drawLine JAVA不会停留在MAC OS X上

Question

First this problem is effective on os x only i don't know why, if anyone can tell me...(he works fine on linux and windows)

When i'm clicking to draw, the old points disappear instead of stay. If you remove the commentary on the super.paintComponent , the result is same on osx but on window and linux this is different.

Click to draw.

import java.awt.Color;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Stack extends JPanel {
    JFrame jf;
    Panneau jp;
    Point p;

    public Stack() {
        p = new Point();
        jf = new JFrame("Window");
        jp = new Panneau();
        jp.setBackground(Color.white);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(800,600);
        jf.setLayout(new GridLayout());
        jf.add(jp);
        jf.setVisible(true);

        jp.addMouseListener(new MouseAdapter(){
            public void mouseClicked(MouseEvent e){
               setPoint(p);
               jp.repaint();
            }
        });
    }

    public void setPoint(Point p) {
        this.p.x += 10;
        this.p.y += 10;
    }

    public class Panneau extends JPanel {
        public void paintComponent(Graphics g) {
            // super.paintComponent(g);
            g.drawLine(p.x, p.y, p.x+5, p.y+5);
        }
    }

    public static void main (String []args) {
        Stack s = new Stack();
    }
}

Answer 1

This happens because youre not re-drawing the areas where you previous points are located, apparently on mac, it does redraw that area for some reason?

But the point is, you shouldnt rely on drawing more points by not drawing over a previous area, and you should always call super.paintComponent(g). I recommend you create a list of points, and then draw all these points when you repaint.

I've taken the liberty of creating this code myself, I hope you understand what im doing here:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Stack extends JPanel {
    JFrame jf;
    Panneau jp;
    //List of points instead of one point
    List<Point> points;

    public Stack() {
        //Instantiating the list
        points = new ArrayList<Point>();
        jf = new JFrame("Window");
        jp = new Panneau();
        jp.setBackground(Color.white);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(800,600);
        jf.setLayout(new GridLayout());
        jf.add(jp);
        jf.setVisible(true);

        jp.addMouseListener(new MouseAdapter(){
            public void mouseClicked(MouseEvent e){
                //Adding a new point to the list
                addPoint();
                jp.repaint();
            }
        });
    }

    public void addPoint() {
        if(points.isEmpty()){
            //If this is the first point to be added, set it to 0,0
            points.add(new Point(0, 0));
        }else{
            //Get the last point currently in the list
            Point lastPoint = points.get(points.size()-1);
            //Create the newpoint, 10px right and 10px down from the current point
            Point newPoint = new Point(lastPoint.x + 10, lastPoint.y + 10);
            //Add the new point to the list
            points.add(newPoint);
        }
    }

    public class Panneau extends JPanel {
        public void paintComponent(Graphics g) {
            //Make sure the background is drawn! Should always be called
            super.paintComponent(g);
            //Iterate over all the points and draw them all
            for(Point p : points){
                g.drawLine(p.x, p.y, p.x + 5, p.y + 5);
            }
        }
    }

    public static void main (String []args) {
        Stack s = new Stack();
    }
}