NullPointerException Issue

I am relatively new to Java programming and I have honestly hit a brick wall with this class lab.

I am creating a program that will do operations on a series of number matrices that will take an array as a parameter. It will then store the result in a third matrix that has an array as a parameter. Think of it as matrix[1] + matrix[1] = matrix[1] and so on. Where I am hitting a brick wall is that I am getting a null pointer exception. I commented it in my code on the line where my JUnit test is seeing it.

So here's the code.

Code Java:

---

/**
 * The DVector class will create a object to be used by the NVector interface
 * to implement its methods
 * 
 * @author
 */
public class DVector implements NVectorADT
{
    private double[] ary1;
    private double[] aryResult;
    private int size;
 
    /**
     * Constructs the first array and the results array. 
     * Will throw an unchecked exception if the length of the first array is 
     * either 0 or array is null.
     * 
     * @param ary1  parameter of the first array
     */
    public DVector(double[] ary1)
    {
        size = ary1.length;
        if (ary1.length == 0 || ary1 == null) {
            throw new IllegalArgumentException ("Illegal Array");
        }
        else {
            aryResult = new double[ary1.length];
        }
    }
 
    /**
     * Adds the elements of the first matrix and the second matrix and stores the
     * result in the result matrix. 
     * @param y
     * @return
     */
    @Override
    public NVectorADT plus(NVectorADT y) {
       DVector ary2 = (DVector) y;
       int i = 0;
       if (this.size != ary1.length) { //null pointer exception here
           throw new IndexOutOfBoundsException("Array indices are of different length");
       }
       else {
           while (i < this.size) {
               aryResult[i] = ary1[i] + y.size();
           }
       }
       i++;
       size++;
       return y;
    }
 
    @Override
    public NVectorADT minus(NVectorADT y) {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public NVectorADT mult(NVectorADT y) {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public NVectorADT divby(NVectorADT y) {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public double sum() {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public NVectorADT sqrt() {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public int size() {
        return size;
    }
 
    @Override
    public double[] getAsArray() {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public boolean equals(NVectorADT y) {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
    @Override
    public String toString()
    {
        throw new UnsupportedOperationException("Not supported yet.");
    }
 
}

---

If you have any questions let me know and I'll answer them to the best of ability. Any help would be appreciated.

The field ary1 was never initialized in your constructor. It therefore takes the default value of null. Note that this.ary1 != ary1 passed to the constructor as a parameter.

okay so I initialized the constructor for ary1 like so, ary1 = new double[size];. The constructor now looks like this

Code :

---

public DVector(double[] ary1)
    {
        size = ary1.length;
        this.ary1 = new double[size];
        if (ary1.length == 0 || ary1 == null) {
            throw new IllegalArgumentException ("Illegal Array");
        }
        aryResult = new double[size];
    }

---

When I added the constructor line another error popped up. It threw the IndexOutOfBoundsException error. The NullPointerException error still gets thrown so that suspects to me they are related. Also I am not seeing your note of this.ary1 != ary1 and perhaps you could explain it to me what you are seeing where I am not.