Why {} is used in this statement for instantiation, Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();

Question

Please explain why {} used in

Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();

This is from Gson documentation link to documentation

Answer 1

It's creating a new anonymous type. This is a workaround for type erasure, because you can get the Collection<Integer> reflectively via Class.getGenericSuperclass() and Class.getGenericInterfaces() if the class is not generic (*).

This code basically implements the same functionality Ideone demo:

abstract class TypeToken<T> {
  Type getType() {
    ParameterizedType t = (ParameterizedType) getClass().getGenericSuperclass();
    return t.getActualTypeArguments()[0];
  }
}

public static void main (String[] args) {
    Collection<Integer> list = new ArrayList<>();
    System.out.println(list.getClass().getGenericInterfaces()[0]);

    TypeToken<Collection<Integer>> tt = new TypeToken<Collection<Integer>>() {};

    System.out.println(tt.getClass().getGenericSuperclass());
    System.out.println(tt.getType());
}

Output:

java.util.List<E>
Ideone.Ideone$TypeToken<java.util.Collection<java.lang.Integer>>
java.util.Collection<java.lang.Integer>

You can see that the "Integer"-ness of the Collection is lost; but it's still present in the TypeToken, so you can get it via the getType() method.

---

(*) An important point here is that you can't (correctly) create a TypeToken in a generic method. For example:

<T> TypeToken<T> getGenericToken() {
  return new TypeToken<T>() {};
}

won't work like you might expect Ideone demo.

TypeToken<Collection<String>> t = getGenericToken();
System.out.println(t.getType()); // T, not Collection<String>.