Java generics for class and new instance

Question

In Java, if I have:

public class Foo<T> {

  public Foo() { }

}

I can do Foo<Integer> foo = new Foo<Integer>(); but I cannot find an equivalent syntax for using the class field, meaning that something like Foo<Integer> foo = Foo<Integer>.class.newInstance(); doesn't work. Is there a way to use generics via the newInstance method?

There is no way to get the class of a generic. Look in all examples they always do somthing like Foo foo = new Foo(Integer.class) — Rene M., Jul 08 '15 at 08:52
`Foo.class` has no sense in Java due to type erasure: you only have the `Foo` type. — Lyubomyr Shaydariv, Jul 08 '15 at 08:58

score 1 · Answer 1 · edited May 23 '17 at 11:51

1

You can do it only without parameterized type. Class.newInstance() is not quite recommendable. You can do this:

//Class<Foo<Integer>> clazz = Foo<Integer>.class; <-- Error
Class<Foo> clazz = Foo.class; // <-- Ok
Foo instance = clazz.getConstructor().newInstance();

Why is Class.newInstance() evil?

Hope it helps.

edited May 23 '17 at 11:51

Community

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answered Jul 08 '15 at 09:01

Héctor

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score 1 · Answer 2 · answered Jul 10 '15 at 00:20

1

.newInstance() (and anything that uses Class) is a reflection API that fundamentally works at runtime. Therefore it has little to do with Generics which operates at compile-time.

answered Jul 10 '15 at 00:20

newacct

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score 0 · Answer 3 · answered Jul 08 '15 at 08:51

newInstance() is an API from the early days of Java - no generics back then. You have to cast the return value.

Also: there will be no future method doing that, newInstance() is used to create objects dynamically at runtime. Due to type erasure in Java there is no generic information at runtime anymore.

Java generics for class and new instance

3 Answers3