init>(ActionFrame.java:20)
at ActionFrame.main(ActionFrame.java:80)

我不知道我做错了什么，所以如果你能指出我正确的方向呢？或者向我解释为什么我在做什么是错的？

public class ActionFrame extends JFrame {
JLabel messageLabel;
JRadioButton rb1;
JRadioButton rb2;
JRadioButton rb3;
JRadioButton rb4;
String a = "Football";
String b = "Basketball";
String c = "Baseball";
String d = "Hockey";

public ActionFrame() {
  setTitle("Favorite Sports");
  setSize (400,200);
  setDefaultCloseOperation(
     JFrame.EXIT_ON_CLOSE );

  JPanel myStuff = makeComponents();
  add(myStuff);
  setVisible(true);
}

private JPanel makeComponents() {
  JPanel myPanel = new JPanel();

  messageLabel = new JLabel("Select your favorite sport: ");

  rb1 = new JRadioButton(a, true);
  rb1 = new JRadioButton(b);
  rb1 = new JRadioButton(c);
  rb1 = new JRadioButton(d);

  ButtonGroup group = new ButtonGroup();

  group.add(rb1);
  group.add(rb2);
  group.add(rb3);
  group.add(rb4);

  myPanel.add(rb1);
  myPanel.add(rb2);
  myPanel.add(rb3);
  myPanel.add(rb4);

  rb1.addActionListener( new BList() );
  rb2.addActionListener( new BList() );
  rb3.addActionListener( new BList() );
  rb4.addActionListener( new BList() );

  myPanel.add(messageLabel);

  return myPanel;
}

private class BList implements ActionListener {
  public void actionPerformed(ActionEvent e) {

     if(e.getSource() == rb1){
        System.out.println("Your favorite sport is " + a +".");
     }

     else if(e.getSource() == rb2){
        System.out.println("Your favorite sport is " + b +".");
     }

     else if(e.getSource() == rb3){
        System.out.println("Your favorite sport is " + c +".");
     }

     else if(e.getSource() == rb4){
        System.out.println("Your favorite sport is " + d +".");
     }

  }
}

public static void main(String[] args) {
  new ActionFrame();
}
}

Answer 1

您已将所有单选按钮命名为rb1，因此其他组件永远不会被初始化

rb1 = new JRadioButton(a, true);
rb2 = new JRadioButton(b);
rb3 = new JRadioButton(c);
rb4 = new JRadioButton(d);

Answer 2

不

  rb1 = new JRadioButton(a, true);
  rb1 = new JRadioButton(b);
  rb1 = new JRadioButton(c);
  rb1 = new JRadioButton(d);

可是：

  rb1 = new JRadioButton(a, true);
  rb2 = new JRadioButton(b);
  rb3 = new JRadioButton(c);
  rb4 = new JRadioButton(d);

糟糕。

这里的关键不是你的错误，拍摄我们都犯了类似的错误。关键是要学习如何调试NPE。当你得到一个，检查抛出异常的行，找出哪个变量为null，然后回头尝试解决它。那是关键。

Answer 3

变化

  rb1 = new JRadioButton(a, true);
  rb1 = new JRadioButton(b);
  rb1 = new JRadioButton(c);
  rb1 = new JRadioButton(d);

到

  rb1 = new JRadioButton(a, true);
  rb2 = new JRadioButton(b);
  rb3 = new JRadioButton(c);
  rb4 = new JRadioButton(d);

原始代码（带有变化）

public class ActionFrame extends JFrame {
JLabel messageLabel;
JRadioButton rb1;
JRadioButton rb2;
JRadioButton rb3;
JRadioButton rb4;
String a = "Football";
String b = "Basketball";
String c = "Baseball";
String d = "Hockey";

public ActionFrame() {
  setTitle("Favorite Sports");
  setSize (400,200);
  setDefaultCloseOperation(
     JFrame.EXIT_ON_CLOSE );

  JPanel myStuff = makeComponents();
  add(myStuff);
  setVisible(true);
}

private JPanel makeComponents() {
  JPanel myPanel = new JPanel();

  messageLabel = new JLabel("Select your favorite sport: ");

/// changes are here...
  rb1 = new JRadioButton(a, true);
  rb2 = new JRadioButton(b);
  rb3 = new JRadioButton(c);
  rb4 = new JRadioButton(d);

  ButtonGroup group = new ButtonGroup();

  group.add(rb1);
  group.add(rb2);
  group.add(rb3);
  group.add(rb4);

  myPanel.add(rb1);
  myPanel.add(rb2);
  myPanel.add(rb3);
  myPanel.add(rb4);

  rb1.addActionListener( new BList() );
  rb2.addActionListener( new BList() );
  rb3.addActionListener( new BList() );
  rb4.addActionListener( new BList() );

  myPanel.add(messageLabel);

  return myPanel;
}

private class BList implements ActionListener {
  public void actionPerformed(ActionEvent e) {

     if(e.getSource() == rb1){
        System.out.println("Your favorite sport is " + a +".");
     }

     else if(e.getSource() == rb2){
        System.out.println("Your favorite sport is " + b +".");
     }

     else if(e.getSource() == rb3){
        System.out.println("Your favorite sport is " + c +".");
     }

     else if(e.getSource() == rb4){
        System.out.println("Your favorite sport is " + d +".");
     }

  }
}

public static void main(String[] args) {
  new ActionFrame();
}
}

单选按钮无法正常显示

3 个答案: