Instance of type generic [duplicate]

Answer 1

If you know the type at compile time, use "new Whatever()". If you don't know the type at compile time but can get a Class object for it, use newInstance().

99% of the time I know the type and I use "new Whatever()".

Answer 2

Due to type erasure: the generic type doesn't know at execution time what T is, so it can't call the right constructor.

See Angelika Langer's FAQ entry on type erasure for (much) more information.

Answer 3

You need to use reflection (newInstance()), because at compile time the class whose constructor would need to be linked is unknown. So the compiler cannot generate the link.

Answer 4

Maybe, you're looking at this pattern (taken from an answer to another question):

private static class SomeContainer<E>
{
    E createContents(Class<E> clazz)
    {
        return clazz.newInstance();
    }
}

Here, when we create a SomeContainer, we parametize the instance with a concrete class (like String). createContents will accept String.class only and String.class.newInstance() will create a new (empty) String.