In this section, we will examine the underlying concept of functions and how they are implemented in Java 8. This includes high-order, first-class, and pure functions.

A first-class function is a function that can be used where other first-class entities can be used. These types of entities include primitive data types and objects. Typically, they can be passed to and returned from functions and methods. In addition, they can be assigned to variables.

A high-order function either takes another function as an argument or returns a function as the return value. Languages that support this type of function are more flexible. They allow a more natural flow and composition of operations. The use of composition is explored in Chapter 3, Function Composition and Fluent Interfaces.

Pure functions have no side effects. The function does not modify nonlocal variables and does not perform I/O.

In this section, we will explore the syntax of lambda expression in more depth. So far, we used them without formally describing them. We will also examine other forms they can take.

As mentioned earlier, a lambda expression is essentially an anonymous function. They can be passed to another function or method, returned from a function or a method, and assigned to variables.

A lambda expression consists of an optional parameter list, followed by the lambda operator, and then a body. The lambda operator is a dash followed by the greater than symbol. The body of a lambda expression may be one or more statements and may optionally return a value.

Let's examine several variations of a simple lambda expression. A single value is passed to the function. This value is incremented and then returned. Several equivalent variations of this function are illustrated in the following table:

x -> x + 1

(x) -&gt...

We used several functional interfaces in the previous examples. In this section, we will examine in more detail how they are created and illustrate a number of predefined functional interfaces available for immediate use in Java 8.

As mentioned earlier, a functional interface is an interface that has one and only one abstract method. It may have zero or more default methods. Since the interface has only one abstract method, the system is able to know which method to match to a lambda expression. This abstract method is called the functional method.

@FunctionalInterface
public interface IntegerConcatenation {
    public String concatenate(Integer n1, Integer n2);
}

In this chapter, we investigated the use of lambda expressions and how they support the functional style of programming in Java 8. When possible, we used examples to contrast the use of classes and methods against the use of functions. This frequently led to simpler and more maintainable functional implementations.

We illustrated how lambda expressions support the functional concepts of high-order, first-class, and pure functions. Examples were used to help clarify the concept of referential transparency. The concepts of closure and currying are found in most functional programming languages. We provide examples of how they are supported in Java 8.

Lambda expressions have a specific syntax, which we examined in more detail. Also, there are several ways of expressing a la­mbda expression which we illustrated. Lambda expressions are based on functional interfaces using type inference. It is important to understand how to create functional interfaces and to know what standard functional...