java - ActionListener上的java.lang.NullPointerException | Java Swing

我确实为创建这个问题而道歉，我也做了.NET，所以我知道问这个问题的感觉，但是我确实进行了搜索并尝试在Eclipse中调试我的程序，但仍然找不到解决方法。 ，因为我是Java GUI的新手，并且是第一次使用Eclipse，所以...

我有一个小程序Java swing GUI程序，其中有2个按钮，其中一个按钮是隐藏的（可见设置为false），当单击一个按钮时，它将显示隐藏的按钮（将Visible设置为true）：

import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class TestButton extends JPanel {

    private JFrame mainFrame;
    private JButton btnShow ;
    private JButton btnNew;

    public TestButton() {
        mainFrame = new JFrame("Test Button");

        JButton btnShow = new JButton("Show New Button");
        JButton btnNew = new JButton("This is New Button");

        Container c = mainFrame.getContentPane();
        c.setLayout(new FlowLayout());

        c.add(btnShow);
        c.add(btnNew);
        btnNew.setVisible(false);

        btnShow.setMnemonic('G');
        btnNew.setMnemonic('N');

        mainFrame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

        ShowButtonHandler ghandler = new ShowButtonHandler();
        btnShow.addActionListener(ghandler);

        mainFrame.setSize(250, 150);
        mainFrame.setLocationRelativeTo(null);
        mainFrame.setResizable(false);
        mainFrame.setVisible(true);
    }

    class ShowButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            btnNew.setVisible(true);
        }
    }

    public static void main(String args[]) {
        TestButton app = new TestButton();
    }
}

但是，当我单击该按钮时，它在线程“ AWT-EventQueue-0” java.lang.NullPointerException中显示异常

错误在此行：btnNew.setVisible（true）;

这是完整的跟踪：

 Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at TestButton$ShowButtonHandler.actionPerformed(TestButton.java:51)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$400(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

因此，通常在未初始化对象时发生Null异常，但是我在声明它时确实初始化了btnNew，不是吗？问题出在哪里？

另外，我想在单击它的位置再添加一个名为“再次运行”的按钮，它会关闭当前窗口并打开新窗口（基本上我想再次运行程序），是否可以通过这种方式或如何对其进行存档？

最佳答案

您的局部变量隐藏了实例变量

更改

JButton btnNew = new JButton("This is New Button");

至

btnNew = new JButton("This is New Button");

编辑

根据您的问题作为评论...但是最好提出一个新问题或将您的代码发布在https://codereview.stackexchange.com/上。

我的意思是您的内部类ShowButtonHandler取决于外部类TestButton，因为它使用外部类的字段btnNew。

class ShowButtonHandler implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        btnNew.setVisible(true);
    }
}

但是这种依赖性不是必需的。 ShowButtonHandler仅需要引用JButton，当执行操作时必须将其设置为可见。

因此，第一步，您只需将按钮作为构造函数参数传递，即可打破对外部类的依赖。

class ShowButtonHandler implements ActionListener {

    private JButton btnNew;

    public ShowButtonHandler(JButton btnNew){
        this.btnNew = btnNew;
    }

    public void actionPerformed(ActionEvent e) {
        btnNew.setVisible(true);
    }
}

现在您意识到ShowButtonHandler可以更灵活地允许重用。您看一下类层次结构，就会发现可以对任何setVisible完成JComponent。因此，您可以使该类更加通用。

class ShowComponentHandler implements ActionListener {

    private JComponent component;

    public ShowComponentHandler(JComponent component){
        this.component = component;
    }

    public void actionPerformed(ActionEvent e) {
        component.setVisible(true);
    }
}

由于ShowButtonHandler现在是独立的，并且具有更通用的API，因此可以将其放置在自己的编译单元（java文件）中并重新使用。

在您的TestButton类中，您仍然可以使用它

ActionListener showComponentAction = new ShowComponentHandler(btnNew);
btnShow.addActionListener(showComponentAction);