exception while creating arrays of Arraylist

Question

I have java program like this

public class Distribution {

    private LinkedList<Integer > group[];    

    public Distribution() {
        group[0] = new LinkedList<>();
        group[0].add(1);
        group[0].add(2);
        group[0].add(3);

        group[1] = new LinkedList<>();
        group[1].add(4);
        group[1].add(5);
        group[1].add(6);
    }

    public static void main(String args[])
    {
        Distribution distribution = new Distribution();
    }
}

but the compiler says Exception in thread "main" java.lang.NullPointerException.

I guess it's not the compiler complaining about NPE but rather the JRE — dotvav, Jul 31 '15 at 12:48

brso05 · Answer 1 · 2015-07-31T13:57:38.440

6

You never initialized your group[].

private LinkedList<Integer> group[] = new LinkedList[2]; //example

You must initialize the array specify the length...

edited Jul 31 '15 at 13:57

answered Jul 31 '15 at 12:48

brso05

13,142
2
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Hoopje · Accepted Answer · 2015-07-31T13:48:35.483

You have not initialized your array. Use the following code to remove the NullPointerException:

private LinkedList<Integer> group[] = new LinkedList[10];

(where you have to replace 10 by the size you need).

Note that generics and arrays are not really convenient to use together. In particular, the above code gives a compiler warning (because it uses a raw type), and

private LinkedList<Integer> group[] = new LinkedList<Integer>[2];

doesn't compile.

In general, using an array of a generic class is not a good idea. Consider using an ArrayList instead of an array:

public class Distribution {
    private ArrayList<LinkedList<Integer>> group = new ArrayList<>();    

    public Distribution() {
        group.add(new LinkedList<>());
        group.get(0).add(1);
        ...
        group.add(new LinkedList<>());
        group.get(1).add(4);
        ...
    }
    ... 
}

score 1 · Answer 3 · edited May 23 '17 at 11:43

1

This will solve your problem

private LinkedList<Integer> group[] = new LinkedList[10];

However it will generate a compiler warning.

You cannot/should not create arrays of lists in java

Use Lists of Lists instead of Arrays of Lists

List< List<IntegerNode>> nodeLists = new LinkedList< List< IntegerNode >>();

See Answers to question - Cannot create an array of LinkedLists in Java...?

edited May 23 '17 at 11:43

Community

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answered Jul 31 '15 at 12:55

CarefreeCrayon

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exception while creating arrays of Arraylist

3 Answers3