Streams

 Streams

    Java 8 introduces a new streaming metaphor to support functional programming. A stream is a sequence of elements that does not save the elements or modify the original source. Functional programming in Java often involves generating a stream from some source of data, passing the elements through a series of intermediate operations (called a pipeline), and completing the process with a terminal expression

Streams can only be used once. After a stream has passed through zero or more intermediate operations and reached a terminal operation, it is finished. To process the values again, you need to make a new stream.

Streams are also lazy. A stream will only process as much data as is necessary to reach the terminal condition. Recipe 3.13 shows this in action.

3.1 Creating Streams

Problem

You want to create a stream from a source of data.

Solution

Use the static factory methods in the Stream interface, or the stream methods on Iterable or Arrays.

Discussion

The new java.util.stream.Stream interface in Java 8 provides several static methods for creating streams. Specifically, you can use the static methods Stream.of, Stream.iterate, and Stream.generate.

The Stream.of method takes a variable argument list of elements:

static <T> Stream<T> of(T... values)

Example 3-1. Reference implementation of Stream.of

@SafeVarargs
public static<T> Stream<T> of(T... values) {
    return Arrays.stream(values);
}

TIP

The @SafeVarargs annotation is part of Java generics. It comes up when you have an array as an argument, because it is possible to assign a typed array to an Object array and then violate type safety with an added element. The @SafeVarargs annotation tells the compiler that the developer promises not to do that. See Appendix A for additional details.