Expressing dependencies in type parameters : Wildcards vs bounded type parameters

This article explains When should I use generic methods, and when should I use wildcard types?
To understand the answer, let’s examine a few methods from the Collection libraries. It has to methods :
containAll and addAll .

Using wildcard:

interface Collection<E> {
   public boolean containsAll(Collection<?> c);
   public boolean addAll(Collection<? extends E> c);

}

Using Bounded type parameters
We could have used generic methods here instead:

interface Collection<E> {
   public <T> boolean containsAll(Collection<T> c);
  public <T extends E> boolean addAll(Collection<T> c);
   // hey, type variables can have bounds too!
}

However, in both containsAll and addAll, the type parameter T is used only once.
The return type doesn’t depend on the type parameter, nor does any other argument to the method (in this case, there simply is only one argument).

This tells us that the type argument is being used for polymorphism; its only effect is to allow a variety of actual argument types to be used at different invocation sites. If that is the case, one should use wildcards. Wildcards are designed to support flexible subtyping, which is what we’re trying to express here.

Generic methods allow type parameters to be used to express dependencies among the types of one or more arguments to a method and/or its return type. If there isn’t such a dependency, a generic method should not be used.


It is possible to use both generic methods and wildcards in tandem. Here is the method Collections.copy():

class Collections {
    public static <T> void copy(List<T> dest, List<? extends T> src){...}
}

Note the dependency between the types of the two parameters. Any object copied from the source list, src, must be assignable to the element type T of the destination list, dst. So the element type of src can be any subtype of T - we don’t care which. The signature of copy expresses the dependency using a type parameter, but uses a wildcard for the element type of the second parameter.

We could have written the signature for this method another way, without using wildcards at all:

class Collections {
     public static <T, S extends T>
     void copy(List<T> dest, List<S> src){...}
}

This is fine, but while the first type parameter is used both in the type of dst and in the bound of the second type parameter, S, S itself is only used once, in the type of src - nothing else depends on it. This is a sign that we can replace S with a wildcard.

Using wildcards is clearer and more concise than declaring explicit type parameters, and should therefore be preferred whenever possible. Wildcards also have the advantage that they can be used outside of method signatures, as the types of fields, local variables and arrays.
Here is an example on this. Returning to our shape drawing problem, suppose we want to keep a history of
drawing requests. We can maintain the history in a static variable inside class Shape, and have drawAll() store its incoming argument into the history field.

static List<List<? extends Shape>> history = 
                    new ArrayList<List<? extends Shape>>();
    public void drawAll(List<? extends Shape> shapes) {

       history.addLast(shapes);
      for (Shape s: shapes) {
           s.draw(this);
    }

}