mousePressed方法将不响应

Question

I'm trying to do something very basic: write program that will draw a line on a frame between two points: the point that the mouse was pressed on and the point where the mouse was released on.

I have these classes:

import java.awt.Graphics;

public class Line implements Drawable{
    private int x1,x2,y1,y2;
    public Line( int x1,int x2,int y1,int y2){
        this.x1=x1;
        this.x2=x2;
        this.y1=y1;
        this.y2=y2;
    }
    public void draw(Graphics g){
        g.drawLine(x1, y1, x2, y2);
    }
}

---

import java.awt.Graphics;

public interface Drawable {
    public void draw(Graphics g);
}

---

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

import javax.swing.JPanel;

@SuppressWarnings("serial")
public class LinePanel extends JPanel {
    ArrayList<Line> lines = new ArrayList<Line>();

    public LinePanel() {
        addMouseListener(new MouseAdapter() {
            Point p1, p2;

            @Override
            public void mousePressed(MouseEvent e) {
                p1 = e.getPoint();
                System.out.println("pressed");
            }

            @Override
            public void mouseClicked(MouseEvent e) {
                // TODO Auto-generated method stub
                System.out.println("clicked");

            }
            @Override
            public void mouseReleased(MouseEvent e) {
                p2 = e.getPoint();
                lines.add(new Line(p1.x, p1.y, p2.x, p2.y));
            }

        });
        System.out.println("after add mouselistener");
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (Line l : lines) {
            l.draw(g);
        }
    }
}

---

import javax.swing.JFrame;

public class LaunchLinesGui {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        JFrame frame=new JFrame();
        frame.setSize(500, 500);
        frame.setVisible(true);

        LinePanel panel=new LinePanel();
        frame.add(panel);
    }

}

The problem is that when I press themouse on any place on the frame I get no response and even the System.out.println 's of mousePressed methods are not printed on the console. What am I doing wrong?

Answer 1

The problem was that I didn't set the size of the panel and the clicks inside the frame were not caught inside the panel .

The fix was to add setSize(500, 500); in LinePanel constructor.

Answer 2

It is the job of the layout manager to determine the size/location of the components added to a frame.

The reason your panel has a size of (0, 0) is because you add the panel to the frame AFTER you make the frame visible so the layout manager is never invoked.

Also, you should not set the size of the frame you should let the layout manager do its job by invoking the pack() method on the frame.

The proper order of your code should be:

    LinePanel panel=new LinePanel();

    JFrame frame=new JFrame();
    frame.add(panel);
    //frame.setSize(500, 500);
    frame.pack();
    frame.setVisible(true);

Also when you do custom painting the panel is responsible for determining its own size, so you also need to override the getPreferredSize() method of the panel:

@Override
public Dimension getPreferredSize()
{
    return new Dimension(500, 500);
}

Now the layout manager can do its job properly.