Thread: 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.

/**
 * 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.