Question

i’m trying to copy an object with a copy constructor, but it outputs an error:

Exception in thread "main" java.lang.NullPointerException
at Polynomial.<init>(Polynomial.java:30)
at Polynomial.showDerivative(Polynomial.java:59)
at Program.main(Program.java:9)

This is my copy constructor:

public Polynomial(Polynomial poly)
{
    for(int i = 0; i < a.length; i++)
        a[i] = poly.a[i];
    for(int i = 0; i < b.length; i++)
        b[i] = poly.b[i];
}

And this is how I instantiate the object:

Polynomial pol = new Polynomial(this);

What do I do?

Thanks.

Answer 1

You would be better with using System.arraycopy for creating a copy of your array.

Further, change your copy-constructor to: –

public Polynomial(Polynomial poly)
{
    int aLen = poly.a.length;
    int bLen = poly.b.length;

    // Initialize arrays for this object
    a = new int[aLen];  // Assuming `a` and `b` are integer arrays
    b = new int[bLen];  // Change accordingly.

    // A better way to create copy of arrays would be to use `System.arraycopy
    System.arraycopy( poly.a, 0, a, 0, aLen);
    System.arraycopy( poly.b, 0, b, 0, bLen);


    /*** You can avoid using below loops ***/

    // Iterate till the `aLen` of `poly` object passed 
    // and add elements to `a` array of this object
    /*for(int i = 0; i < aLen; i++)
        a[i] = poly.a[i];

    // Iterate till the `bLen` of `poly` object passed 
    // and add elements to `b` array of this object
    for(int i = 0; i < bLen; i++)
        b[i] = poly.b[i]; */
}

Your for loop should run till the length of poly.a and poly.b and not a and b, because they are not yet initialized, and hence NPE.