Java 单选按钮赢得'；不能正确显示

我只是在学习单选按钮，并认为我终于得到了它，因为它编译得很好。但当我尝试运行程序时，会出现以下错误：

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)

不是：

但是：

哎呀

---

这里的关键不是你的错误，我们都会犯类似的错误。关键是学习如何调试NPE。当您得到一个异常时，检查抛出异常的行，找出哪个变量为null，然后回过头来尝试解决它这是关键。

您已经命名了所有单选按钮

rb1

，因此其他组件永远不会初始化

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

改变

进入

原始代码（带更改）

---

@用户2877117：不，你不是白痴；您只是还没有学会调试NPE。请阅读我的编辑。

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

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

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

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

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

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();
}
}