除非最小化或调整JFrame的大小，否则我不能将其称为KeyEvent事件

Question

i have 3 classes inside a class. the main class will call one of the 3 classes, depending on the input of the user, if it is B, R or G. B for Blue class, r for red and g for green.

after i call a class, lets say Blue class, the blue will be added to the container and the class that calls this, a panel class will be removed from the container.

when i press the button "exit", the blue class will be removed and the panel class will be added to the container again.

my problem is, after i've done this scenario above, i can't call another class again, by pressing B, R or G. i must minimize or resize first my jframe to call a class. again

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class practice implements KeyListener{
JFrame frame = new JFrame("practice");
JLabel label = new JLabel("Press B, R, or G");
JPanel panel = new JPanel();

public class BLUE implements ActionListener{
    JPanel bluePanel = new JPanel();
    JButton exit = new JButton("exit");
    public BLUE(){
        bluePanel.setPreferredSize(new Dimension(300, 300));
        bluePanel.setFocusable(true);
        bluePanel.setVisible(true);
        bluePanel.requestFocus();
        bluePanel.setBackground(Color.blue);
        bluePanel.add(exit);
        exit.addActionListener(this);
        frame.remove(panel);
        frame.add(bluePanel);
        frame.revalidate();
        frame.repaint();
    }   

    public void actionPerformed(ActionEvent e){
        if(e.getSource().equals(exit)){
            frame.remove(bluePanel);
            frame.add(panel);
            frame.revalidate();
            frame.repaint();
        }
    }           
}


public class RED extends JPanel implements ActionListener{
    JPanel redPanel = new JPanel();
    JButton exit = new JButton("exit");
    public RED(){
        redPanel.setPreferredSize(new Dimension(300, 300));
        redPanel.setFocusable(true);
        redPanel.setVisible(true);
        redPanel.requestFocus();
        redPanel.setBackground(Color.red);
        redPanel.add(exit);
        exit.addActionListener(this);
        frame.remove(panel);
        frame.add(redPanel);
        frame.revalidate();
        frame.repaint();
    }

    public void actionPerformed(ActionEvent e){
        if(e.getSource().equals(exit)){
            frame.remove(redPanel);
            frame.add(panel);
            frame.revalidate();
            frame.repaint();
        }
    }           
}

public class GREEN extends JPanel implements ActionListener{
    JPanel greenPanel = new JPanel();
    JButton exit = new JButton("exit");
    public GREEN(){
        greenPanel.setPreferredSize(new Dimension(300, 300));
        greenPanel.setFocusable(true);
        greenPanel.setVisible(true);
        greenPanel.requestFocus();
        greenPanel.setBackground(Color.green);
        greenPanel.add(exit);
        exit.addActionListener(this);
        frame.remove(panel);
        frame.add(greenPanel);
        frame.revalidate();
        frame.repaint();
    }   

    public void actionPerformed(ActionEvent e){
        if(e.getSource().equals(exit)){
            frame.remove(greenPanel);
            frame.add(panel);
            frame.revalidate();
            frame.repaint();
        }
    }           

}

public practice(){  
    frame.setVisible(true);
    frame.setPreferredSize(new Dimension(300, 300));
    frame.setDefaultCloseOperation(3);
    frame.setResizable(false);
    frame.pack();
    frame.setLocationRelativeTo(null);

    initUI();
}

public void initUI(){
    frame.addKeyListener(this);
    panel.add(label);
    panel.setBackground(Color.black);
    frame.add(panel);
}

public void keyReleased(KeyEvent e){
}

public void keyTyped(KeyEvent e){   
}

public void keyPressed(KeyEvent e){
    if(e.getKeyCode() == KeyEvent.VK_B){
        BLUE blue = new BLUE();
    }
    if(e.getKeyCode() == KeyEvent.VK_R){
        RED red = new RED();
    }
    if(e.getKeyCode() == KeyEvent.VK_G){
        GREEN green = new GREEN();
    }
}

public static void main(String[] args){
    practice p = new practice();
}
}

Answer 1

I think the problem is that the focus is given to the panel after the call, and the JFrame then can't listen for input. You want to requestFocus() from the panel after the call is made.

Answer 2

Your jframe (where you added the keyListener) lost focus as you request focus on the jpanels which do not have a keyListener. Use requestFocus() on the jframe after every change of jpanels, i cant test it myself atm though so not completely sure .

Answer 3

Not 100% sure but I think your problem lies with what component is in focus. Once you click the exit button the focus shifts to the exit button and even though it's removed later focus is never given back to the original frame. At the end of your exit button code add:

frame.requstFocus();

Yup, checked and this works.