How to implement Hilbert Curve in JFrame

Question

I am trying to make a project based on Hilbert Curve. I was able to use the code in Applet, but I need it to work in JFrame, because I will be needing to open more than 1 frame at once to present my project. I have the code in applet below, but I don't know how to change to JFrame.

In the applet form, it is like the code below:

import java.awt.*;
import java.applet.*;   

public class HilbertCurve extends Applet{
private SimpleGraphics sg = null;
private int dist0=256;
private int dist=dist0;

public void init(){
    resize(dist0, dist0);
    sg = new SimpleGraphics(getGraphics());}

public void paint(Graphics g){
    int level = 4;
    dist = dist0;
    for(int i=level; i>0; i--) dist/=2;
    sg.goToXY (dist/2, dist/2);
    HilbertU(level);
}

private void HilbertU(int level){
    if(level>0){
        HilbertD(level-1); sg.lineRel(0,dist);
        HilbertU(level-1); sg.lineRel(dist,0);
        HilbertU(level-1); sg.lineRel(0,-dist);
        HilbertC(level-1);
    }
}

private void HilbertD(int level){
    if(level>0){
        HilbertU(level-1); sg.lineRel(dist,0);
        HilbertD(level-1); sg.lineRel(0,dist);
        HilbertD(level-1); sg.lineRel(-dist,0);
        HilbertA(level-1);
    }
}
private void HilbertC(int level){
    if(level>0){
        HilbertA(level-1); sg.lineRel(-dist,0);
        HilbertC(level-1); sg.lineRel(0,-dist);
        HilbertC(level-1); sg.lineRel(dist,0);
        HilbertU(level-1);
    }
}
private void HilbertA(int level){
    if(level>0){
        HilbertC(level-1); sg.lineRel(0,-dist);
        HilbertA(level-1); sg.lineRel(-dist,0);
        HilbertA(level-1); sg.lineRel(0,dist);
        HilbertD(level-1);
    }
}

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

class SimpleGraphics{
private Graphics g = null;
private int x =0, y = 0;

public SimpleGraphics(Graphics g) {
    this.g = g;}

public void goToXY(int x, int y){
    this.x =x;
    this.y= y;
}

public void lineRel(int deltaX, int deltaY){
    g.drawLine(x, y, x+deltaX, y+deltaY);
    x+=deltaX;
    y+=deltaY;}
}

I tried putting it to JFrame by myself, but I wasn't able to. The JFrame opens, but the HilbertCurve doesn't start. Below is my code, and I didn't change the class SimpleGraphics

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

public class HilbertCurve extends JPanel {
JFrame frame;

private SimpleGraphics sg = null;
private int dist0=256;
private int dist=dist0;


public static void main(String[] args){
    HilbertCurve exemplo1 = new HilbertCurve();
    exemplo1.fireUpScreen();
}

public void fireUpScreen(){
    frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(512,512);
    frame.setVisible(true);
    frame.add(this);
}

public void paint(Graphics g){

    int level = 4;
    dist = dist0;
    for(int i=level; i>0; i--) dist/=2;
    sg.goToXY (dist/2, dist/2);
    HilbertU(level);

}

private void HilbertU(int level){
    if(level>0){
        HilbertD(level-1); sg.lineRel(0,dist);
        HilbertU(level-1); sg.lineRel(dist,0);
        HilbertU(level-1); sg.lineRel(0,-dist);
        HilbertC(level-1);
    }
}

private void HilbertD(int level){
    if(level>0){
        HilbertU(level-1); sg.lineRel(dist,0);
        HilbertD(level-1); sg.lineRel(0,dist);
        HilbertD(level-1); sg.lineRel(-dist,0);
        HilbertA(level-1);
    }
}
private void HilbertC(int level){
    if(level>0){
        HilbertA(level-1); sg.lineRel(-dist,0);
        HilbertC(level-1); sg.lineRel(0,-dist);
        HilbertC(level-1); sg.lineRel(dist,0);
        HilbertU(level-1);
    }
}

 private void HilbertA(int level){
    if(level>0){
        HilbertC(level-1); sg.lineRel(0,-dist);
        HilbertA(level-1); sg.lineRel(-dist,0);
        HilbertA(level-1); sg.lineRel(0,dist);
        HilbertD(level-1);
    }
}

