Generic Type Class initiating

Question

We write a library that use generics, i am not sure how to create an instance of generic type. Shortly how to write create method?

public class Lib<V extends Base> {

    private HashMap<Integer, V> map = new HashMap<Integer, V>();    

    public V get(int key) {
        return map.get(key);
    }

    public void add(int key, V value) {
        map.put(key, value);
    }

    public void create(int key) {
        // V v = new V();  // ?? How to implement this line
        // add(key++, v);
    }
}

(You might want to remove that static field.) (Edit: Oh, I'll do it.) — Tom Hawtin - tackline, Apr 15 '13 at 08:16

score 2 · Answer 1 · edited May 23 '17 at 12:28

2

From a language point of view, there are no guarantees that V has a non-throwing, accessible, no-args constructor. The standard way to deal with this situation is to introduce an Abstract Factory as in, for instance, my answer here.

edited May 23 '17 at 12:28

Community

1
1

answered Apr 15 '13 at 08:15

Tom Hawtin - tackline

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@TomHawtintackline your approach is simply stunning! Thx for the suggestion. – Eugene Apr 15 '13 at 09:01

score 0 · Accepted Answer · edited May 23 '17 at 11:57

You cannot use new with a generic type; generics in java have type erasure. See http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead.

There are a couple of dirty ways to acquire an instance of your generic class though; for example: -

void create(Class<V> foo)
{
    foo.newInstance(); // Create a new instance of generic type V
}

Alternatively, you could create foo as a private member and do this without having to pass 'type.class' to the method whereby you pass the class in the constructor: -

class bar<V>
{
    private Class<V> baz;

    private V qux;

    public bar(Class<V> foo)
    {
        baz = foo;
    }

    void create()
    {
        qux = baz.newInstance();
    }
}

bar<Integer> quxx = new bar<Integer>(Integer.class);

As others have said, you can create a factory that will do this for you. There's another stack overflow post that has further information Create instance of generic type in Java?

FFS, don't use reflection for this. (Thankfully, Java is good enough to prevent your code compiling. — Tom Hawtin - tackline, Apr 15 '13 at 08:08
Is the down vote entirely necessary? I've already suggested that a factory can be used to do this and provided a link to a question that has an up voted answer for resolving this using a factory.. — ed_me, Apr 15 '13 at 08:20
Suggesting using reflection for this is dangerous. Yes it deserves downvotes. — Tom Hawtin - tackline, Apr 15 '13 at 08:59

score 0 · Answer 3 · answered Apr 15 '13 at 08:58

You should really consider @Tom Hawtin - tackline answer. It is the best and the approach is simply beautiful! All it takes is to add another factory implementation all the time, but it's quiet small payment.

  public class InstanceOfGenericClass {
      public static void main(String[] args) {
          new GenericClass<VPassed>(new FactoryImpl()).createInstance();
      }
  }

//------------------------------------------------------------

 class GenericClass<V>{

     private final Factory<V> factory;
     public GenericClass(Factory<V> factory){
         this.factory = factory;
     }

     V createInstance(){
         return factory.create();
     }
 }

//-------------------------------------------------------------

interface Factory<V>{
V create();

}

//-------------------------------------------------------------

 class FactoryImpl implements Factory<VPassed>{
     @Override
     public VPassed create() {
         return new VPassed();
     }
 }

//--------------------------------------------------------------

class VPassed{}

Generic Type Class initiating

3 Answers3