Java Graphics trying to draw rectangle

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.

Rect rect = new Rect();

window.add(rect);

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"

Rect.drawRect(g);

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);
}
}


2 Answers
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:

paintComponent

@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:

Rect

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.

Rect

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:

Rect.repaint()

repaint()

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


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

void paintComponent(Graphics g)

void drawRect(Graphics g)

super.paintComponent(g)

window.add(rect);

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