What is the use and point of unbound wildcards generics in Java?

Question

I don't understand what is the use of unbound wildcards generics. Bound wildcards generics with upper boundary <? extends Animal> makes perfect sense, because using polymorphism I can work with that type or collection. But what is the point of having generics that can be of any type? Doesn't it defeat the purpose of generics? Compiler doesn't find any conflict and after type erasure it would be like no generics was used.

Answer 1

An unbound type can be useful when your method doesn't really care about the actual type.

A primitive example would be this:

public void printStuff(Iterable<?> stuff) {
  for (Object item : stuff) {
    System.out.println(item);
  }
}

Since PrintStream.println() can handle all reference types (by calling toString()), we don't care what the actual content of that Iterable is.

And the caller can pass in a List<Number> or a Set<String> or a Collection<? extends MySpecificObject<SomeType>>.

Also note that not using generics (which is called using a raw type) at all has a quite different effect: it makes the compiler handle the entire object as if generics don't exist at all. In other words: not just the type parameter of the class is ignored, but also all generic type parameters on methods.

Another important distinctions is that you can't add any (non-null) value to a Collection<?>, but can add all objects to the raw type Collection:

This won't compile, because the type parameter of c is an unknown type (= the wildcard ?), so we can't provide a value that is guaranteed to be assignable to that (except for null, which is assignable to all reference types).

Collection<?> c = new ArrayList<String>();
c.add("foo");    // compilation error

If you leave the type parameter out (i.e. use a raw type), then you can add anything to the collection:

Collection c = new ArrayList<String>();
c.add("foo");
c.add(new Integer(300));
c.add(new Object());

Note that the compiler will warn you not to use a raw type, specifically for this reason: it removes any type checks related to generics.