Modern Java Recipes

Chapter 2. The java.util.function Package

The previous chapter discussed the basic syntax of lambda expressions and method references. One basic principle is that for either, there is always a context. Lambda expressions and method references are always assigned to functional interfaces, which provide information about the single abstract method being implemented.

While many interfaces in the Java standard library contain only a single, abstract method and are thus functional interfaces, there is a new package that is specifically designed to contain only functional interfaces that are reused in the rest of the library. That package is called java.util.function.

The interfaces in java.util.function fall into four categories: (1) consumers, (2) suppliers, (3) predicates, and (4) functions. Consumers take a generic argument and return nothing. Suppliers take no arguments and return a value. Predicates take an argument and return a boolean. Functions take a single argument and return a value.

For each of the basic interfaces, there are several related ones. For example, Consumer has variations customized for primitive types (IntConsumer, LongConsumer, and Dou⁠bleConsumer) and a variation (BiConsumer) that takes two arguments and returns void.

Although by definition the interfaces in this chapter only contain a single abstract method, most also include additional methods that are either static or default. Becoming familiar with these methods will make your job as a developer easier. ...