In Java, why can't you use a constructor on generic classes?

Question

What's the logic behind the limitaion of doing the following in Java?

public class genericClass<T>{
    void foo(){
       T t = new T(); //Not allowed
    }
}

`T` is undefined at runtime, because there's only one copy of the byte code. So the JVM can't tell which class to instantiate. — Dawood ibn Kareem, Jul 19 '14 at 12:20
If you could, what would happen if `T` didn't have a no-args constructor, or it was private, or `T` was an interface? (What would `new genericClass().foo()` do?) — user253751, Jul 19 '14 at 12:20
Because `T` is not real -- it's just a fig newton of the compiler's imagination. There is no constuctor to call -- the type of `T` is not even known at runtime. — Hot Licks, Jul 19 '14 at 12:22
possible duplicate of [Create instance of generic type in Java?](http://stackoverflow.com/questions/75175/create-instance-of-generic-type-in-java) — Drazen Bjelovuk, Aug 23 '14 at 01:16

score 6 · Accepted Answer · answered Jul 19 '14 at 12:20

6

Because of type erasure.

The runtime does not know the "real" type of T, it is Object for it. You code would be more or less read like this:

public class genericClass {
    void foo(){
       Object t = new Object(); //Not allowed
    }
}

If you need to do such a thing, you need to use reflection:

public class GenericClass<T> {
  private final Class<? extends T> clazz;
  public GenericClass(Class<? extends T> clazz) {
    this.clazz = clazz;
  }
  void foo() {
    T t = clazz.newInstance();
  }
}

answered Jul 19 '14 at 12:20

NoDataFound

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2

But you can do new Object() – Angelo Fuchs Jul 19 '14 at 12:22
5

@AngeloNeuschitzer - But if you do `new Object()` you can't cast that to anything else. – Hot Licks Jul 19 '14 at 12:23
Maybe it should be `T t = new Object()` – Tony Sep 19 '14 at 11:23
You will have `ClassCastException` when using t. Example: if you have a `T get()` method, and `GenericClass a`: `Foobar foobar = a.get()` will fail because the runtime type would be `Object`, and the expected type `Foobar` (on the left). – NoDataFound Sep 19 '14 at 12:03

score 1 · Answer 2 · answered Jul 19 '14 at 12:22

1

In Java, generics is implemented using type erasure which means generic type information is erased at compile time. This means that the information for constructing the object isn't available at runtime.

answered Jul 19 '14 at 12:22

svenslaggare

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In Java, why can't you use a constructor on generic classes?

2 Answers2