Java Graphics trying to draw rectangle

Question

My goal is to make a class that contains rectangle and then use it and change it in other classes. I tried to write this code and make an object Rect rect = new Rect(); but when i start the program the rectangle doesn't show up. I also tried to add it with window.add(rect); but had same problem i'm sure im doing something wrong but i don't really know what.

One more thing that i tried was calling method from other class Rect.drawRect(g); but then it asks for "Argument" and if i add Argument g like i had in method drawRect it says "g cannot be resolved to a variable"

I hope someone can explain and tell me what did i do wrong, also would be great to know how to make rectangle or a circle and later use it in other classes and maybe change its color and size, I only found how to do it in one class.

import javax.swing.JFrame;

public class Main extends Rect{

    public static void main(String[] args ) {

        JFrame window = new JFrame("test");
        window.setSize(1000, 800);
        window.setVisible(true);
        window.setResizable(false);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Rect rect = new Rect();
    }   
}

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

public class Rect extends JPanel{

    public void drawRect(Graphics g){
        g.setColor(Color.RED);
        g.fillRect(100, 100, 200, 200); 
    }
}

Answer 1

UDP: You need override void paintComponent(Graphics g) instead void drawRect(Graphics g) and call super.paintComponent(g) inside method. Then you can use window.add(rect); . Thanks @FredK for correction

Answer 2

The most important thing is that you need to write some code to do the painting. This is done by overriding the paintComponent method inside your Rect class a bit like this:

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.RED);
        g.fillRect(100, 100, 200, 200); 
    }

Your second issue is that you want to be able to change the colour and size of your rectangle from other classes. For a simple example, this can easily be done by adding some static values inside your Rect class:

    public static Color myColor = Color.RED;
    public static int myX = 100;
    public static int myY = 100;
    public static int myWidth = 200;
    public static int myHeight = 200;

Now update your paint method to use the static values:

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(myColor);
        g.fillRect(myX, myY, myWidth, myHeight); 
    }

Now, whenever or wherever you use the Rect panel it will now show the rectangle according to the static values.

For example, below is a simple and working program, note how it uses the following:

    //create Rect
    Rect rect = new Rect();
    //set the size of the new panel
    rect.setPreferredSize(new Dimension(800, 600));
    //add the rect to your JFrame
    window.add(rect);
    //now you can change the color for all Rect instances
    //Note how I use Rect instead of rect, however, both will work.
    Rect.myColor = Color.BLUE;
    //And dont forget to repaint it if you want to see the changes immediatly
    rect.repaint();

Full example, main class:

import javax.swing.JFrame;
import java.awt.Color;

public class Main{

    //Note how we don't need to extend the Rect class (It just adds confusion)
    public static void main(String[] args ) {

        JFrame window = new JFrame("test");
        window.setSize(1000, 800);
        window.setVisible(true);
        window.setResizable(false);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //create Rect
        Rect rect = new Rect();
        //set the size of the new panel
        rect.setPreferredSize(new Dimension(800, 600));
        //add the rect to your JFrame
        window.add(rect);


        //now you can change the color for all Rect instances
        //Note how I use Rect instead of rect, however both will work.
        Rect.myColor = Color.BLUE;
        //And dont forget to update it
        rect.repaint();
    }
}

And the Rect class:

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

public class Rect extends JPanel{
    public static Color myColor = Color.RED;
    public static int myX = 10;
    public static int myY = 10;
    public static int myWidth = 200;
    public static int myHeight = 200;

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(myColor);
        g.fillRect(myX, myY, myWidth, myHeight); 
    }
}

Note, if you don't want to call Rect.repaint() every time you make a color/size change then just make a new method that changes each value and include repaint() , for example:

public void changeWidth(int width){
    myWidth  = width;
    repaint();
}