Convert Integer List to int array [duplicate]

Answer 1

Here is a utility method that converts a Collection of Integers to an array of ints. If the input is null, null is returned. If the input contains any null values, a defensive copy is created, stripping all null values from it. The original collection is left unchanged.

public static int[] toIntArray(final Collection data){
    int[] result;
    // null result for null input
    if(data == null){
        result = null;
    // empty array for empty collection
    } else if(data.isEmpty()){
        result = new int[0];
    } else{
        final Collection effective;
        // if data contains null make defensive copy
        // and remove null values
        if(data.contains(null)){
            effective = new ArrayList(data);
            while(effective.remove(null)){}
        // otherwise use original collection
        }else{
            effective = data;
        }
        result = new int[effective.size()];
        int offset = 0;
        // store values
        for(final Integer i : effective){
            result[offset++] = i.intValue();
        }
    }
    return result;
}

Update: Guava has a one-liner for this functionality:

int[] array = Ints.toArray(data);

Reference:

• Ints.toArray(Collection)