Java Generics and Collections, 2nd Edition

Chapter 2. Subtyping and Wildcards

Now that we’ve covered the basics, we can start to explore more advanced features of generics, such as subtyping and wildcards. In this chapter, we’ll review how subtyping works and see how wildcards let us use subtyping in connection with generics. The Java Collections Framework will serve as the source of our examples; see Part II for details on specific features of the Collections API.

Note

The code examples for this chapter can be found at:

https://github.com/MauriceNaftalin/JGC_2e_Book_Code/tree/main/src/main/java/org/jgcbook/chapter02

Subtyping and the Substitution Principle

Subtyping is a key feature of object-oriented languages such as Java. In Java, one type is a subtype of another if they are related by an extends or implements clause. Here are some examples:

Integer	is a subtype of	Number
Double	is a subtype of	Number
ArrayList<E>	is a subtype of	List<E>
List<E>	is a subtype of	Collection<E>
Collection<E>	is a subtype of	Iterable<E>

Subtyping is transitive, meaning that if one type is a subtype of a second, and the second is a subtype of a third, then the first is also a subtype of the third. So from the last two lines in the preceding list, it follows that List<E> is a subtype of Iterable<E>. If one type is a subtype of another, we also say that the second is a supertype of the first. Subtyping is reflexive, meaning that every type is a subtype—​and therefore also a supertype—​of itself. Every reference type is a ...