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The Complete Java Crash Course

Classes contain methods

Classes contain the code for methods that act on objects; methods have access to the object using the special variable "this".

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Here is some code to create a custom Ball class that is used to store information about a ball that will bounce on the screen. The main method of BallExample creates a Ball object using the new keyword and a call to the constructor. Read the code carefully now:

Java
import com.educative.graphics.*;
class Ball {
  public String color;
  public int x;
  public int y;
  public int vx;
  public int vy;
  
  public Ball(String color, int x, int y, int vx, int vy) {
    this.color = color;
    this.x = x;
    this.y = y;
    this.vx = vx;
    this.vy = vy;
  }
}
class BallExample {
  public static void drawBall(Canvas canvas, Ball ball) {
    canvas.fill(ball.color);
    canvas.stroke("black");
    canvas.circle(ball.x, ball.y, 10);
  }
  
  public static void main( String args[] ) {
    Canvas c = new Canvas(200, 200);
    
    // create a red ball at location (20, 30) with 0
    //  x and y velocity:
    Ball b = new Ball("red", 20, 30, 0, 0); 
    
    drawBall(c, b);
  }
}

The BallExample class also has a method drawBall that accesses instance variables of the Ball object and uses them, together with a Canvas object to draw a circle in the right place on the screen.

Classes group together methods. It seems like the code for drawBall might naturally be in the Ball class. Like this:

Java
import com.educative.graphics.*;
class Ball {
  public String color;
  public int x;
  public int y;
  public int vx;
  public int vy;
  
  public Ball(String color, int x, int y, int vx, int vy) {
    this.color = color;
    this.x = x;
    this.y = y;
    this.vx = vx;
    this.vy = vy;
  }
  
  public void draw(Canvas canvas) {
    canvas.fill(this.color);
    canvas.stroke("black");
    canvas.circle(this.x, this.y, 10);
    
  }
}
class BallExample {  
  public static void main( String args[] ) {
    Canvas c = new Canvas(200, 200);
  
    Ball b = new Ball("red", 20, 30, 0, 0); 
    
    // call the draw method of the ball class
    b.draw(c);
  }
}

The BallExample class ...