Java GUI 计算器上未显示文本的问题

Question

When i run the program the GUI shows up. When i click on the buttons nothing populates in the screen. In my action performed method, screen does not resolve because i have not declared it in the method. screen is declared in my init method at the beginning.

I don't understand how my code in the second method will be applied to the first method (the GUI).

any tips?

import acm.program.*;
import javax.swing.*;
import java.awt.*;    // for graphics 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import acm.gui.*;
public class Calculator extends DialogProgram implements ActionListener{ 
    public void init() {


    JButton b0,b1,b2,b3,b4,b5,b6,b7,b8,b9,bm,bd,ba,bs,bdec,bc;
    DoubleField screen;


    //create a panel 


    JPanel P= new JPanel();     // main panel 

    JPanel P1= new JPanel();    // panel for buttons 

    JPanel P2= new JPanel();    // panel for screen




    // adding panel to dialog box

    add(P);        // main panel 
    P.add(P2);     // adding screen
    P.add(P1);     // adding panel of buttons 

    // set panel color 
    P.setBackground(Color.yellow);                 



    // grid layout for buttons 

    P1.setLayout(new GridLayout(4,4,4,4));           // layout for button panel 
    P.setLayout(new GridLayout(2,1));                // layout for main panel 

    // create screen 
    screen = new DoubleField();                      // creating screen 
    screen.setPreferredSize(new Dimension(200,40));  // setting screen size 
    screen.setEditable(false);                       // screen can not be edited 




    // integer buttons 
    b0 = new JButton("0");
    b1 = new JButton("1");
    b2 = new JButton("2");
    b3 = new JButton("3");
    b4 = new JButton("4");
    b5 = new JButton("5");
    b6 = new JButton("6");
    b7 = new JButton("7");
    b8 = new JButton("8");
    b9 = new JButton("9");

    // operator buttons 
    bm = new JButton("*");
    bd= new  JButton("/");
    ba = new JButton("+");
    ba.addActionListener(this); 
    bs = new JButton("-");

    // decimal button 
    bdec = new JButton(".");

    // clear button
    bc = new JButton("C");


    // adding buttons to panel 
    P2.add(screen);
    P1.add(b7);
    P1.add(b8);
    P1.add(b9);
    P1.add(bm);
    P1.add(b4);
    P1.add(b5);
    P1.add(b6);
    P1.add(bd);
    P1.add(b1);
    P1.add(b2);
    P1.add(b3);
    P1.add(ba);
    P1.add(bc);
    P1.add(b0);
    P1.add(bdec);
    P1.add(bs);

    // adding action listeners 
     bm.addActionListener(this); 
     bd.addActionListener(this); 
     bs.addActionListener(this); 

     b9.addActionListener(this); 
     b8.addActionListener(this); 
     b7.addActionListener(this); 
     b6.addActionListener(this); 
     b5.addActionListener(this); 
     b4.addActionListener(this); 
     b3.addActionListener(this); 
     b2.addActionListener(this); 
     b1.addActionListener(this); 
     b0.addActionListener(this); 
     bdec.addActionListener(this); 
     bc.addActionListener(this); 




     addActionListeners();


    } // close 


    public void actionPerformed (ActionEvent e) {
         String clicked =e.getActionCommand();
         if (clicked.equals("ba")) {
             screen.setText("+");
         }

    }   



} /*Class*/

Answer 1

Move the declarations of the variables for all your GUI components out of the init() method, but still within the Calculator class, so they they will be visible to other methods (like actionPerformed ) that might need to access them:

public class Calculator extends DialogProgram implements ActionListener{ 

    JButton b0,b1,b2,b3,b4,b5,b6,b7,b8,b9,bm,bd,ba,bs,bdec,bc;
    DoubleField screen;
    JPanel P, P1, P2;

    public void init() { 
        P= new JPanel();     // main panel 
        P1= new JPanel();    // panel for buttons 
        P2= new JPanel();    // panel for screen
        // continue setting up the rest of the GUI...
    }

    public void actionPerformed (ActionEvent e) {
         String clicked = e.getActionCommand();
         if (clicked.equals("ba")) {
             screen.setText("+");
         }
    }   
}