joptionpane.showmessagedialog在if-Statement(事件处理程序程序)内部工作

Question

上下文

我正在尝试构建百分比计算器使用 Jframe 和事件处理通过具有空白文本字段和"输入"按钮和显示答案取决于他们单击的按钮（每个按钮的两个不同计算）。

问题

的事情是，当我将joptionpane.showmessagedialog线放入if/else if语句的内部（所有操作效果的方法中）时，程序运行，但是单击按钮无能为力。但是，如果我将其放在语句之外，它说答案变量是未宣布的。

我意识到可以以其他更简单的方式来完成一个百分比的计算器，但这只是我想的，想看看我是否可以做到这一点（我是一个完整的初学者）。

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

您具有具有相同名称的成员变量和本地变量。请阅读ERE和此处。

这是您程序的校正变体

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

对您的按钮使用单独的ActionListeners。两者都使用具有不同参数的相同计算方法：

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);
}