使用动作启动的 for 循环不工作（重新绘制）更新 JFrame 上的 JLabel

Question

import java.awt.Color;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.IOException;
import javax.swing.*;

public class show{
    
    static JFrame frame = new JFrame();
    static JPanel panel = new JPanel();
    
    public static void main(String[] args) throws IOException, InterruptedException {
        
                //setup Frame
        frame.setVisible(true);
        frame.setSize(1500, 500);
        frame.setLayout(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.getContentPane().setBackground(new Color(0,120,240));
        frame.addKeyListener(new KeyListener() {

            @Override
            public void keyPressed(KeyEvent e) {
                
                if(e.getKeyCode() == KeyEvent.VK_SPACE) {
                    try {
                        move();
                    } catch (InterruptedException e1) {
                        e1.printStackTrace();
                    }
                }
                
            }

            @Override
            public void keyTyped(KeyEvent e) {
                // TODO Auto-generated method stub
                
            }

            @Override
            public void keyReleased(KeyEvent e) {
                // TODO Auto-generated method stub
                
            }
            
        });
        
        //setup Panel
        panel.setVisible(true);
        panel.setLocation(150,150);
        panel.setSize(150, 50);
        panel.setBackground(new Color(0,0,0));
        frame.add(panel);
        
        frame.repaint();
    }
    
    public static void move() throws InterruptedException {
        for(int i=0;i<20;i++) {
            panel.setLocation(panel.getLocation().x+20,panel.getLocation().y+20);
            Thread.sleep(100);
            frame.repaint();
        }
    }
    
}

Hi guys,

when I try to update a JLabel/JButton/JPanel using a action-started (ex. key-press) for-loop, it first updates the frame after the for loop is over, even though I update the frame in the for-loop with frame.repaint()...

I couldn't find any posts to this problem...

What can I do?

pls help!

Answer 1

As mentioned in comments, you will need aSwing Timer to carry out the task. Right now, the for loop within the move() method is holding up the EDT and you won't see the panel move. You will only see the final panel position once the EDT is free and can update the GUI.

In order to get your code working I would suggest the following:

Get rid of the move() method and its for loop. Instead, utilize a counter within a Swing Timer's ActionPerformed event to carry out the similar thing. Below is your code quickly modified to work with a Swing Timer. Read the comments in code:

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.IOException;
import javax.swing.*;

public class show{
    static JFrame frame = new JFrame();
    static JPanel panel = new JPanel();
    static Timer animationTimer;       // Swing Timer.
    static int animationSpeed = 100;
    static int counter; 
    
    
    public static void main(String[] args) throws IOException, InterruptedException {
        frame.setVisible(true);
        frame.setSize(1500, 500);
        frame.setLayout(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.getContentPane().setBackground(new Color(0, 120, 240));
        
        frame.addKeyListener(new KeyListener() {
            @Override
            public void keyPressed(KeyEvent evt) {
                if(evt.getKeyCode() == KeyEvent.VK_SPACE) {
                    /* Set the panel for location for another restart.
                       Every time the space-Bar is hit the Panel will
                       reset and move down again.                 */
                    panel.setLocation(150,150); 
                    
                    // Counter instead of `for` loop:
                    counter = 0;
                    
                    /* Start the Swing Timer. The real work is done within
                       the actionPerformed event of the timer
                    */
                    animationTimer.start();
                }
            }

            @Override
            public void keyTyped(KeyEvent e) {
                // TODO Auto-generated method stub
            }

            @Override
            public void keyReleased(KeyEvent e) {
                // TODO Auto-generated method stub
            }
            
        });
        
        //setup Panel
        panel.setVisible(true);
        panel.setLocation(150,150);
        panel.setSize(150, 50);
        panel.setBackground(new Color(0,0,0));
        frame.add(panel);
        
        
        // The ActionListern for the Swing Timer ... Used for Animation of Panel.
        ActionListener performTask = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                panel.setLocation(panel.getLocation().x + 20, panel.getLocation().y + 20);
                
                // If the counter reaches 20 then stop the Swing Timer.
                if (counter == 20) {
                    animationTimer.stop();
                }
                /* Incremant the counter by 1 upon each timer cycle. 
                   The Timer Delay is set with the animationSpeed 
                   member variable which is currently set to 100 
                   milliseconds. So, every100 milliseconds the 
                   counter is incremented and the panel is moved
                   upon the next cycle.                */
                counter++;
            }
        };
        
        /* The swing timer delay time (100 milliseconds) and the ActionListener
           (performTask) which contains the ActionPerformed event that will be 
           fired on each timer cycle. The Timer will continuously cycle every 
           100 milliseconds until told to Stop.        */
        animationTimer = new Timer(animationSpeed, performTask);
    }
}

---

Here is another version of your code which allows you to move the panel in different directions within the Window. The panel is always in continuous motion unless you select the Keyboard Number Pad 5 button which pauses the animation or the ESCAPE key which stops the animation altogether. The Space Bar restarts the animation after it has been stopped. Other Number Pad keys and cursor keys move the panel in specific directions. If you just let the panel float around, when it hits a window edge, it will randomly change to any one of three opposing directions. The Number Pad + and - keys increases and decreases the movement increments of the Panel and the Keyboard = and - keys will increase and decrease the Swing Timer Delay Rate (in essance, the Animation Speed).

