0

我只想将组合框中的选定选项打印到文本字段。请解释什么是错的，因为我必须完成它并在课堂上解释它。任何帮助将不胜感激。提前致谢。


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

public class App3 extends JFrame implements ActionListener
{
    private JPanel boxPanel,textPanel;
    private JLabel selectName,selectedName;
    private JComboBox nameCombo;
    private JTextField valueOfSelectedName;
    private Container c;

    public App3()
    {
        super("Combo example");
        setup();
        setSize(200,200);
        setLocation(50,50);
        show();
    }

    public void setup()
    {
        c = getContentPane();

        boxPanel = new JPanel();
        c.add(boxPanel,BorderLayout.NORTH);

        selectName = new JLabel("Select Name : ");
        selectedName = new JLabel("The selected Name : ");

        String[] names = {"Ramila","Hashan","Shaad","Gus","Mahasen","Hasaru","Shabba"};
        nameCombo = new JComboBox(names);
        nameCombo.addActionListener(this);

        valueOfSelectedName = new JTextField(10);

        boxPanel.add(selectName);
        boxPanel.add(nameCombo);

        c.add(textPanel,BorderLayout.CENTER);

        textPanel.add(selectedName);
        textPanel.add(valueOfSelectedName);

    }

    public void actionPerformed(ActionEvent e) 
    {
        JComboBox nameCombo = (JComboBox)e.getSource();
        String newSelection = (String)nameCombo.getSelectedItem();
        valueOfSelectedName.setText(newSelection);
    }

    public static void main(String args[])
    {
        App3 a = new App3();
    }
}

我没有得到任何编译时错误，当我运行它时我得到了这些错误。 

线程“主”java.lang.NullPointerException 中的异常
    在 java.awt.Container.addImpl(Container.java:1041)
    在 java.awt.Container.add(Container.java:927)
    在 App3.setup(App3.java:42)
    在 App3.(App3.java:16)
    在 App3.main(App3.java:58)

过程完成。
4

3 回答 3

1 
private JPanel boxPanel,textPanel;
...
textPanel = new JPanel();

您还没有创建 JPanel 对象，因此 textPanel 指向 null 这就是抛出异常的原因。创建对象，一切正常

于 2010-10-31T08:59:57.643 回答
0

当您尝试将 textPanel 添加到内容窗格时，它为 null：

c.add(textPanel, BorderLayout.CENTER);
于 2010-10-31T08:25:05.840 回答
0 
import java.awt.*;

import java.awt.event.*;

import javax.swing.*;


public class App3 extends JFrame implements ActionListener
{
    private JPanel boxPanel,textPanel;

    private JLabel selectName,selectedName;

    private JComboBox nameCombo;

    private JTextField valueOfSelectedName;

    private Container c;


    public App3()
    {
        super("Combo example");
        setup();
        setSize(200,200);
        setLocation(50,50);
        show();
    }

    public void setup()
    {
        c = getContentPane();

        boxPanel = new JPanel();
        c.add(boxPanel,BorderLayout.NORTH);

        selectName = new JLabel("Select Name : ");
        selectedName = new JLabel("The selected Name : ");

        String[] names = {"Ramila","Hashan","Shaad","Gus","Mahasen","Hasaru","Shabba"};
        nameCombo = new JComboBox(names);
        nameCombo.addActionListener(this);

        valueOfSelectedName = new JTextField(10);

        boxPanel.add(selectName);
        boxPanel.add(nameCombo);

        textPanel = new JPanel(); // panel is not init that why its give error

        c.add(textPanel,BorderLayout.CENTER);

        textPanel.add(selectedName);
        textPanel.add(valueOfSelectedName);

    }

    public void actionPerformed(ActionEvent e) 
    {
        JComboBox nameCombo = (JComboBox)e.getSource();
        String newSelection = (String)nameCombo.getSelectedItem();
        valueOfSelectedName.setText(newSelection);
    }

    public static void main(String args[])
    {
        App3 a = new App3();
    }

}
于 2011-04-22T11:39:08.283 回答