Java GUI计算器JButton转换setText（双）

Question

我正在尝试制作一个Infix到Postfix GUI计算器。我需要将正常的计算器按钮添加到计算器中，我希望有＆＃34; PI＆＃34;保存pi值的按钮。当动作被调用时，我想要＆＃34; Pi＆＃34;出现在input.setText字段中。但是，当我尝试这样做时，我遇到了将double转换为textField接受的类型的问题。

import acm.program.*;
import acm.graphics.*;  // GCanvas and G-friends
import javax.swing.*;   // JButton and J-friends
import java.awt.event.ActionEvent;
import java.awt.Color;
import java.awt.Font;

public class Calc extends Program
{
JTextField input;
JTextField output;
JTextField result;


public Calc()
{
    start();
    setSize(500, 500);
}

public void init()
{
    GCanvas canvas = new GCanvas();
    add(canvas);

    canvas.setBackground(Color.lightGray);

    JButton conversionButton = new JButton("     DEG     ");
    canvas.add(conversionButton, 85, 60);

    JButton conversionButton2 = new JButton("RAD");
    canvas.add(conversionButton2, 170, 60);
    conversionButton2.setSize(conversionButton.getSize());

    JButton goButton = new JButton("Go!");
    canvas.add(goButton, 280, 350);
    goButton.setBackground(Color.green);
    goButton.setSize(conversionButton.getSize());

    JButton clearButton = new JButton("Clear");
    canvas.add(clearButton, 380, 350);
    clearButton.setBackground(Color.red);
    clearButton.setSize(conversionButton.getSize());

    JButton graphButton = new JButton("Graph");
    canvas.add(graphButton, 0, 120);
    graphButton.setSize(conversionButton.getSize());

    JButton factorialButton = new JButton("!");
    canvas.add(factorialButton, 85, 120);
    factorialButton.setSize(conversionButton.getSize());

    JButton ANSButton = new JButton("ANS");
    canvas.add(ANSButton, 0, 90);
    ANSButton.setSize(conversionButton.getSize());

    JButton LnButton = new JButton("Ln");
    canvas.add(LnButton, 85, 90);
    LnButton.setSize(conversionButton.getSize());

    JButton AbsButton = new JButton("|   |");
    canvas.add(AbsButton, 170, 90);
    AbsButton.setSize(conversionButton.getSize());

    JButton PIButton = new JButton("PI");
    canvas.add(PIButton, 170, 120);
    PIButton.setSize(conversionButton.getSize());

    JLabel infixLabel = new JLabel("Infix");
    canvas.add(infixLabel, 275, 40); 

    JLabel postfixLabel = new JLabel("Postfix");
    canvas.add(postfixLabel, 275, 160);

    JLabel resultLabel = new JLabel("Result");
    canvas.add(resultLabel, 275, 280);

    JLabel messageLabel = new JLabel("Note: this calculator 
    can only graph     functions of the 3rd degree or lower");
    canvas.add(messageLabel, 25, 10);


    input = new JTextField();
    canvas.add(input, 275, 60);
    input.setSize(200, 30);

    output = new JTextField();
    canvas.add(output, 275, 180);
    output.setSize(200, 30);

    result = new JTextField();
    canvas.add(result, 275, 300);
    result.setSize(200, 30);

    addActionListeners();
    }


    public void actionPerformed(ActionEvent ae)
    {

    String action = ae.getActionCommand();

    if (action.equals("Clear"))
    {
        input.setText("");
        output.setText("");
        result.setText("");
    }
    else if (action.equals("Go!"))
    {
        // Get input from user
        String c = input.getText();
        result.setText(input.getText());
    }

    if (action.equals("PI"))
    {
        double Pi = 3.145926;
        Double.toString(Pi);
        input.setText(String.Pi);


    }
}
}