Java AWT如何延迟绘制对象

Question

I would like to draw a new random shape every 2 seconds.

I already have a window, that shows immediately some shapes. I tried to mess around with Timer to make new things appear in the window after a few seconds, but it didn't work, or the whole program freezes. Is it a good idea to use Timer? How should I implement it, to make it work?

import javax.swing.*;
import java.awt.*;
import java.util.Random;

class Window extends JFrame {

    Random rand = new Random();
    int x = rand.nextInt(1024);
    int y = rand.nextInt(768);
    int shape = rand.nextInt(2);

    Window(){
        setSize(1024,768);
        setVisible(true);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    }

    public void paint(Graphics g) {
        super.paint(g);
        g.setColor(new Color(0, 52, 255));
        switch(shape) {
            case 0:
                g.fillOval(x, y, 50, 50);
                break;
            case 1:
                g.fillRect(x,y,100,100);
                break;
        }
        repaint();
    }
}

public class Main {
    public static void main(String[] args) {
        Window window = new Window();
    }
}

I would also like to draw some random shapes. Is it ok, to use switch in paint method for this purpose? I would make a random variable, if it's 1 it would paint rectangle, if it's 2 it would paint oval etc.

Answer 1

First, don't subclass JFrame; subclass JPanel instead, and place that panel in a JFrame. Second, don't override paint() - override paintComponent() instead. Third, create a Swing Timer, and in its actionPerformed() method make the changes you want and then call yourPanel.repaint()

Answer 2

First of all, do not change the way JFrame gets painted (with other words, do not override paintComponent() of a JFrame ). Create an extending class of JPanel and paint the JPanel instead. Secondly, do not override paint() method. Override paintComponent() . Thirdly, always run Swing applications with SwingUtilities.invokeLater() since they should run in their own thread named EDT (Event dispatch thread). Finally, javax.swing.Timer is what you are looking for.

Take a look at this example. It paints an oval shape in random X,Y every 1500ms.

Preview:

Source code:

import java.awt.BorderLayout;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class DrawShapes extends JFrame {
    private ShapePanel shape;

    public DrawShapes() {
        super("Random shapes");
        getContentPane().setLayout(new BorderLayout());
        getContentPane().add(shape = new ShapePanel(), BorderLayout.CENTER);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(500, 500);
        setLocationRelativeTo(null);

        initTimer();
    }

    private void initTimer() {
        Timer t = new Timer(1500, e -> {
            shape.randomizeXY();
            shape.repaint();
        });
        t.start();
    }

    public static class ShapePanel extends JPanel {
        private int x, y;

        public ShapePanel() {
            randomizeXY();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.fillOval(x, y, 10, 10);
        }

        public void randomizeXY() {
            x = (int) (Math.random() * 500);
            y = (int) (Math.random() * 500);
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new DrawShapes().setVisible(true));
    }
}