A stream seems superficially similar to a collection, allowing you to transform and retrieve
data. But there are significant differences:
1. A stream does not store its elements. They may be stored in an underlying collection
or generated on demand.
2. Stream operations don’t mutate their source. For example, the filter method does not
remove elements from a new stream, but it yields a new stream in which they are not present.
3. Stream operations are
lazy
when possible. This means they are not executed until their
result is needed. For example, if you only ask for the first five long words instead of
all, the filter method will stop filtering after the fifth match. As a consequence, you
can even have infinite streams!
Let us have another look at the example. The stream and parallelStream methods yield a
stream
for the words list. The filter method returns another stream that contains only the words
of length greater than twelve. The count method reduces that stream to a result.
This workflow is typical when you work with streams. You set up a pipeline of operations
in three stages:
1. Create a stream.
2. Specify
intermediate operations
for transforming the initial stream into others,
possibly in multiple steps.
3. Apply a
terminal operation
to produce a result. This operation forces the execution of
the lazy operations that precede it. Afterwards, the stream can no longer be used.
In the example in Listing 1.1, the stream is created with the stream or parallelStream method.
The filter method transforms it, and count is the terminal operation.
In the next section, you will see how to create a stream. The subsequent three sections
deal with stream transformations. They are followed by five sections on terminal operations.
Listing 1.1 streams/CountLongWords.java
1 package streams;
2
3 import java.io.IOException;
4 import java.nio.charset.StandardCharsets;
5 import java.nio.file.Files;
6 import java.nio.file.Paths;
7 import java.util.Arrays;
8 import java.util.List;
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