Here is the example code:

import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.Arrays;
import java.util.concurrent.ThreadLocalRandom;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.Timer;


public class Show {

    JFrame frame = new JFrame();
    JPanel panel = new JPanel();
    JLabel animationSpeedLbl;
    JLabel label;
    Timer animationTimer;
    
    int animationSpeed = 5, increment = 1;
    int curX = 0, curY = 0, key = 0;
    java.util.List<Integer> allowablekeys = Arrays.asList(
            KeyEvent.VK_SPACE, KeyEvent.VK_ESCAPE, KeyEvent.VK_UP, KeyEvent.VK_DOWN,
            KeyEvent.VK_LEFT, KeyEvent.VK_RIGHT, KeyEvent.VK_NUMPAD7, KeyEvent.VK_NUMPAD8,
            KeyEvent.VK_NUMPAD9, KeyEvent.VK_NUMPAD4, KeyEvent.VK_NUMPAD5, KeyEvent.VK_NUMPAD6,
            KeyEvent.VK_NUMPAD1, KeyEvent.VK_NUMPAD2, KeyEvent.VK_NUMPAD3, KeyEvent.VK_ADD,
            KeyEvent.VK_SUBTRACT, KeyEvent.VK_EQUALS, KeyEvent.VK_MINUS);

    public static void main(String[] args) {
        new Show().startApp(args);
    }

    private void startApp(String[] args) {
        //setup Frame
        frame.setLayout(new FlowLayout());
        frame.setPreferredSize(new java.awt.Dimension(800, 700));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.getContentPane().setBackground(new Color(0, 120, 240));
        frame.addKeyListener(new KeyListener() {
            @Override
            public void keyPressed(KeyEvent e) {
                int curKey = -1;
                if (animationTimer.isRunning()) {
                    curKey = key;
                }
                int keyNum = e.getKeyCode();

                if (!allowablekeys.contains(keyNum)) {
                    return;
                }
                key = keyNum;
                System.out.println(key);

                switch (key) {
                    case KeyEvent.VK_SPACE:
                        if (!animationTimer.isRunning()) {
                            key = KeyEvent.VK_DOWN;
                            label.setText("<html><center><font size='2' color=#DCDCDC>"
                                    + "Press Numpad Keys To<br>Change Direction</html>");
                            animationTimer.start();
                        } break;
                    case KeyEvent.VK_ESCAPE:
                        if (animationTimer.isRunning()) {
                            animationTimer.stop();
                        } break;
                        
                    case KeyEvent.VK_ADD:
                        if (animationTimer.isRunning()) {
                            increment++;
                            if (increment > 50) {
                                // Max allowable is an increment of 50. You can change this.
                                increment = 50;
                            }
                            key = curKey;
                        } break;
                        
                    case KeyEvent.VK_SUBTRACT:
                        if (animationTimer.isRunning()) {
                            increment--;
                            if (increment < 0) {
                                // Min allowable is an increment of 0. You can change this.
                                increment = 0;
                            }
                            key = curKey;
                            key = curKey;
                        } break;
                        
                    case KeyEvent.VK_EQUALS:
                        if (animationTimer.isRunning()) {
                            animationSpeed++;
                            animationTimer.setDelay(animationSpeed);
                            if (animationSpeed > 1000) {
                                // Max allowable is an increment of 50. You can change this.
                                animationSpeed = 1000;
                            }
                            key = curKey;
                        } break;
                    
                    case KeyEvent.VK_MINUS:
                        if (animationTimer.isRunning()) {
                            animationSpeed--;
                            if (animationSpeed < 0) {
                                /* Min allowable is an Animation Speed of 0. You can not 
                                   go less than 0. Since the value is a delay time ( in 
                                   milliseconds), the lower the number, the faster the
                                   animation.                               */
                                animationSpeed = 0;
                            }
                            animationTimer.setDelay(animationSpeed);
                            key = curKey;
                        } break;
                    default:
                        break;
                }
                frame.setTitle("Animation Speed: " + animationSpeed  + "  (Keyboard =/- to change)"
                             + "  |  Increment Level: " + increment + "  (Number Pad +/- to change)");
            }

            @Override
            public void keyTyped(KeyEvent e) {
                // Do nothing

            }

            @Override
            public void keyReleased(KeyEvent e) {
                // Do Nothing
            }
        });
        
        //setup Panel
        panel.setPreferredSize(new java.awt.Dimension(150, 50));
        panel.setBackground(new Color(0, 0, 0));
        
        label = new JLabel("<html><center><font size='2' color=#DCDCDC>"
                         + "Press Space-Bar To<br>Start Animation</html>");
        label.setForeground(Color.white);
        panel.add(label);
        frame.add(panel);
        frame.pack();

