Getting an instance of a parameterized class in Java

Question

I have these 2 classes

abstract class A
{
//some abstract methods and variables
}

class B<E extends A>
{
}

Now in a method of B, I want to get a dummy instance of E. How to do that? Is this ok -

E temp = (E)(new Object());

I have control over the class definitions and hence they're flexible.

score 3 · Accepted Answer · answered Jun 21 '13 at 13:54

You will need to pass in a factory object (or an instance) into the constructor for B. There is no way of specifying that a generic type parameter has a specific constructor, and the static type is not available at runtime.

(You could fake a distinguished value with E temp = (E)new A() { };, but that is well dodgy. Some of the collection code does something similar only with arrays.)

Arnaud Denoyelle · Answer 2 · 2013-06-21T14:21:41.313

You have 2 solutions :

If E has a no-arg constructor, you can use reflection to build one.
You can make an abstract method abstract E buildE();

For the first solution, you will have to pass the class as a parameter of your constructor :

class B<E extends A>{

  public B(Class<E> clazz){
    //Create instance of E:
    E e=clazz.newInstance();
  }
}

For the second solution, it means that class B has an abstract method to build the object (it is more or less equivalent to passing a factory):

public class B<E extends A>{

  public abstract E buildE();

  private void foo(){
    E e = buildE();
    //Do generic stuff with E
  }

}

In this way, if you create a sub class of B, you need to implement this method

public class B1 extends B<Bar>{

  //You are required to implement this method as it is abstract.
  public Bar buildE(){
    ..
  }

  private void anyMethod(){
    //call B.foo() which will use B1.buildE() to build a Bar
    super.foo();
  }

}

IMHO, 2nd solution is far cleaner.

Can you please elaborate on your second proposed solution? – ksb Jun 21 '13 at 14:13 — ksb, Jun 21 '13 at 14:13

Alan · Answer 3 · 2013-06-21T14:27:28.870

-1

You can use reflection in B to get Class<E>, then get the desired constructor and call newInstance

(Class<E>)((ParameterizedType)this.getClass().getGenericSuperclass()).getActualTypeArguments()[0]

edited Jun 21 '13 at 14:27

answered Jun 21 '13 at 14:16

Alan

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-1 This assumes `this` is an instance of a class extending `B` and resolving its type parameter with a concrete type, e.g. `class C extends B`. – Paul Bellora Jun 21 '13 at 16:09
@Paul Bellora The questioner asks "in a method of B, I want to get a dummy instance of E", so `this` is of course a concrete class – Alan Jun 21 '13 at 16:29
Please reread my comment to understand it. Try it with a `new B` - it will fail on `(ParameterizedType)this.getClass().getGenericSuperclass()`. Make sure to understand patterns before you use them. – Paul Bellora Jun 21 '13 at 17:44
it will fail with `new B` but not `new B` where `E` extends `A` and `A` is an abstract class the questioner creates. I'm using this code in production so wanna give an alternative suggestion. – Alan Jun 22 '13 at 03:40

Getting an instance of a parameterized class in Java

3 Answers3