如何让一个类创建自己的Graphics实例

Question

I have a class called CharMove,in it are the paint(Graphics g) method, and some custom methods. The class should create a square,then move that square randomly around the screen. However,when I create two instances of this class in my World Class,only one square appears. First the square doesn't move but the new coord.'s are displayed,then after 5 runs the square begins to move randomly. I think the program is getting caught on the Graphics method because only one square is being created,when the CharMove class should be creating another instance of Graphics.I have searched online but can't find a way to create different instances of Graphics.Thanks in advance.

CharMove Class

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

public class CharMove extends JPanel {
    int x = 250;
    int y = 250;  
    public void paint(Graphics g) { 
        Graphics pane = (Graphics2D) g;
        pane.setColor(Color.blue);
        pane.fillRect(x, y, 10, 10);  

    }

    public void movement(JFrame frame) { 
        for (int i=0;i<5;i++) {
            try {
                TimeUnit.SECONDS.sleep(1);
                this.x = Getx(this.x,frame); 
                this.y = Gety(this.y,frame);
                frame.repaint();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public int Getx(int a, JFrame frame) { 
        Random rn = new Random();
        int xnum = rn.nextInt(10)-5; 
        a += xnum; 
        System.out.println("x:" + a); 
        return a;
    } 
    public int Gety(int b, JFrame frame){ 
        Random rn = new Random();
        int ynum = rn.nextInt(10)-5; 
        b += ynum; 
        System.out.println("y:" + b); 
        return b;
    } 
}

World Class

import java.awt.*;
import java.util.concurrent.TimeUnit;
import javax.swing.*;
import java.util.Random;
public class World {


    public static void main(String[] args) {  

        JFrame game = new JFrame();
        game.setTitle("Matrix");
        game.setSize(500, 500);;
        game.getContentPane().setBackground(Color.white);
        game.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        game.setVisible(true);  
        CharMove char1 = new CharMove(); 
        CharMove char2 = new CharMove();
        game.add(char1);  
        game.add(char2);
        char1.movement(game);  
        char2.movement(game);
    } 
}

Answer 1

In swing, all of your painting should be down in paintComponent(Graphics g) (rename your method)

To do animation, you should use a Swing Timer (w/ an ActionListener) to update the positions of your animated items. Once that's done, the timer should call repaint();

public void actionPerformed(ActionEvent ae) {
    this.x = Getx(this.x,frame); 
    this.y = Gety(this.y,frame);
    frame.repaint();

}

Answer 2

However,when I create two instances of this class in my World Class,only one square appears.

The default layout manager for a JFrame is a BorderLayout.

    game.add(char1);  
    game.add(char2);

When you add components without specifying a constraint then both components are added to the CENTER. However, only one component can be added to the CENTER so only the last one added is displayed.

Try:

    game.add(char1, BorderLayout.PAGE_START);  
    game.add(char2, BorderLayout.PAGE_END);

However when you do this the componens won't be displayed because they have a (0, 0) preferredSize. So you will also need to override the getPreferredSize() method of your CharMove class.

@Override
public Dimension getPreferredSize()
{
    return new Dimension(300, 200);
}

Also, custom painting should be done in the paintComponent(...) method and you need to invoke super.paintComponent(...) at the start to clear the background.

The repaint() method in your movement() method should be on the panel, not the frame, since you are changing properties of the panel.

Answer 3

Each CharMove is essentially a JPanel which draws a single square of size 10 somewhere on itself when it is painted. You are adding two CharMove panels to the game JFrame (which in fact adds them to the default content pane, which has a subclassed BorderLayout ). As you are not providing a layout constraints object, in fact both panels are being added to the BorderLayout.CENTER of the content pane, and the second is completely covering the first.

To correct this you should modify CharMove so that it paints all of the squares (eg by maintaining an array or some sort of collection of squares, and painting all of them in the paint method) and just add that one panel to the JFrame.

Apart from this issue, while you are animating the squares in the movement method you are blocking the Event Dispatch Thread, meaning that during the animation you won't be able to move any other windows or respond to any mouse clicks or other inputs. Take ControlAltDel's advice about using the Swing Timer for animation to correct this problem.