create instance of generic type in java

Tags: java generics
Question!

Is it possible to create an instance of a generic type in Java? I'm thinking based on what I've seen that the answer is no (due to type erasure), but I'd be interested if anyone can see something I'm missing:

class SomeContainer<E>
{
    E createContents()
    {
        return what???
    }
}

EDIT: It turns out that Super Type Tokens could be used to resolve my issue, but it requires a lot of reflection-based code, as some of the answers below have indicated.

I'll leave this open for a little while to see if anyone comes up with anything dramatically different than Ian Robertson's Artima Article.



Answers

You can do this now and it doesn't require a bunch of reflection code.

import com.google.common.reflect.TypeToken;

public class Q26289147
{
    public static void main(final String[] args) throws IllegalAccessException, InstantiationException
    {
        final StrawManParameterizedClass<String> smpc = new StrawManParameterizedClass<String>() {};
        final String string = (String) smpc.type.getRawType().newInstance();
        System.out.format("string = \"%s\"",string);
    }

    static abstract class StrawManParameterizedClass<T>
    {
        final TypeToken<T> type = new TypeToken<T>(getClass()) {};
    }
}

Of course if you need to call the constructor that will require some reflection, but that is very well documented, this trick isn't!

Here is the JavaDoc for TypeToken.



In Java 8 you can use the Supplier functional interface to achieve this pretty easily:

class SomeContainer<E> {
  private Supplier<E> supplier;

  SomeContainer(Supplier<E> supplier) {
    this.supplier = supplier;
  }

  E createContents() {
    return supplier.get();
  }
}

You would construct this class like this:

SomeContainer<String> stringContainer = new SomeContainer<>(String::new);

The syntax String::new on that line is a constructor reference.

If your constructor takes arguments you can use a lambda expression instead:

SomeContainer<BigInteger> bigIntegerContainer
    = new SomeContainer<>(() -> new BigInteger(1));


Java unfortunatly does not allow what you want to do. See http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#createObjects

You cannot create an instance of a type parameter. For example, the following code causes a compile-time error:

public static <E> void append(List<E> list) {
    E elem = new E();  // compile-time error
    list.add(elem);
}

As a workaround, you can create an object of a type parameter through reflection:

public static <E> void append(List<E> list, Class<E> cls) throws Exception {
    E elem = cls.newInstance();   // OK
    list.add(elem);
}

You can invoke the append method as follows:

List<String> ls = new ArrayList<>();
append(ls, String.class);


This video can help you solving your question :)
By: admin