How to properly return generic array in Java generic method?

Question

I have below generic method that returns a generic array:

public static  T[] genericMethod1(List input) {
    T[] res = (T[]) new Object[input.s         


        

Answer 1

The explanation for what you are seeing is due to something called type erasure. Here is what your genericMethod() will look like after the compiler performs type erasure:

public static Object[] genericMethod(List input) {
    Object[] res = new Object[input.size()];

    int i = 0;
    for (Object t : input) {
        res[i] = t;
        i++;
    }
    return res;
}

In other words, this method will return an array of type Object. There is no way to cast an Object[] to an Integer[] because they are not the same type. If you want your method to be able to dynamically return the type you want, then you can use Array.newInstance(). This will require also passing in the type of the array you want as an input parameter:

public static  T[] genericMethod(Class clazz, List input) {
    @SuppressWarnings("unchecked")
    T[] res = (T[]) Array.newInstance(clazz, input.size());

    int i = 0;
    for (T t : input) {
        res[i] = t;
        i++;
    }
    return res;
}

Now your code snippet will run without error:

LinkedList list = new LinkedList();    
Integer[] i = genericMethod(Integer.class, list);

Update:

Your second method, genericMethod2(), will look like this after type erasure:

public static Object genericMethod2(List input) {
    return input.get(0);
}

It will return the first element of the input list, cast to Object. Here is your usage of that method:

Integer j = genericMethod2(list);

The compiler will try to cast the output from genericMethod2() to Integer:

Integer j = (Integer)genericMethod2(list);

This cast is legal, because every Integer is also an Object, and furthermore it succeeds here because you passed in a collection of Integer. This second method is not the same scenario as the first one you highlighted for us.