The parameter is ArrayList<T> and how can I get the T's className

Question

The parameter is ArrayList<T> how can I get the T's className

public static <T extends Object> void test(ArrayList<T> list){
        T temp;
        Class classType=temp.getClass(); 
        System.out.println(classType.getName());
}

It will be failed to compile that:he local variable temp may not have been initialized.

But how can I get the className of the template class.

Answer 1

You cannot get the generic type. This is due to how generics are implemented in Java (using type erasure). Basically, you can only use generics to catch type-errors at compile-time.

Your code fails to compile, because you are trying to call getClass on a local variable that has not been initialized.

What you could do instead is:

• pass in a Class<T> parameter in addition to the list to tell the method about the type to be used
• or look at the first element of the list (if present) and trust that its runtime type is representative enough (which it may not)

Answer 2

You cannot, because of type erasure in the implementation of Java generics. When you need to know the class, the typical trick is to pass Class as a separate parameter called "type token", like this:

public static <T extends Object> void test(ArrayList<T> list, Class<T> classType) {
}

This trick is discussed at some length in the tutorial on Java generics (see the bottom of the page for an example).