Java Awt和Swing

Question

Pong.Java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;

public class Pong {
    private static final int WIDTH = 900, HEIGHT = 700;
    JFrame win = new JFrame();
    Paddle paddleOne = new Paddle(1);
    Paddle paddleTwo = new Paddle(2);
    Graphics g;

    Pong(){
        init();
    }

    void init(){
        win.setTitle("PONG");
        win.setSize(WIDTH, HEIGHT);
        win.addKeyListener(keyListener);
        win.setLocationRelativeTo(null);
        win.getContentPane().setBackground(Color.black);
        win.setVisible(true);
        win.getContentPane().validate();
        win.repaint();
    }


    public void paintComponent(Graphics g){
        g.setColor(Color.white);
        g.fillRect(paddleOne.getX(), paddleOne.getY(), paddleOne.getHEIGHT(), paddleOne.getWIDTH());

        System.out.println("drawn");
    }

    KeyListener keyListener = new KeyListener() {
        public void keyPressed(KeyEvent e) {
            int key = e.getKeyCode();

            switch(key){
            case 87:
                System.out.println("Player 1 Up");
                break;
            case 83:
                System.out.println("Player 1 Down");
                break;
            case 38:
                System.out.println("Player 2 Up");
                break;
            case 40:
                System.out.println("Player 2 Down");
                break;
            }

            win.getContentPane().validate();
            win.repaint();

        }

        public void keyReleased(KeyEvent arg0) {
            // TODO Auto-generated method stub

        }

        public void keyTyped(KeyEvent arg0) {
            // TODO Auto-generated method stub

        }

    };

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

Paddle.Java

public class Paddle{
private int WIDTH = 50, HEIGHT = 150, X, Y;


int playerNum;

Paddle(int playerNum){
    if(playerNum == 1){
        X = 10;
        Y = 10;
    }else if (playerNum == 2){
        X = 500;
        Y = 10;
    }
}

public void setX(int x){
    X = x;
}

public void setY(int y){
    Y = y;
}

public int getWIDTH() {
    return WIDTH;
}

public int getHEIGHT() {
    return HEIGHT;
}

public int getX() {
    return X;
}

public int getY() {
    return Y;
}

}

I'm relatively new to Java programming, or more specifically Awt & Swing, what my question is, why isn't my rectangle drawing? Any help is appreciated. Thank you so much!

Answer 1

You need to overriding paintComponents to draw. Here is your Pong.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class Pong {
    private static final int WIDTH = 900, HEIGHT = 700;
    JFrame win = new JFrame();
    Paddle paddleOne = new Paddle(1);
    Paddle paddleTwo = new Paddle(2);
    Graphics g;

    Pong() {
        init();
    }

    void init() {
        win.setTitle("PONG");
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        win.setSize(WIDTH, HEIGHT);
        win.addKeyListener(keyListener);
        win.setLocationRelativeTo(null);
        win.add(new Panel(paddleOne));
        win.getContentPane().setBackground(Color.black);
        win.setVisible(true);
        win.getContentPane().validate();
        win.repaint();
    }

    KeyListener keyListener = new KeyListener() {
        public void keyPressed(KeyEvent e) {
            int key = e.getKeyCode();

            switch (key) {
            case 87:
                System.out.println("Player 1 Up");
                break;
            case 83:
                System.out.println("Player 1 Down");
                break;
            case 38:
                System.out.println("Player 2 Up");
                break;
            case 40:
                System.out.println("Player 2 Down");
                break;
            }

            win.getContentPane().validate();
            win.repaint();

        }

        public void keyReleased(KeyEvent arg0) {
            // TODO Auto-generated method stub

        }

        public void keyTyped(KeyEvent arg0) {
            // TODO Auto-generated method stub

        }

    };

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Pong();

            }
        });
    }
}

Here Panel.java in where paintComponent overridden.

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

public class Panel extends JPanel{
    private Paddle paddleOne;
    public Panel(Paddle pdl) {
        paddleOne = pdl;
    }

    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        g.setColor(Color.white);
        g.fillRect(paddleOne.getX(), paddleOne.getY(), paddleOne.getHEIGHT(), paddleOne.getWIDTH());

        //System.out.println("drawn");   //Should not put something here which may overhead.
    }

} 

Answer 2

In order for something to be painted within in Swing, it first must extend from something Swing know's how to paint, this commonly means a JComponent (or more typically a JPanel ).

Then you can override one of the paint methods, which is called by the painting subsystem, in this case, it's generally preferred to override paintComponent , but don't forget to call super.paintComponent before you do any custom painting, or you're setting yourself up for some weird and generally unpredictable painting issues.

Take a look at Painting in AWT and Swing and Performing Custom Painting for more details.

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Pongo {

    public static void main(String[] args) {
        new Pongo();
    }

    public Pongo() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new PongPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class PongPane extends JPanel {

        private static final int WIDTH = 900, HEIGHT = 700;
        Paddle paddleOne = new Paddle(1);
        Paddle paddleTwo = new Paddle(2);

        public PongPane() {
            setBackground(Color.BLACK);
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.white);
            g.fillRect(paddleOne.getX(), paddleOne.getY(), paddleOne.getHEIGHT(), paddleOne.getWIDTH());

            //System.out.println("drawn");   //Should not put something here which may overhead.
        }

    }

    public class Paddle {

        private int WIDTH = 50, HEIGHT = 150, X, Y;

        int playerNum;

        Paddle(int playerNum) {
            if (playerNum == 1) {
                X = 10;
                Y = 10;
            } else if (playerNum == 2) {
                X = 500;
                Y = 10;
            }
        }

        public void setX(int x) {
            X = x;
        }

        public void setY(int y) {
            Y = y;
        }

        public int getWIDTH() {
            return WIDTH;
        }

        public int getHEIGHT() {
            return HEIGHT;
        }

        public int getX() {
            return X;
        }

        public int getY() {
            return Y;
        }
    }
}

I would also discourage the use of KeyListener within this context and inside advice the use of the key bindings API, it doesn't suffer from the same focus related issues that KeyListener does. See How to Use Key Bindings for more details