Question

I keep get this exception error Exception in thread "main" java.lang.NullPointerException
   at Quadrilateral.returnCoordsAsString(Quadrilateral.java:44)
   at Quadrilateral.toString(Quadrilateral.java:51)
   at tester1.main(tester1.java:39)

In this program I needed to make a Point class to create a coordinate square from x and y. I also needed to make a Quadrilateral class that has an instance reference variable to Point

.The Quadrilateral class then inherits itself to other classes or in this case other shapes like square, trapazoid. I thought I did it right but for some reason there seems to be a problem with the toString class.

Here is Point class

public class Point {
private double x;
private double y;

public Point(double point1, double point2)
{
x = point1;
y = point2;
}
public Point()
{
x = 0.00;
y = 0.00;
}
public void setx(double x)
{
this.x = x;
}
public void sety(double y)
{
this.y = y;
}

public double getx()
{
return x;
}
public double gety()
{
return y;
}

@Override
public String toString()
{
return "(" + getx() + " , " + gety() + ")";
  
}
  
}

here is the Quadrilateral class

public class Quadrilateral {
Point Corner1;
Point Corner2;
Point Corner3;
Point Corner4;
public Quadrilateral(double x1, double y1, double x2, double y2, double x3, double y3 , double x4, double y4)
{
Point Corner1 = new Point(x1, y1);
Point Corner2 = new Point(x2, y2);
Point Corner3 = new Point(x3, y3);
Point Corner4 = new Point(x4, y4);
  
}

public Point getCorner1()
{
return Corner1;
}
public Point getCorner2()
{
return Corner2;
  
}
public Point getCorner3()
{
return Corner3;
}
public Point getCorner4()
{
return Corner4;
}
public String returnCoordsAsString()
{
String name1 = getCorner1().toString() + " , " + getCorner2().toString() + " , " + getCorner3().toString() + " , " + getCorner4().toString();
return name1;
}

@Override
public String toString()
{
String name = " coordinates od Quadrilateral are : " + returnCoordsAsString();
return name;

}

  
}

and this is the diver class

public class tester1 {

/**
* @param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
Quadrilateral quadrilateral = new Quadrilateral(
1.1, 1.2, 6.6, 2.8, 6.2, 9.9, 2.2, 7.4 );
quadrilateral.toString();
  
}
}

Answer 1

If you have any doubts, please give me comment...

Point.java

public class Point {

    private double x;

    private double y;

    public Point(double point1, double point2) {

        x = point1;

        y = point2;

    }

    public Point() {

        x = 0.00;

        y = 0.00;

    }

    public void setx(double x) {

        this.x = x;

    }

    public void sety(double y) {

        this.y = y;

    }

    public double getx() {

        return x;

    }

    public double gety() {

        return y;

    }

    @Override

    public String toString() {

        return "(" + getx() + " , " + gety() + ")";

    }

}

Quadralateral.java

public class Quadrilateral {

    Point Corner1;

    Point Corner2;

    Point Corner3;

    Point Corner4;

    public Quadrilateral(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {

        Corner1 = new Point(x1, y1);

        Corner2 = new Point(x2, y2);

        Corner3 = new Point(x3, y3);

        Corner4 = new Point(x4, y4);

    }

    public Point getCorner1() {

        return Corner1;

    }

    public Point getCorner2() {

        return Corner2;

    }

    public Point getCorner3() {

        return Corner3;

    }

    public Point getCorner4() {

        return Corner4;

    }

    public String returnCoordsAsString() {

        String name1 = getCorner1().toString() + " , " + getCorner2().toString() + " , " + getCorner3().toString()

                + " , " + getCorner4().toString();

        return name1;

    }

    @Override

    public String toString() {

        String name = " coordinates od Quadrilateral are : " + returnCoordsAsString();

        return name;

    }

}

tester1.java

public class tester1 {

    /**

     * @param args the command line arguments

     */

    public static void main(String[] args) {

        // TODO code application logic here

        Quadrilateral quadrilateral = new Quadrilateral(1.1, 1.2, 6.6, 2.8, 6.2, 9.9, 2.2, 7.4);

        System.out.println(quadrilateral.toString());

    }

}