}

Start by making `frame.setVisible(true);` the last thing you call in `fireUpScreen` — MadProgrammer, Feb 07 '18 at 03:50
Nope, second last thing. Next, you should be getting a `NullPointerException` because `sg` is `null — MadProgrammer, Feb 07 '18 at 04:03
Method names should NOT start with an upper case character. Learn and follow Java naming conventions. Custom painting is done by overriding `paintComponent(...)` of the JPanel and don't forget to invoke super.paintComponent(..). Painting should be done with the Graphics object passed to the paintComponent() method. — camickr, Feb 07 '18 at 04:04

MadProgrammer · Accepted Answer · 2018-02-07T09:44:03.500

First, I would suggest you start by reading through Performing Custom Painting and Painting in AWT and Swing to better understand how painting works in Swing.

The example Applet is actually a bad example of how to perform painting, as you should never maintain a reference to Graphics and it should be passed to the methods which need it when the component is painted.

You should also end up with a NullPointerException as sg is never initialised.

As a general rule, establish the UI first, then call setVisible last, it will solve a number of simple issues.

There's no reason HilbertCurve needs a reference to the JFrame, the frame can be instantiated in the main method instead.

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class HilbertCurve extends JPanel {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                HilbertCurve exemplo1 = new HilbertCurve();
                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(exemplo1);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    private SimpleGraphics sg = null;
    private int dist0 = 256;
    private int dist = dist0;

    public HilbertCurve() {
        sg = new SimpleGraphics();
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(512, 512);
    }

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

        int level = 4;
        dist = dist0;
        for (int i = level; i > 0; i--) {
            dist /= 2;
        }
        sg.goToXY(dist / 2, dist / 2);
        Graphics2D g2d = (Graphics2D) g.create();
        hilbertU(g2d, level);
        g2d.dispose();

    }

    private void hilbertU(Graphics2D g, int level) {
        if (level > 0) {
            hilbertD(g, level - 1);
            sg.lineRel(g, 0, dist);
            hilbertU(g, level - 1);
            sg.lineRel(g, dist, 0);
            hilbertU(g, level - 1);
            sg.lineRel(g, 0, -dist);
            hilbertC(g, level - 1);
        }
    }

    private void hilbertD(Graphics2D g, int level) {
        if (level > 0) {
            hilbertU(g, level - 1);
            sg.lineRel(g, dist, 0);
            hilbertD(g, level - 1);
            sg.lineRel(g, 0, dist);
            hilbertD(g, level - 1);
            sg.lineRel(g, -dist, 0);
            hilbertA(g, level - 1);
        }
    }

    private void hilbertC(Graphics2D g, int level) {
        if (level > 0) {
            hilbertA(g, level - 1);
            sg.lineRel(g, -dist, 0);
            hilbertC(g, level - 1);
            sg.lineRel(g, 0, -dist);
            hilbertC(g, level - 1);
            sg.lineRel(g, dist, 0);
            hilbertU(g, level - 1);
        }
    }

    private void hilbertA(Graphics2D g, int level) {
        if (level > 0) {
            hilbertC(g, level - 1);
            sg.lineRel(g, 0, -dist);
            hilbertA(g, level - 1);
            sg.lineRel(g, -dist, 0);
            hilbertA(g, level - 1);
            sg.lineRel(g, 0, dist);
            hilbertD(g, level - 1);
        }
    }

    class SimpleGraphics {

//        private Graphics g = null;
        private int x = 0, y = 0;

        public SimpleGraphics() {
        }

        public void goToXY(int x, int y) {
            this.x = x;
            this.y = y;
        }

        public void lineRel(Graphics2D g, int deltaX, int deltaY) {
            g.drawLine(x, y, x + deltaX, y + deltaY);
            x += deltaX;
            y += deltaY;
        }
    }
}

Thank you man, appreciate it very much. I bet this was piece of cake for you, but it was a hard struggle here. Cheers! — Ian Tupiara, Feb 07 '18 at 20:37
How would I be able to draw something different in a new Frame. I was able to create the new Frame, but instead of doing the curve, I need to write the count of each point (1, 2, 3..) — Ian Tupiara, Feb 08 '18 at 00:15

How to implement Hilbert Curve in JFrame

1 Answers1