Question

Alright so my program is supposed draw a rectangle automatically based on the coordinates (X, Y, Width, Length) that the user types in. When I run my program, i get an Exception in thread main error.

Here is the exact error:

Exception in thread "main" java.lang.NullPointerException
at rectangle.draw(rectangle.java:31)
at rectangle.main(rectangle.java:52)

Please tell me what I’m doing wrong!

Thanks!

Code: `import gpdraw.*;
import java.util.Scanner;

public class rectangle {

private static double myX;
private static double myY;
private static double myWidth;
private static double myHeight;
private DrawingTool myPencil;
private SketchPad myPaper;

public double getPerimeter(){

    double perimeter;
    perimeter = myWidth * 2 + myHeight * 2;


    return perimeter;   
}
public double Area(){
    double area;

    area = myHeight * myWidth;
    System.out.println("Area: " + area);

    return area;

}
public void draw(){
    myPencil.up();
    myPencil.move(myX , myY);
    myPencil.down();
    myPencil.move(myX + myWidth, myY);
    myPencil.move(myX + myWidth, myY + myHeight);
    myPencil.move(myX , myY);
}


public static void main(String[] args){

    Scanner input = new Scanner(System.in);
    System.out.println("Enter X Value: ");
    myX = input.nextInt();
    System.out.println("Enter Y Value: ");
    myY = input.nextInt();
    System.out.println("Enter Width: ");
    myWidth = input.nextInt();
    System.out.println("Enter Height: ");
    myHeight = input.nextInt();
    rectangle picture = new rectangle();
    picture.draw();
}

}
`

Line 51: picture.draw();
Line 31: myPencil.up();

Answer 1

You never assign a value to the myPencil field, so it will have the default value of null. When you then try to dereference it here:

myPencil.up();

… that will throw an exception.

Presumably you meant to give myPencil a value, e.g.

private DrawingTool myPencil = new Pencil();

… or perhaps do so in the constructor?