How can I pass a generic class to a method in Java?

Question

Suppose we have a function that creates objects given a particular class:

public static <T> T createObject(Class<T> generic) {
    try {
        return generic.newInstance();
    } catch (Exception ex) {
        return null;
    }
}

We can use the function easily to create instances of non-generic types.

public static void main(String[] args) {
    Foo x = createObject(Foo.class);
}

Is it possible to do the same thing with a generic type?

public static void main(String[] args) {
    ArrayList<Foo> x = createObject(ArrayList<Foo>.class); // compiler error
}

Answer 1

Generics, in Java, are implemented through type erasure.

That means that an ArrayList<T>, at run time, is an ArrayList. The compiler simply inserts casts for you.

You can test this with the following:

ArrayList<Integer> first = new ArrayList<Integer>();
ArrayList<Float> second = new ArrayList<Float>();

if(first.getClass() == second.getClass())
{
    // should step in the if
}

Answer 2

Also, as long as you are sure of what you are doing, and as long as you are pretty sure your code cannot incur heap pollution you can suppress the warning.

You may also take into consideration that your code cannot instantiate arrays, which by chance also requires an unchecked exception:

@SuppressWarnings("unchecked")
public static <T> T[] createArray(Class<T> anyClass, int size){
    T[]result = null;
    try{
        result = (T[]) Array.newInstance(anyClass, size);
    }catch(Exception e){
        e.printStackTrace();
    }
    return result;
}

This is how I used it:

public static void main(String[] args) throws Exception{
    @SuppressWarnings("unchecked")
    List<String> jediNames = (List<String>) createObject(ArrayList.class);
    jediNames.add("Obiwan");

    String[] moreJedis = createArray(String.class, 10);
    System.out.println(moreJedis.length);
    moreJedis[0] = "Anakin";
}