Java - How to copy each element of array to ArrayList?

Question

Why this didn't work

import java.util.ArrayList;
public class Vector 
{
    ArrayList<Long> vector;
    public Vector(long ...vector)
    {
        for (long value : vector)
            this.vector.add(new Long(value));
    }
}

when I make a new objet like Vector a = new Vector(4,7,8); it says

java.lang.NullPointerException

I've tried Arrays.asList(array); and

for (int i=0;i<vector.lenght;i++)
    this.vector.add(vector[i])

and same error

nanofarad · Accepted Answer · 2016-02-01 00:41:52Z

1

Your code is almost correct, in that the code to copy to an arraylist is properly written. However, the fact that the error is a NullPointerException clues us into knowing that something is null where it shouldn't be. In this case, vector itself is null, since it never got assigned a value. You can either declare vector and assign it using ArrayList<Long> vector = new ArrayList<>();, or assign vector = new ArrayList<>(); in the constructor.

Note: The <> diamond shorthand is a shorthand for generic types available in Java 7 and higher. If targeting Java 6, use new ArrayList<Long>(); on the right-hand-side.

answered Feb 1, 2016 at 0:41

nanofarad

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