draw lines

Class Graphics (from package java.awt) provides various methods for drawing text and shapes onto the screen.

Class JPanel (from package javax.swing) provides an area on which we can draw.

The keyword extends creates a so-called inheritance relationship.

The class from which DrawPanel inherits, JPanel, appears to the right of keyword extends.
In this inheritance relationship, JPanel is called the superclass and DrawPanel is called the subclass.

JPanel has a paintComponent method, which the system calls every time it needs to display the JPanel.

super.paintComponent( g );

JPanel methods getWidth and getHeight return the JPanel’s width and height, respectively.
Graphics method drawLine draws a line between two points represented by its four arguments. The first two are the x- and y-coordinates for one endpoint, and the last two arguments are the coordinates for the other endpoint.

To display the DrawPanel on the screen, place it in a window.

Create a window with an object of class JFrame.
JFrame method setDefaultCloseOperation with the argument JFrame.EXIT_ON_CLOSE indicates that the application should terminate when the user closes the window.
JFrame’s add method attaches the DrawPanel (or any other GUI component) to a JFrame.
JFrame method setSize takes two parameters that represent the width and height of the JFrame, respectively.
JFrame method setVisible with the argument true displays the JFrame.
When a JFrame is displayed, the DrawPanel’s paintComponent method is implicitly called.