NullPointerException用于动作侦听器中的实例化按钮 [英] NullPointerException for instantiated button in action listener

问题描述

以下代码是我从中获取nullpointer异常的程序的摘要。当我按下GUI上的添加按钮时，错误消息指向此行：

The following code is a snippet of a program I am getting a nullpointer exception from. When I press the "Add" button on the GUI, an error message pointing to this line:

  buttonPanel.addButton.setEnabled(false);

。我猜测由于某些原因，addButton为空，尽管我在buttonPanel的构造函数中将其实例化了：

is displayed. I'm guessing that the addButton is null for some reason although I instantiated it in the constructor of buttonPanel:

 addButton = new JButton("Add");
 addButton.addActionListener(buttonListener);

为什么是空指针错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AddButtonListener.actionPerformed(AddButtonListener.java:21)

出现？当侦听器在buttonPanel类中进行编码时，程序可以正常运行且没有错误。预先感谢您的帮助！

appearing? When the listener is coded inside the buttonPanel class, the program runs fine with no errors. Thanks in advance for your help!

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

public class ButtonPanel extends JPanel{
    public JButton addButton,
                   editButton,
                   deleteButton,
                   acceptButton,
                   cancelButton,
                   exitButton;

    public JPanel topPanel,
                   exitPanel;

    private ParentFrame parentFrame;

    public static String buttonStatus;

    public ButtonPanel(ParentFrame parent){
        parentFrame = parent;

        buttonStatus = "idle";
        //Create Buttons

        AddButtonListener buttonListener = new AddButtonListener(parent);
        addButton = new JButton("Add");
        addButton.addActionListener(buttonListener);
        editButton = new JButton("Edit");
        deleteButton = new JButton("Delete");
        acceptButton = new JButton("Accept");
        cancelButton = new JButton("Cancel");
        exitButton = new JButton("Exit");

        //Manipulate Buttons
        acceptButton.setEnabled(false);
        cancelButton.setEnabled(false);

        //Add to panels
        topPanel = new JPanel();
        topPanel.add(addButton);
        topPanel.add(editButton);
        topPanel.add(deleteButton);
        topPanel.add(acceptButton);
        topPanel.add(cancelButton);

        exitPanel = new JPanel();
        exitPanel.add(exitButton);

        this.setLayout(new GridLayout(2,1));
        this.add(topPanel);
        this.add(exitPanel);
    }
}

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class AddButtonListener implements ActionListener{

    private ParentFrame myFrame;

    private ButtonPanel buttonPanel;

    public AddButtonListener(ParentFrame parent){
        myFrame = parent;
        buttonPanel = parent.buttonPanel;
    }

    @Override
    public void actionPerformed(ActionEvent ae) {
        buttonPanel.buttonStatus = "add";

        buttonPanel.addButton.setEnabled(false);
        buttonPanel.editButton.setEnabled(false);
        buttonPanel.deleteButton.setEnabled(false);

        buttonPanel.acceptButton.setEnabled(true);
        buttonPanel.cancelButton.setEnabled(true);
    }

}

import java.awt.BorderLayout;
import javax.swing.JFrame;

public class ParentFrame extends JFrame{

    public ButtonPanel buttonPanel;

    public ParentFrame(){
        this.setResizable(false);

        buttonPanel = new ButtonPanel(this);

        this.add(buttonPanel, BorderLayout.SOUTH);
        this.pack();

        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(400, 300);
    }

    public static void main(String[] args){
        ParentFrame frame = new ParentFrame();
        frame.setVisible(true);
    }
}

推荐答案

您的buttonpanel要求引用要构造的 ParentFrame 。 ButtonPanel 正在父框架中构建并创建一个侦听器，该侦听器引用父框架的按钮面板。

Your buttonpanel requires a reference to ParentFrame to be constructed. ButtonPanel is being constructed in parent frame and creating a listener, which references the parent frame's button panel.

不幸的是，此时尚未分配按钮面板，因此您的操作侦听器为其按钮面板实例分配了空值。

Unfortunately, the button panel hasn't been assigned at that point, and so your action listener has a null value assigned to its instance of button panel.

我认为问题在于您的 AddButtonListener 中的按钮面板实例为空。

I think the problem is your button panel instance is null in your AddButtonListener.

您可以通过将ButtonPanel实例传递到您的<$中来解决此问题。 c $ c> AddButtonListener 构造函数。由于 AddButtonListener 没有使用 ParentFrame ，所以根本不用传递它。

You can fix this by passing a ButtonPanel instance into your AddButtonListener constructor. As AddButtonListener isn't using ParentFrame, don't bother passing it at all.

private ButtonPanel buttonPanel;
public AddButtonListener(ButtonPanel panel){
    myFrame = parent;
    buttonPanel = panel;
}

然后在按钮面板构造函数中：

And then in your button panel constructor:

public ButtonPanel(ParentFrame parent){
    parentFrame = parent;

    buttonStatus = "idle";
    //Create Buttons

    AddButtonListener buttonListener = new AddButtonListener(this);
    //rest the code

此外，您不应该构建这样的结构。您正在做的是将 ButtonPanel 与 ParentFrame 紧密耦合。这意味着，如果您的父框架发生更改，则可能导致 ButtonPanel 发生另一次更改，从而减少了可维​​护的代码。

Aside, you shouldn't be structuring things like this. What you're doing is tightly coupling your ButtonPanel with your ParentFrame. This means if your parent frame changes it could cause another change in ButtonPanel, which makes for less maintanable code.

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