Pass the return type as a parameter in java?

Question

I have some files that contain logs of objects. Each file can store objects of a different type, but a single file is homogeneous -- it only stores objects of a single type.

I would like to write a method that returns an array of these objects, and have the array be of a specified type (the type of objects in a file is known and can be passed as a parameter).

Roughly, what I want is something like the following:

public static <T> T[] parseLog(File log, Class<T> cls) throws Exception {
    ArrayList<T> objList = new ArrayList<T>();
    FileInputStream fis = new FileInputStream(log);
    ObjectInputStream in = new ObjectInputStream(fis);
    try {
        Object obj;
        while (!((obj = in.readObject()) instanceof EOFObject)) {
            T tobj = (T) obj;
            objList.add(tobj);
        }
    } finally {
        in.close();
    }
    return objList.toArray(new T[0]);
}

The above code doesn't compile (there's an error on the return statement, and a warning on the cast), but it should give you the idea of what I'm trying to do. Any suggestions for the best way to do this?

Answer 1

You can't have generic arrays, so just return the ArrayList from your method:

    public static <T> ArrayList<T> parseLog(File log, Class<T> cls) throws Exception {
        ArrayList<T> objList = new ArrayList<T>();
        FileInputStream fis = new FileInputStream(log);
        ObjectInputStream in = new ObjectInputStream(fis);
        try {
            Object obj;
            while (!((obj = in.readObject()) instanceof EOFObject)) {
                T tobj = (T) obj;
                objList.add(tobj);
            }
        } finally {
            in.close();
        }
        return objList;
    }

Then, when you know the actual type, do the cast then. So assuming you know that T is actually an Integer at some point, do this:

Integer[] array = ((ArrayList<Integer>) myList).toArray(new Integer[myList.size()]);

Answer 2

I can't see how this would be a good programming practice. Essentially you're trying to say "I want to arbitrarily determine the return type without sacrificing type-safety." which is a bit awkward.

I would say your 'best' option would be to just return ArrayList<T> instead of going for an array.