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Wildcard Types

In the Java API, you might see type argument notation that uses a question mark, like this method from the Collections class:

reverse(List<?> list)

The question mark is called the wildcard type. A wildcard represents some type, but one that is not known at compile time. In this case, it means that the reverse method can accept a List of any type. It might be a List of Integer, for example, or of String.

You might think that List<?> is the same as List<Object>. It is not. If the reverse method had been defined as accepting type List<Object> then the compiler would not allow a List<Integer>, for example, to be passed to the method. When the method is defined with List<?> you can pass a List<Integer>, a List<String>, or a list of any type and they all work.

Constraining a Type with a Bound

It is also possible to constrain the wildcard type with an upper or lower bound (but not both). To illustrate how to define a bound, look at the following classes from the JDK API:

Hierarchy of some of the Number classes.

[PENDING: This quick and dirty oversized graphic will be replaced by a polished one from the art department.]

As the figure shows, Object is a supertype of Number, which is a supertype of Integer, Long and Float (to name three of Number's subtypes).

Here is the syntax for constraining a wildcard type with a bound:

• super className
  The type is constrained with a lower bound. In the case of "List<? super Number>" the List must contain either Numbers or Objects.

• extends className
  The type is constrained with an upper bound. In the case of "List<? extends Number>" the List must contain Numbers, Integers, Longs, Floats or one of the other subtypes of Number.

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