Question

上下文

我尝试使用 JFrame 和事件处理来构建百分比计算器，方法是使用空白文本字段和＆＃34;输入＆＃34;按钮和显示答案取决于他们点击的按钮（每个按钮两个不同的计算）。

问题

事实是，当我将JOptionPane.showMessageDialog行放在if / else if语句中（全部在actionPerformed方法中）时，程序会运行，但单击按钮不会执行任何操作。但如果我把它放在陈述之外，它就说答案变量是未声明的。

我意识到百分比计算器可以用其他一百万种更简单的方式完成，但这只是我想到的，并且想看看我是否可以做到（我是完全初学者）。

代码

public class PercentageCalc extends JFrame{
private JTextField item2, item4, item5, item7;
private JButton button, button2;

public PercentageCalc(){
    super("Percentage Calculator");
    setLayout(new FlowLayout());

    JLabel item1 = new JLabel("What is");
    add(item1);

    JTextField item2 = new JTextField(3);
    add(item2);

    JLabel item3 = new JLabel("% of");
    add(item3);

    JTextField item4 = new JTextField(2);
    add(item4);
    button = new JButton("Enter");
    add(button);
    JLabel spacer = new JLabel("                       ");
    add(spacer);

    JTextField item5 = new JTextField(3);
    add(item5);

    JLabel item6 = new JLabel("is what % of");
    add(item6);

    JTextField item7 = new JTextField(3);
    add(item7);
    button2 = new JButton("Enter");
    add(button2);


    thehandler handler = new thehandler();
    button.addActionListener(handler);
    button2.addActionListener(handler); 
}
private class thehandler implements ActionListener{
    public void actionPerformed(ActionEvent e){
        String strx;
        String stry;
        double x, y, answer;
        if(e.getSource()==button){
            strx = item2.getText();
            stry = item4.getText();
            x = Double.parseDouble(strx);
            y = Double.parseDouble(stry);
            answer = y * 0.01 * x;
            JOptionPane.showMessageDialog(null, answer);
        }
            else if(e.getSource()==button2){
            strx = item5.getText();
            stry = item7.getText();
            x = Double.parseDouble(strx);
            y = Double.parseDouble(stry);
            answer = x/y*100;
            JOptionPane.showMessageDialog(null, answer);
            }
}

Answer 1

您具有相同名称的成员变量和局部变量。请阅读ere和here。

以下是您修正后的程序

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

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JTextField;
import javax.swing.WindowConstants;

public class PrecentageCalc extends JFrame {
    private JTextField item2, item4, item5, item7;

    private JButton button, button2;

    public PrecentageCalc() {
        super("Percentage Calculator");
        setLayout(new FlowLayout());

        JLabel item1 = new JLabel("What is");
        add(item1);

        item2 = new JTextField(3);
        add(item2);

        JLabel item3 = new JLabel("% of");
        add(item3);

        item4 = new JTextField(2);
        add(item4);
        button = new JButton("Enter");
        add(button);
        JLabel spacer = new JLabel("                       ");
        add(spacer);

        item5 = new JTextField(3);
        add(item5);

        JLabel item6 = new JLabel("is what % of");
        add(item6);

        item7 = new JTextField(3);
        add(item7);
        button2 = new JButton("Enter");
        add(button2);

        thehandler handler = new thehandler();
        button.addActionListener(handler);
        button2.addActionListener(handler);
    }

    private class thehandler implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            String strx;
            String stry;
            double x, y, answer;
            if (e.getSource() == button) {
                strx = item2.getText();
                stry = item4.getText();
                x = Double.parseDouble(strx);
                y = Double.parseDouble(stry);
                answer = y * 0.01 * x;
                JOptionPane.showMessageDialog(null, answer);
            } else if (e.getSource() == button2) {
                strx = item5.getText();
                stry = item7.getText();
                x = Double.parseDouble(strx);
                y = Double.parseDouble(stry);
                answer = x / y * 100;
                JOptionPane.showMessageDialog(null, answer);
            }
        }
    }

    public static void main(String[] args) {
        JFrame frm = new PrecentageCalc();
        frm.pack();
        frm.setLocationRelativeTo(null);
        frm.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frm.setVisible(true);
    }
}

Answer 2

为按钮使用单独的ActionListeners。两者都使用相同的计算方法和不同的参数：

public PercentageCalc() { 

  // Part of your code   

  button.addActionListener(new ActionListener() {

    @Override
    public void actionPerformed(ActionEvent e) {
      handleInput(item2.getText(), item4.getText());
    }
  });

  button2.addActionListener(new ActionListener() {

    @Override
    public void actionPerformed(ActionEvent e) {
      handleInput(item5.getText(), item7.getText());
    }
  });

}

private void handleInput(final String strx, final String stry) {
  final double x = Double.parseDouble(strx);
  final double y = Double.parseDouble(stry);
  final double answer = y * 0.01 * x;
  JOptionPane.showMessageDialog(null, answer);
}

JOptionpane.showMessageDialog在if语句中没有工作（事件处理程序程序）

2 个答案: