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In this post, we will cover following topics.

• What are Streams?
• What is a pipeline?
• Key points to remember for Streams.
• How to create Streams?

What are Streams?

Java 8 introduced new package java.util.stream which contains classes to perform SQL-like operations on elements. Stream is a sequence of elements on which you can perform aggregate operations (reduction, filtering, mapping, average, min, max etc.). It is not a data structure that stores elements like collection but carries values often lazily computed from source through pipeline.

What is a pipeline?

A pipeline is sequence of aggregate (reduction and terminal) operations on the source. It has following components.

• A source: Collections, Generator Function, array, I/O channel etc.
• zero or more intermediate operations: filter, map, sequential, sorted, distinct, limit, flatMap, parallel etc. Intermediate operations returns/produces stream.
• a termination operation: forEach, reduction, noneMatch, allMatch, count, findFirst, findAny, min, max etc.

Key points to remember for Streams

• No storage.
• Functional in nature.
• Laziness-seeking.
• Possibly unbounded. Operations, for example, limit(n) or findFirst() can permit calculations on infinite streams to finish in finite time.
• Consumable. The elements can be visited only once. To revisit, you need to create a new stream.

How to create Streams?

1. In Collection, you can create streams by calling stream(), parallelStream().

Collection<Person> persons = StreamSamples.getPersons();
persons.stream().forEach(System.out::println);

// parallel stream
persons.parallelStream().forEach(System.out::println);

2. From Stream interface, calling static factory method of() which takes varargs of T type.

Stream.of("This", "is", "how", "you", "create", "stream", "from", "static", "factory",
      "method").map(s -> s.concat(" ")).forEach(System.out::print);

3. From Arrays class, by calling stream() static method.

Arrays.stream(new String[] { "This", "is", "how", "you", "create", "stream", ".",
      "Above", "function", "use", "this" }).map(s -> s.concat(" "))
      .forEach(System.out::print);

4. From Stream by calling iterate(). It is infinite stream function.

// iterate return infinite stream... beware of infinite streams
Stream.iterate(1, i -> i++).limit(10).forEach(System.out::print);

5. From IntStream by calling range.

int sumOfFirst10PositiveNumbers = IntStream.range(1, 10).reduce(0, Integer::sum);
System.out.println(sumOfFirst10PositiveNumbers);

6. From Random by calling ints(). It is infinite stream function.

// random.ints for random number
new Random().ints().limit(20).forEach(System.out::println);

7. From BufferedReader by calling lines(). Streams of file paths can be obtained by calling createDirectoryStream of Files class and some other classes like JarFile.stream(), BitSet.stream() etc.

try (BufferedReader br = new BufferedReader(new StringReader(myValue))) {
  br.lines().forEach(System.out::print);
  System.out.println();
}
catch (IOException io) {
  System.err.println("Got this:>>>> " + io);
}

I hope the post is informative and helpful in understanding Streams. You can find the full example code on Github.

You can also read on Aggregate opeations on Stream.