java swing单选按钮 - java.lang.NullPointerException

Question

我正试图掌握java swing并测试单选按钮.我的代码是:

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:

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

这是:"if(bandButton.isSelected()){"

我认为'bandButton'已被创建并标记为"已选中" - 或者我误解了某些内容？

非常感谢,Curly.

Answer 1

你正在影响bandButton变量 - 你在构造函数中重新声明它并初始化本地重新声明的变量,而不是类字段使类字段为null.解决方案 - 不要重新声明变量.

要明确,改变这个:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

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

对此:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

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

请注意,您正在为您在类中声明的所有三个变量执行此操作.