Java 单击JButton时如何更改JLabel？

程序说明：

实现具有以下功能的简单整数计算器：

启动时，将计算器置于默认屏幕的中心。 显示器必须有边框。 所有操作均采用以下形式： 操作数1运算符1操作数2=运算符2操作数3= [C]按钮将清除计算器，例如初始状态。必须支持Alt+[C] Ctrl+[C]Ctrl+按钮组合将显示“C您的姓名” 只有[C]按钮可用于恢复当前显示。 [=]按钮是默认按钮。 结果可能是负面的。 最多只允许8位数字。 显示错误，如“溢出”、“0除以除法”、…，并且[C]按钮将清除错误并重置计算器。 使用JCalculator.png作为程序图标。 我正试图编写一个程序来创建一个简单计算器的swing gui。我希望点击的号码显示在标签上。但是，每当我单击数字时，我总是收到NullPointerException错误。我认为动作表演部分是错误的，但我不太确定

您正在隐藏button_Shapes变量

您声明了一个名为button_Shapes的实例字段，但在构造函数中，您创建了另一个名为button_Shapes的变量，该变量仅具有构造函数的上下文，它只能在构造函数的上下文中使用

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

public class JCalculator implements ActionListener {

private JButton[] buttons;
private JLabel display;
private String[] button_Shapes;
//private String s0, s1, s2;

//Create new form Calculator
public JCalculator(){

    //Create new JFrame container
    JFrame jfrm = new JFrame("Calculator");

    //Set the initial size for frame
    jfrm.setSize(500,500);

    //Terminate the program when the user closes the application.
    jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Set the calculator to center on the screen
    jfrm.setLocationRelativeTo(null);

    //Set the icon
    ImageIcon icon = new ImageIcon("C:/Users/zetsu/Desktop/JCalculator.png");
    jfrm.setIconImage(icon.getImage());

    //Create label
    display = new JLabel("0", JLabel.RIGHT); 

    //Put border around display label
    display.setBorder(BorderFactory.createLineBorder(Color.black));

    //Create a grid layout
    GridLayout layout = new GridLayout(4,4);

    //Create a panel and set layout
    JPanel bottom_Panel = new JPanel();
    bottom_Panel.setLayout(layout);

    //Create an array of buttons
    buttons = new JButton[16];        

    //Put buttons in an array
    String[] button_Shapes = {"7", "8", "9", "/", "4", "5", "6", "x", "1",
                     "2", "3", "-", "0", "C", "=", "+"};

    for(int i = 0; i < button_Shapes.length; i++){
        //make new button name
        JButton btn = new JButton("" + button_Shapes[i]);
        buttons[i] = btn;
        //add action listener for each button
        btn.addActionListener(this);
        //add each button to panel
        bottom_Panel.add(btn);
    }

    //Set [=] button to default
    jfrm.getRootPane().setDefaultButton(buttons[14]);

    //Set Mnemonic to Alt+[C]
    buttons[13].setMnemonic(KeyEvent.VK_C);

    //Add label to content pane
    jfrm.add(display, BorderLayout.NORTH);

    //Add panel to content pane
    jfrm.add(bottom_Panel, BorderLayout.CENTER);

    jfrm.setVisible(true);

}

@Override
public void actionPerformed(ActionEvent e) {
    //try left operand, operation, right operand
    Object event = e.getSource();
    if(event == buttons[0]){
        display.setText(""+button_Shapes[0]);
    }
}

public static void main(String[] args){
    SwingUtilities.invokeLater(new Runnable(){
        public void run(){
            new JCalculator();
        }
    });
}  
}

删除第二个声明

public class JCalculator implements ActionListener {

    private String[] button_Shapes;
    //Create new form Calculator
    public JCalculator(){
        //...    
        //Put buttons in an array
        String[] button_Shapes = {"7", "8", "9", "/", "4", "5", "6", "x", "1",
                     "2", "3", "-", "0", "C", "=", "+"};
        //...

---

您可能还想通读一些详细信息

是的，这很有效。我想这可能是问题所在，但不知道如何解决。非常感谢。

private String[] button_Shapes;
//Create new form Calculator
public JCalculator(){
    //...    
    //Put buttons in an array
    button_Shapes = {"7", "8", "9", "/", "4", "5", "6", "x", "1",
                 "2", "3", "-", "0", "C", "=", "+"};