Nick Rossetti Nick Rossetti - 29 days ago 11
Java Question

How to draw in jPanel? (swing/graphics Java)

I'm working on a project in which I am trying to make a paint program.
So far I've used netbeans to create a GUI and set up the program.

As of right now I am able to call all the coordinated necessary to draw inside it but I am very confused with how to actually paint inside it.

Towards the end of my code I have a failed attempt at drawing inside the panel.

Can anyone explain/show how to use graphics in a example like this?

All examples I have found make a class and extend it with

JPanel
but I don't know if I can do this since it was generated in netbeans.

I need to draw inside a
JPanel
, inside my
JFrame
. I don't know where to put the graphics class.


JavaPaintUI Class



package javapaint;

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

public class JavaPaintUI extends javax.swing.JFrame {

public JavaPaintUI() {
initComponents();
}


private void initComponents() {


jPanel2 = new javax.swing.JPanel();

jPanel2.setBackground(new java.awt.Color(255, 255, 255));
jPanel2.setBorder(javax.swing.BorderFactory.createBevelBorder(javax.swing.border.BevelBorder.RAISED));
jPanel2.addMouseListener(new java.awt.event.MouseAdapter() {
public void mousePressed(java.awt.event.MouseEvent evt) {
jPanel2MousePressed(evt);
}
public void mouseReleased(java.awt.event.MouseEvent evt) {
jPanel2MouseReleased(evt);
}
});
jPanel2.addMouseMotionListener(new java.awt.event.MouseMotionAdapter() {
public void mouseDragged(java.awt.event.MouseEvent evt) {
jPanel2MouseDragged(evt);
}
});
pack();
}// </editor-fold>

int currentX, currentY, oldX, oldY;

private void jPanel2MouseDragged(java.awt.event.MouseEvent evt) {
if (tool == 1) {
currentX = evt.getX();
currentY = evt.getY();
oldX = currentX;
oldY = currentY;
System.out.println(currentX + " " + currentY);
System.out.println("PEN!!!!");
}

}

private void jPanel2MousePressed(java.awt.event.MouseEvent evt) {
oldX = evt.getX();
oldY = evt.getY();
System.out.println(oldX + " " + oldY);
}


//mouse released//
private void jPanel2MouseReleased(java.awt.event.MouseEvent evt) {
if (tool == 2) {
currentX = evt.getX();
currentY = evt.getY();
System.out.println("line!!!! from" + oldX + "to" + currentX);
}
}

//set ui visible//
public static void main(String args[]) {
java.awt.EventQueue.invokeLater(new Runnable() {

public void run() {
new JavaPaintUI().setVisible(true);
}
});
}

// Variables declaration - do not modify
private javax.swing.JPanel jPanel2;
// End of variables declaration

class jPanel2 extends JPanel {

@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);

g.drawString("BLAH", 20, 20);
g.drawRect(200, 200, 200, 200);
}
}
}


Screen shot



The whole thing is a
JFrame
and the white section in the center is
jPanel2
which is what I want to draw on.
screen shot of some code that is not this

Answer

Note the extra comments.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;

class JavaPaintUI extends JFrame {

    private int tool = 1;
    int currentX, currentY, oldX, oldY;

    public JavaPaintUI() {
        initComponents();
    }

    private void initComponents() {
        // we want a custom Panel2, not a generic JPanel!
        jPanel2 = new Panel2();

        jPanel2.setBackground(new java.awt.Color(255, 255, 255));
        jPanel2.setBorder(BorderFactory.createBevelBorder(BevelBorder.RAISED));
        jPanel2.addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent evt) {
                jPanel2MousePressed(evt);
            }
            public void mouseReleased(MouseEvent evt) {
                jPanel2MouseReleased(evt);
            }
        });
        jPanel2.addMouseMotionListener(new MouseMotionAdapter() {
            public void mouseDragged(MouseEvent evt) {
                jPanel2MouseDragged(evt);
            }
        });

        // add the component to the frame to see it!
        this.setContentPane(jPanel2);
        // be nice to testers..
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        pack();
    }// </editor-fold>

    private void jPanel2MouseDragged(MouseEvent evt) {
        if (tool == 1) {
            currentX = evt.getX();
            currentY = evt.getY();
            oldX = currentX;
            oldY = currentY;
            System.out.println(currentX + " " + currentY);
            System.out.println("PEN!!!!");
        }
    }

    private void jPanel2MousePressed(MouseEvent evt) {
        oldX = evt.getX();
        oldY = evt.getY();
        System.out.println(oldX + " " + oldY);
    }


    //mouse released//
    private void jPanel2MouseReleased(MouseEvent evt) {
        if (tool == 2) {
            currentX = evt.getX();
            currentY = evt.getY();
            System.out.println("line!!!! from" + oldX + "to" + currentX);
        }
    }

    //set ui visible//
    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                new JavaPaintUI().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    private JPanel jPanel2;
    // End of variables declaration

    // This class name is very confusing, since it is also used as the
    // name of an attribute!
    //class jPanel2 extends JPanel {
    class Panel2 extends JPanel {

        Panel2() {
            // set a preferred size for the custom panel.
            setPreferredSize(new Dimension(420,420));
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);

            g.drawString("BLAH", 20, 20);
            g.drawRect(200, 200, 200, 200);
        }
    }
}

Screen Shot

enter image description here

Other examples - more tailored to multiple lines & multiple line segments

HFOE put a good link as the first comment on this thread. Camickr also has a description of active painting vs. drawing to a BufferedImage in the Custom Painting Approaches article.

See also this approach using painting in a BufferedImage.

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