[SOLVED] trouble in sorting...please help

[jacline]

Add an interface, SizeComparable, that includes only the boolean isBiggerThan( Shape other) method. In your ShapeSet class, provide a method void sort(), that sorts the shapes of the shape set by class Shape implementing the SizeComparable interface.
This is the problem I wrote codes but it gives run-time error.Could you help me?

public abstract class Shape implements SizeComparable {
Shape a;

public abstract double getArea();

public boolean isBiggerThan(Shape other){
if(other.getArea()>a.getArea())
return true;
else
return false;	
}
}

public interface SizeComparable {
public boolean isBiggerThan(Shape other);
}


public class ShapeSet {
//properties
Shape[] shapeList;
int number;
//constructor
public ShapeSet(){
shapeList=new Shape[19];
number=0;
}
............
.........
public void sort()
{
int in, out;

for(out=1; out<number; out++) // out is dividing line
{
Shape temp = shapeList[out]; // remove marked Shape
in = out; // start shifting at out

while(in>0 && // until smaller one found,
temp.isBiggerThan(shapeList[in]))
{
shapeList[in] = shapeList[in-1]; // shift item to the right
--in; // go left one position
}
shapeList[in]= temp; // insert marked item
} // end for
} // end Sort()

}

[Fubarable]

what line of code causes the error and what error is specifically thrown? Please post the error code itself. Also, when posting code, you would benefit by using code tags in order to make your code retain its formatting and thus be readable. You can do this by placing the tag [code] above your code block and [/code] below it. Again this will improve the readability of your code and should mean more folks will actually take the time to read it. Best of luck.

[jacline]

thanks for your helping.. it gives run-time error so i couldnt understand the exact line.

[Fubarable]

Is it possible for you to post here the text of the error message?

[jacline]

it gives this error:

Exception in thread "main" java.lang.NullPointerException
at Shape.isBiggerThan(Shape.java:8)
at ShapeSet.sort(ShapeSet.java:45)
at Lab03.main(Lab03.java:33)

[Fubarable]

so it does give line numbers for you to use. Look at the Shape class, line 8, the ShapeSet class, line 45, ...

You are somehow using an object that has not been constructed yet, and which object will be one of the ones on the lines above (look at the first line first of all).

[Fubarable]

I think I see your problem. Your Shape class has a Shape field, "a" that is probably null and probably shouldn't even be there. Instead compare the "other" shape's area size with "this"'s area's size.

public abstract class Shape implements SizeComparable {
// Shape a; // get rid of

  public abstract double getArea();

  public boolean isBiggerThan(Shape other){
    if(other.getArea() > getArea()) // use "this's" getArea, not a's
       return true;
    else
       return false;	
  }
}

[jacline]

thanks for your helping...:D

[Fubarable]

You're entirely welcome!