Java JOptionPane 对话框 NullPointerException

Question

以下代码运行完美，但是如果您在对话框上单击“x”或“取消”，则会抛出 NullPointerException 。我试图调用 TestMessage 方法并返回到同一个对话框。

import javax.swing.JOptionPane;

public class Alpha {

  public void TestMessage() {

    Object [] tM1={"Option 1","Option 2","Option 3"};

    String mA2 = (String) JOptionPane.showInputDialog(null,"This is a test Message",null, JOptionPane.QUESTION_MESSAGE, null,tM1,tM1[0]);

    if (mA2.equals("Option 1")) {JOptionPane.showMessageDialog(null,"You chose Option 1");}
    if (mA2.equals("Option 2")) {JOptionPane.showMessageDialog(null,"You chose Option 2");}
    if (mA2.equals("Option 3")) {JOptionPane.showMessageDialog(null,"You chose Option 3");}

    }

    public static void main (String [] args) {

    Alpha A1=new Alpha();
    A1.TestMessage();

    }

    }

错误消息:

java.lang.NullPointerException
at Alpha.TestMessage(Alpha.java:11)
at Alpha.main(Alpha.java:20)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

Answer 1

showInputDialog 如果您按“取消”或“X”，则返回 null，以便您检查 if (mA2.equals("Option 1") 访问null 指针。只需先检查 mA2 != null 即可。