Java newInstance of Generic Class

Question

I am working with Java Generic classes (in this example, these are the Collection classes), and Reflection. I would like to be able to use reflection to take in a Class, check if it is an instance of a List, and then invoke the add method to it.

However, I've faced some difficulties in trying to put as the parameters to invoke the method call, and getting the declared method (shown where I put-what???). Both of those method parameter calls, require an object of type Class<?> which is the parameter type of needed for the add methods being invoked, which I don't know, since T itself is a generic.

Any help is appreciated! I apologize if the question is unclear, I tried the best I could to clarify.

static <T> void TestACollection(Class<T> clazz) {
     T element=clazz.newInstance();
     if(element instanceof List<?>)
     Method m=clazz.getDeclaredMethod("add", what???  ); 
     m.invoke(element, what???);
}

First what??? is Object. Second what??? is whatever you are adding. — Dima
– Dima, Commented Dec 9, 2015 at 22:58
Due to type erasure, this isn't very possible. (it depends how you got that Class<T>) — SLaks
– SLaks, Commented Dec 9, 2015 at 22:58
So @Dima, if I passed in a List.class , calling clazz.getDeclaredMethod("add", Object.class) would work? And for the second part, if I added a String object, would T element be an instance of List<String> ? — deepmindz
– deepmindz, Commented Dec 9, 2015 at 23:05
element is an instance of List. There are no generics in runtime. — Dima
– Dima, Commented Dec 9, 2015 at 23:17

live2 · Accepted Answer · 2021-04-01 20:46:02Z

1

I'm guessing what you are trying to do is this:

public static <T> List<T> makeList() {
   List<T> list = (List<T>) new ArrayList();
   return list;
}

//...
{
   List<String> list = makeList();
   list.add( "Howdy" );
}

Which works as-is in Java 8. In earlier versions you may have to add @SuppressWarnings("unchecked") to the assignment.

edited Apr 1, 2021 at 20:46

live2

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answered Dec 9, 2015 at 23:29

markspace

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2 Comments

newacct Over a year ago

The argument type is unused and unnecessary. What you have is just a really verbose way of doing public static <T> List<T> makeList() { return new ArrayList<T>(); }

markspace Over a year ago

Oh yeah. I copied the OP's method spec and ended up not needing the type parameter, which I didn't notice. Good catch.

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