Java 向JComboBox添加操作侦听器_Java_Jcombobox_Actionlistener

Java 向JComboBox添加操作侦听器

java

Java 向JComboBox添加操作侦听器,java,jcombobox,actionlistener,Java,Jcombobox,Actionlistener,我只想将组合框中选定的选项打印到文本字段中。请解释什么是错的，因为我必须在课堂上完成并解释。任何帮助都将不胜感激。提前谢谢 import java.awt.*; import java.awt.event.*; import javax.swing.*; public class App3 extends JFrame implements ActionListener { private JPanel boxPanel,textPanel; private JLabel s

我只想将组合框中选定的选项打印到文本字段中。请解释什么是错的，因为我必须在课堂上完成并解释。任何帮助都将不胜感激。提前谢谢


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();
    }
}

Exception in thread "main" java.lang.NullPointerException at java.awt.Container.addImpl(Container.java:1041) at java.awt.Container.add(Container.java:927) at App3.setup(App3.java:42) at App3.(App3.java:16) at App3.main(App3.java:58) Process completed. 线程“main”java.lang.NullPointerException中出现异常位于java.awt.Container.addImpl（Container.java:1041）在java.awt.Container.add处（Container.java:927）在App3.setup（App3.java:42） App3.（App3.java:16）位于App3.main（App3.java:58）进程已完成。

尝试将textPanel添加到内容窗格时，textPanel为空：

c.add(textPanel, BorderLayout.CENTER);

您尚未创建JPanel对象，因此textPanel指向null，这就是引发异常的原因。创建对象，一切都应该正常运行

什么不正常？你有什么错误？ Exception in thread "main" java.lang.NullPointerException at java.awt.Container.addImpl(Container.java:1041) at java.awt.Container.add(Container.java:927) at App3.setup(App3.java:42) at App3.(App3.java:16) at App3.main(App3.java:58) Process completed.

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();
    }

}