        // Using a Swing Timer...for animation
        ActionListener performTask = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                // Random directional Options:
                int[] upOptions = {KeyEvent.VK_NUMPAD7, KeyEvent.VK_NUMPAD8, KeyEvent.VK_NUMPAD9};
                int[] downOptions = {KeyEvent.VK_NUMPAD1, KeyEvent.VK_NUMPAD2, KeyEvent.VK_NUMPAD3};
                int[] leftOptions = {KeyEvent.VK_NUMPAD7, KeyEvent.VK_NUMPAD4, KeyEvent.VK_NUMPAD1};
                int[] rightOptions = {KeyEvent.VK_NUMPAD9, KeyEvent.VK_NUMPAD6, KeyEvent.VK_NUMPAD3};
                switch (key) {
                    case KeyEvent.VK_NUMPAD7:
                        curX = panel.getLocation().x - increment;
                        curY = panel.getLocation().y - increment;
                        break;
                    case KeyEvent.VK_NUMPAD8:
                    case KeyEvent.VK_UP:
                        curX = panel.getLocation().x;
                        curY = panel.getLocation().y - increment;
                        break;
                    case KeyEvent.VK_NUMPAD9:
                        curX = panel.getLocation().x + increment;
                        curY = panel.getLocation().y - increment;
                        break;
                    case KeyEvent.VK_NUMPAD2:
                    case KeyEvent.VK_DOWN:
                        curX = panel.getLocation().x;
                        curY = panel.getLocation().y + increment;
                        break;
                    case KeyEvent.VK_NUMPAD1:
                        curX = panel.getLocation().x - increment;
                        curY = panel.getLocation().y + increment;
                        break;
                    case KeyEvent.VK_NUMPAD4:
                    case KeyEvent.VK_LEFT:
                        curX = panel.getLocation().x - increment;
                        curY = panel.getLocation().y;
                        break;
                    case KeyEvent.VK_NUMPAD6:
                    case KeyEvent.VK_RIGHT:
                        curX = panel.getLocation().x + increment;
                        curY = panel.getLocation().y;
                        break;
                    case KeyEvent.VK_NUMPAD3:
                        curX = panel.getLocation().x + increment;
                        curY = panel.getLocation().y + increment;
                        break;
                    default:
                        return;
                }

                if (panel.getLocation().x <= 0 + increment) {
                    key = rightOptions[ThreadLocalRandom.current().nextInt(0, rightOptions.length)];
                    switch (key) {
                        case KeyEvent.VK_NUMPAD6:
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y;
                            break;
                        case KeyEvent.VK_NUMPAD9:
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y + increment;
                            break;
                        case KeyEvent.VK_NUMPAD3:
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y - increment;
                            break;
                        default:
                            key = KeyEvent.VK_NUMPAD6;
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y;
                            break;
                    }
                }

                else if (panel.getLocation().x >= (frame.getContentPane().getWidth() - panel.getWidth() - increment)) {
                    key = leftOptions[ThreadLocalRandom.current().nextInt(0, leftOptions.length)];
                    switch (key) {
                        case KeyEvent.VK_NUMPAD4:
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y;
                            break;
                        case KeyEvent.VK_NUMPAD7:
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y - increment;
                            break;
                        case KeyEvent.VK_NUMPAD1:
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y + increment;
                            break;
                        default:
                            key = KeyEvent.VK_NUMPAD4;
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y;
                            break;
                    }
                }
                else if (panel.getLocation().y <= 0) {
                    key = downOptions[ThreadLocalRandom.current().nextInt(0, downOptions.length)];
                    switch (key) {
                        case KeyEvent.VK_NUMPAD2:
                            curX = panel.getLocation().x;
                            curY = panel.getLocation().y + increment;
                            break;
                        case KeyEvent.VK_NUMPAD1:
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y + increment;
                            break;
                        case KeyEvent.VK_NUMPAD3:
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y + increment;
                            break;
                        default:
                            key = KeyEvent.VK_NUMPAD2;
                            curX = panel.getLocation().x;
                            curY = panel.getLocation().y + increment;
                            break;
                    }
                }
                else if (panel.getLocation().y >= (frame.getContentPane().getHeight() - panel.getHeight() - increment)) {
                    key = upOptions[ThreadLocalRandom.current().nextInt(0, upOptions.length)];
                    switch (key) {
                        case KeyEvent.VK_NUMPAD8:
                            curX = panel.getLocation().x;
                            curY = panel.getLocation().y - increment;
                            break;
                        case KeyEvent.VK_NUMPAD7:
                            curX = panel.getLocation().x - increment;
                            curY = panel.getLocation().y - increment;
                            break;
                        case KeyEvent.VK_NUMPAD9:
                            curX = panel.getLocation().x + increment;
                            curY = panel.getLocation().y - increment;
                            break;
                        default:
                            key = KeyEvent.VK_NUMPAD8;
                            curX = panel.getLocation().x;
                            curY = panel.getLocation().y - increment;
                            break;
                    }
                }
                panel.setLocation(curX, curY);
            }
        };
        animationTimer = new Timer(animationSpeed, performTask);
        
        java.awt.EventQueue.invokeLater(() -> {
            frame.setVisible(true);
            frame.setLocationRelativeTo(null);
        });
    }
}