Learning Java, 4th Edition

Generic Methods

Thus far in this chapter, we’ve talked about generic types and the implementation of generic classes. Now, we’re going to look at a different kind of generic animal: generic methods. Generic methods essentially do for individual methods what type parameters do for generic classes. But as we’ll see, generic methods are smarter and can figure out their parameter types from their usage context without having to be explicitly parameterized. (In reality, of course, it is the compiler that does this.) Generic methods can appear in any class (not just generic classes) and are very useful for a wide variety of applications.

First, let’s quickly review the way that we’ve seen regular methods interact with generic types. We’ve seen that generic classes can contain methods that use type variables in their arguments and return types in order to adapt themselves to the parameterization of the class. We’ve also mentioned that generic types themselves can be used in most of the places that any other type can be used. So methods of generic or nongeneric classes can use generic types as argument and return types as well. Here are examples of those usages:

    // Not generic methods

    class GenericClass< T > {
         // method using generic class parameter type
        public void T cache( T entry ) { ... }
    }
    class RegularClass {
         // method using concrete generic type
        public List<Date> sortDates( List<Date> dates ) { ... }
        // method using wildcard generic type
        public List<?> reverse( List<?> dates ) {