package calculator;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Example extends JPanel{
private JButton display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;

public Example() {
    setLayout(new BorderLayout());

    result = 0;
    lastCommand = "=";
    start = true;

    display = new JButton("0");
    display.setEnabled(false);
    add(display, BorderLayout.NORTH);

    ActionListener insert = new InsertAction();
    ActionListener command = new CommandAction();

    panel = new JPanel();
    panel.setLayout(new GridLayout(4, 4));

    addButton("7", insert);
    addButton("8", insert);
    addButton("9", insert);
    addButton("/", insert);

    addButton("4", insert);
    addButton("5", insert);
    addButton("6", insert);
    addButton("*", insert);

    addButton("1", insert);
    addButton("2", insert);
    addButton("3", insert);
    addButton("-", insert);

    addButton("0", insert);
    addButton(".", insert);
    addButton("=", insert);
    addButton("+", insert);

    add(panel, BorderLayout.CENTER);
}

private void addButton(String label, ActionListener listener) {
    JButton button = new JButton(label);
    button.addActionListener(listener);
    panel.add(button);
}

private class InsertAction implements ActionListener{
    public void actionPerformed(ActionEvent event) {
        String input = event.getActionCommand();
        if(start) {
            display.setText("");
            start = false;
        }
        display.setText(display.getText() + input);
    }
}

private class CommandAction implements ActionListener{
    public void actionPerformed(ActionEvent event) {
        String command = event.getActionCommand();

        if(start) {
            if(command.equals("-")) {
                display.setText(command);
                start = false;
            }
            else lastCommand = command;
        }
        else {
            calculate(Double.parseDouble(display.getText()));
            lastCommand = command;
            start = true;
        }
    }
}
public void calculate(double x) {
    if(lastCommand.contentEquals("+")) result += x;
    else if(lastCommand.equals("-")) result -=x;
    else if(lastCommand.equals("*")) result *=x;
    else if(lastCommand.equals("/")) result /=x;
    else if(lastCommand.equals("=")) result =x;
    display.setText("" + result);
}

public static void main(String[] args){

}
}
double转String类型报错For input string

qq_38705728

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