How to initialize the elements of an ArrayList?

Question

How do I initialize the values of an ArrayList?

I found this on the internet, but it doesn't work.

ArrayList<Kaart>[] hand = (ArrayList<Kaart>[]) new ArrayList[AANTALSPELERS];

All elements of hand are null. I get a nullPointerException because of that. Kaart is a class I created. AANTALSPELERS is a private static final int.

possible duplicate http://stackoverflow.com/questions/5244268/how-to-store-arraylist-into-an-array-in-java — Gonzalo Garcia Lasurtegui, Nov 30 '11 at 11:02
Arrays of references get initialized to `null` everywhere... So you'll have to assign `new ArrayList()` to the array "cells" before you use these. — Romain, Nov 30 '11 at 11:03
Refer this http://stackoverflow.com/questions/358546/casting-between-arraylists-in-java — Siva Charan, Nov 30 '11 at 11:04

score 6 · Answer 1 · answered Nov 30 '11 at 11:02

An array of Objects has elements initialized to null (just like how an array of ints is initialized to zeros).

So before you can use the elements of the array, you have to initialize each element.

ArrayList[] al = new ArrayList[5];

for( int i = 0; i < al.length; i++ )
    al[i] = new ArrayList();

score 3 · Accepted Answer · answered Nov 30 '11 at 11:11

This is the correct way, using generics. Notice that the warning is unavoidable (you can use a @SuppressWarnings annotation if that's a problem):

ArrayList<Kaart>[] array = (ArrayList<Kaart>[]) new ArrayList[AANTALSPELERS];
for (int i = 0; i < AANTALSPELERS; i++)
    array[i] = new ArrayList<Kaart>();

score 0 · Answer 3 · answered Nov 30 '11 at 11:13

0

Consider using Guava's ListMultimap where the key is the index.

ListMultimap<Integer, Kaart>

It will take care of all the list initialization for you.

answered Nov 30 '11 at 11:13

John B

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score 0 · Answer 4 · edited Nov 30 '11 at 12:54

0

you created a Array of AANTALSPELERS elements and each element can hold an ArrayList.

Since you have not added any ArrayList to the Array, the Array will have the default element null.

You also need to do something like this to populate the Array with ArrayList

for(int i = 0; i < hand.length; i++)
    hand[i] = new ArrayList();// or the arraylist you have

edited Nov 30 '11 at 12:54

Sastrija

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answered Nov 30 '11 at 11:13

M S

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How to initialize the elements of an ArrayList?

4 Answers4