casting ArrayList.toArray() with ArrayList of Generic Arrays

Question

A difficult question which I\'m close to giving up all hope on. I\'m trying to make a function, but am having problems getting ArrayList.toArray() to return the ty

Answer 1

toArray() always returns an object array.

To return an array of a specific type, you need to use toArray(Object[]).

To provide the correct type for the array argument, you need to use the specific class or in this case use reflections.

try

 return (T[]) list.toArray(Array.newInstance(one.getClass(), list.size()));

Answer 2

The problem on your second code sample is caused because of your T[] typeVar: Input is a two dim array, so input[0] will return a one dim array (a String[], instead a String, as expected).

If your are to convert your List<T> to a T[], you'll need a T typeVar,

To fix it:

public static <T> T[] unTreeArray(T[][] input) {
    java.util.List<T> outList = new ArrayList<T>();
    java.util.List<T> tempList;

    if (input.length == 0) {
        return null;
    }


    T typeVar=null;
    for (T[] subArray : input) {
        if(typeVar==null && subArray.length>0) {
            typeVar=subArray[0];
        }
        tempList = java.util.Arrays.asList(subArray);
        outList.addAll(tempList);
    }

    return outList.toArray((T[]) Array.newInstance(typeVar.getClass(),0));
}

public static void main(String[] args) {
    String[][] lines = { { "111", "122", "133" }, { "211", "222", "233" } };
    String[] result=unTreeArray(lines);

    System.out.println(result);

    System.out.println(result.getClass());

        //added for completion:
    System.out.println( Arrays.toString(result));

}

The resulting output:

[Ljava.lang.String;@5a405a4 class
[Ljava.lang.String;
[111, 122, 133, 211, 222, 233]

Answer 3

You can't create an array of a generic type parameter. The simple reason is the fact that java usually erases the generic type information at runtime so the runtime has no idea what T is (when T is a generic type parameter).

So the solution is to actually get a hold of the type information at runtime. Usually this is done by passing around a Class<T> type parameter but in this case you can avoid that:

@peter's answer looks good and i would use it :).

return (T[]) list.toArray(Array.newInstance(one.getClass(), list.size()));