Is it possible to instantiate an object of generic type in Java or C# ?

Question

For example, in Java, if I have a parameterized class, is it possible to do the following?

    public class MyClass<T>{
       public void aMethod {
       ..
       T object = new T();
       ..
       }
       ..

    }

In Java, I think it is not possible, because the compiler doesn't know what constructor to call. But in C#? I don't know the language and I've read that many dark things can be done, so I was just wondering..

Answer 1

Yes, it's possible. You can use Activator.CreateInstance<T>();

From MSDN:

Activator.CreateInstance(T) Method
Creates an instance of the type designated by the specified generic type parameter, using the parameterless constructor.

Answer 2

this is possible in C# if you are setting constraint on T type to have parameterless constructor.

public class MyClass<T> where T:new(){
   public void aMethod {
   ..
   T object = new T();
   ..
   }
   ..

}

See here.

Answer 3

Like Donut said, C# allows you to instantiate arbitrary types at runtime using Activator.CreateInstance which works a bit like Class<?>.newInstance in Java. Since C#’s Generics are preserved after compilation, C#, unlike Java, allows you to get hold of a Type instance for a given type parameter and thus call CreateInstance or any other constructor.

For the parameterless constructor, this is considerably easier, using a generic constraint:

void <T> T createT() where T : new()
{
    return new T();
}

The key here is where T : new().

In the general case, using reflection, you can use the following:

void <T> T createT()
{
    var typeOfT = typeof(T);
    return (T) Activator.CreateInstance(typeOfT, new object[] { arg1, arg2 … });
}

Answer 4

In c# you can use the where keyword

private class MyClass<T> where T : new()
{
  private void AMethod()
  {
   T myVariable = new T();
  }
}

Answer 5

To summarize, in java there is no way to instantiate a generic type T. In C#, there are several ways.

Am I understanding this correctly?

Answer 6

You can do this in Java via reflection:

public class Example {
    public static void main(String[] args) throws Exception {
        String s = create(String.class);
    }

    // Method that creates a new T
    public static <T> T create(Class<T> c) throws InstantiationException, IllegalAccessException {
        return c.newInstance();
    }
}

Answer 7

One of the workarounds in Java that's less ugly: Javassist.