Check out example codes for "how to draw a line in java". It will help you in understanding the concepts better.

Code Example 1

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

class Scratch extends JComponent{
    public static void main(String[] args) {
        JFrame window = new JFrame();   // creates a JFrame
        window.setTitle("Example");     //sets the title
        window.setSize(600, 600);   //sets the size
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  // sets the default close operation
        window.setVisible(true);    //sets the JFrame visible
        Scratch drawing = new Scratch();    //indirectly calls the paint method
        window.add(drawing);    //adds the paint method onto the JFrame

    public void paint(Graphics g){
        Graphics2D g2 = (Graphics2D) g; //casting
        g2.drawLine(100, 200, 300, 400);    //Draws a line from (100, -200) -> (300, -400) on the x and y plane
        g2.draw(new Rectangle(10, 10, 10, 10));     //draw a rectangle
        g2.setColor(Color.GREEN);   //Changes colour to Green
        g2.fill(new Rectangle(100, 100, 100, 100)); //fills a rectangle

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