Generic method to convert an array to ArrayList

Question

I have a method to convert an array to an ArrayList as follows:

    public static <T> ArrayList<T> getArrayList(T[] a){
        ArrayList<T> retList = new ArrayList<T>();
        for (T i : a){ 
            retList.add(i); 
        }
        return retList;
    }

which works fine for object arrays such as:

    String[] arr = {"String","'nother string","etc"};
    ArrayList<String> stringList = initArrayList(arr);

But not with primitive arrays:

    int[] arr2 = {1,2,3};
    ArrayList<Integer> intList = initArrayList(arr2); //Compiler is insulted by this.

I guess I have to convert the array to an Integer array if I want the method to work, but is there a way to make the method a little smarter about how it handle's this?

The Java tutorials site has the following:

    static <T> void fromArrayToCollection(T[] a, Collection<T> c) {
        for (T o : a) {
            c.add(o); // Correct
        }
    }

Which would work, but I'd like the method to be creating the ArrayList.

Also, this is just for my own amusement, so the type of Collection doesn't really matter, I just used ArrayList when I wrote the method.

Thanks

No, there really isn't. Primitive arrays and generics aren't going to mix. — Louis Wasserman
– Louis Wasserman, Commented Mar 19, 2013 at 23:53

rgettman · Accepted Answer · 2013-03-19 23:59:19Z

2

There's no generic method that can work with primitives, because primitives can't be a generic type. The closest you could come in pure Java is to overload getArrayList with an int[]...

public static ArrayList<Integer> getArrayList(int[] a){
    ArrayList<Integer> retList = new ArrayList<Integer>();
    for (int i : a){ 
        retList.add(i); 
    }
    return retList;
}

which will box your int for you. It's not generic, and you'd need a new overload for each primitive array type, but that's the closest solution I can think of.

answered Mar 19, 2013 at 23:59

rgettman

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2 Comments

Kraiden Over a year ago

Had a feeling this might be the case. Kinda sucks, but oh well. Thanks.

Paul Bellora Over a year ago

+1 and note that you could have it take int... for varargability and initialize capacity by using new ArrayList<Integer>(a.length).

kan · Accepted Answer · 2013-03-19 23:58:41Z

2

There are ready to use methods java.util.Arrays.asList and com.google.common.primitives.Ints.asList.

answered Mar 19, 2013 at 23:58

kan

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2 Comments

rgettman Over a year ago

Arrays.asList returns a List, not an ArrayList. Also, even if List<Integer> list = Arrays.asList(1, 2, 3); compiles, List<Integer> list = Arrays.asList(new int[]{1, 2, 3}); doesn't compile. With an array argument, the compiler will think that the return type is a List<int[]>.

Paul Bellora Over a year ago

@rgettman is right. Only Guava's Ints.asList would work correctly. See my answer here for example.

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