ArrayList nullPointerException

Question

I am getting a NullPointerException while adding objects to hello ArrayList. I want to add objects at specific indices, so if I don't add null objects to the array before hand, I get IndexOutOfBoundsException when I try to add at an index whose previous index hasn't been populated yet. Why am I getting a NullPointerException and is there any other way to achieve it?

    public void test()
    {
        ArrayList<TEST> temp = new ArrayList<>(4);

        temp.add(0,new TEST(2));
        temp.add(1,new TEST(3));
        temp.add(2,new TEST(1));
        temp.add(3,new TEST(0));


        for(int i=0; i<4; i++)
            Log.e("test", "i: "+i+ " index: "+temp.get(i).x);


        ArrayList<TEST> hello = new ArrayList<>(4);
        hello.add(null);
        hello.add(null);
        hello.add(null);
        hello.add(null);

        hello.add(temp.get(0).x, temp.get(0));
        hello.add(temp.get(1).x, temp.get(1));
        hello.add(temp.get(2).x, temp.get(2));
        hello.add(temp.get(3).x, temp.get(3));


        Log.e("test", "___________________________");
        for(int i=0; i<4; i++)
            Log.e("test", "i: "+i+ " index: "+hello.get(i).x);

    }

    public class TEST
    {
        int x;

        public TEST(int x) {
            this.x = x;
        }


    }

Eran · Accepted Answer · 2015-01-25 08:14:32Z

6

When you write

hello.add(temp.get(0).x, temp.get(0));

you don't replace the null you put in the temp.get(0).x index. You just move that null to the next index.

Therefore, in the loop :

    for(int i=0; i<4; i++)
        Log.e("test", "i: "+i+ " index: "+hello.get(i).x);

you encounter a null value, so hello.get(i).x throws NullPointerException.

change it to

    hello.set(temp.get(0).x, temp.get(0));
    hello.set(temp.get(1).x, temp.get(1));
    hello.set(temp.get(2).x, temp.get(2));
    hello.set(temp.get(3).x, temp.get(3));

in order to replace all the null values with non null values.

answered Jan 25, 2015 at 8:14

Eran

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1 Comment

Koen Over a year ago

And it would also be helpful to make the output more robust, like this: Log.e("test", "i: "+i+ " index: "+(hello.get(i)==null ? "" : hello.get(i).x));

bedrin · Accepted Answer · 2015-01-25 08:30:37Z

0

You can override ArrayList.set() method to add objects at specific indexes:

public class SparseArrayList<E> extends ArrayList<E> {

    @Override
    public E set(int index, E element) {
        while (size() <= index) add(null);
        return super.set(index, element);
    }

}

answered Jan 25, 2015 at 8:30

bedrin

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Comments

Frank Why · Accepted Answer · 2015-01-25 08:35:17Z

-4

I think you get the error because you try to add more then 4 elements to the list that initials to 4 maximum capacity.

answered Jan 25, 2015 at 8:35

Frank Why

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1 Comment

Ingo Bürk Over a year ago

The initial capacity is just that, an initial capacity. It is not a maximum capacity.

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