java Swing 单选按钮 - java.lang.NullPointerException [英] java swing radio buttons - java.lang.NullPointerException

问题描述

我正在尝试使用 java swing 并测试单选按钮.我的代码是:

I'm trying to get to grips with java swing and was testing out radio buttons. My code is:

import java.awt.*;
import javax.swing.*;
import javax.swing.ButtonGroup;

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    private JRadioButton gelButton;
    private JButton jbtnRun;

    public Scafhome() {
        JFrame jfrm = new JFrame("Scaffold search ...");
        jfrm.setLayout (new GridLayout(8,2));
        jfrm.setSize(320,220);
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JRadioButton bandButton = new JRadioButton();
        bandButton.setText("Band-id");
        bandButton.setSelected(true);

        JRadioButton gelButton = new JRadioButton();
        gelButton.setText("Gelc-ms");

        ButtonGroup group = new ButtonGroup();
        group.add(bandButton);
        group.add(gelButton);

        JButton jbtnRun = new JButton("RUN");


        jbtnRun.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                RunActionPerformed(evt);
            }
        });


        jfrm.add(bandButton);
        jfrm.add(gelButton);
        jfrm.add(jbtnRun);

        jfrm.setVisible(true);

    }


    private void RunActionPerformed(java.awt.event.ActionEvent evt) {

        String radioText="";
        if (bandButton.isSelected()) {
            radioText=bandButton.getText();
        }

        if (gelButton.isSelected()) {
            radioText=gelButton.getText();
        }

        javax.swing.JOptionPane.showMessageDialog( Scafhome.this, radioText );

    }


    public static void main(String args[]) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Scafhome();
            }
        });
    }

}

不幸的是，我收到以下错误消息:

Unfortunately I get the following error message:

线程AWT-EventQueue-0"中的异常 java.lang.NullPointerException在 Scafhome.RunActionPerformed(Scafhome.java:50)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at Scafhome.RunActionPerformed(Scafhome.java:50)

这是在:"if (bandButton.isSelected()) {"

This is at: "if (bandButton.isSelected()) {"

我认为bandButton"已被创建并标记为已选择" - 还是我误解了什么?

I thought 'bandButton' had been created and marked as 'selected' - or have I misunderstood something?

非常感谢，卷曲.

推荐答案

您正在隐藏 bandButton 变量——您是在构造函数中重新声明它并初始化本地重新声明的变量，而不是离开类的类字段字段为空.解决方案——不要重新声明变量.

You're shadowing the bandButton variable -- you're re-declaring it in the constructor and initializing the local redeclared variable, not the class field leaving the class field null. Solution -- don't re-declare the variable.

为了明确，改变这个:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

        // re-declared variable!
        JRadioButton bandButton = new JRadioButton();

为此:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

        // variable not re-declared    
        bandButton = new JRadioButton();

请注意，您正在为类中声明的所有三个变量执行此操作.

Note that you're doing this for all three variables that you've declared in the class.

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