java swing radio buttons - java.lang.NullPointerEx

问题:

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();
            }
        });
    }

}

Unfortunately I get the following error message:

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

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

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

Many thanks, Curly.

回答1:

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.

To be explicit, change this:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

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

to this:

public class Scafhome extends javax.swing.JFrame {

    private JRadioButton bandButton;
    //...

    public Scafhome() {
        //...

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

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