Question

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

public class Calculator extends JFrame implements ActionListener{
    JButton[] nums;
    JButton eq, mult, div, clr, clrEnt, sub, add, dot;
    JTextArea txtArea = new JTextArea();

    public Calculator() {
        super("Calculator");
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel pane = new JPanel(new GridLayout(6, 1, 0, 5));
        add(pane);
        JPanel paneSecond = new JPanel(new GridLayout(1, 3, 5, 5));
        JPanel paneThird = new JPanel(new GridLayout(1, 3, 5, 5));
        JPanel paneFourth = new JPanel(new GridLayout(1, 3, 5, 5));
        JPanel paneFifth = new JPanel(new GridLayout(1, 3, 5, 5));
        JPanel paneSixth = new JPanel(new GridLayout(1, 3, 5, 5));
        JPanel paneSeventh = new JPanel(new GridLayout(1, 3, 5, 5));
        nums = new JButton[11];
        //add = new JButton();
        addOpButton(eq, "=");
        addOpButton(mult, "x");
        addOpButton(div, "/");
        addOpButton(clr, "C");
        addOpButton(clrEnt, "CE");
        addOpButton(sub, "-");
        addOpButton(add, "+");
        addOpButton(dot, ".");

        for (int i = 0; i < nums.length; i++) {
            nums[i] = new JButton("" + i);
            nums[i].setActionCommand(nums[i].toString());
            nums[i].addActionListener(this);
        }

        addPanel(paneSecond, 1, 4);
        addPanel(paneThird, 4, 7);
        addPanel(paneFourth, 7, 10);
        //addButtons(paneFifth, add, nums[0], sub);
        pane.add(paneSecond);
        pane.add(paneThird);
        pane.add(paneFourth);
        pane.add(paneFifth);
        pane.add(paneSixth);
        pane.add(paneSeventh);
        pack();
    }

    void addPanel(JComponent pane, int start, int condition) {
        for (int i = start; i < condition; i++) {
            pane.add(nums[i]);
        }
    }

    void addButtons(JComponent pane, JButton btn1, JButton btn2, JButton btn3) {
        pane.add(btn1);
        pane.add(btn2);
        pane.add(btn3);
    }

    void addOpButton(JButton btn, String op) {
        btn = new JButton(op);
        btn.setActionCommand(op);
        btn.addActionListener(this);
    }

    public void actionPerformed(ActionEvent e) {
    }

    public static void main(String[] args) {
        Calculator calc = new Calculator();
    }
}

在addOpButton方法中，我的按钮都没有被初始化。我不知道为什么。字符串正在传递和使用，但不是按钮。即使在初始化其中一个按钮（构造函数中的“添加”按钮并删除了注释）之后，该方法中的按钮仍然为null。我无法弄清楚为什么。任何帮助表示赞赏。

Answer 1

Java是＆＃34;传递值＆＃34;。这意味着如果您尝试传递引用，则复制引用然后传递。

在实践中，这意味着您为addOpButton提供的btn参数是一个副本，而另一个副本由调用该方法的人保留。然后当你执行btn = new时，你要为副本分配一些内容，给它一个命令和一个actionlistener，然后它就被丢弃了。来电者仍然可以参考他自己的副本。

JButton没有在方法中初始化

1 个答案: