Java Stream API Tricks

Java 10: Top 10 Stream API Tricks

Java 10 introduced several enhancements to the Stream API which is part of the java.util.stream package. Here are the top 10 tricks you can use to get the most out of the Stream API.

1. Collecting to Unmodifiable List

In Java 10, you can collect a stream of elements into an unmodifiable list. This is useful when you want to ensure that the data in your list remains constant and cannot be modified.

List<String> list = Stream.of("Java", "Python", "C++")
                           .collect(Collectors.toUnmodifiableList());

2. Take While

The takeWhile method is a short-circuiting stateless intermediate operation. It stops processing once the predicate returns false. This is useful when you want to limit the stream to elements that match a given condition.

Stream.of(1, 2, 3, 4, 5)
      .takeWhile(n -> n < 4)
      .forEach(System.out::println);

3. Drop While

The dropWhile method is also a short-circuiting stateless intermediate operation. It drops elements until the predicate returns false. This is useful when you want to ignore the elements that match a given condition.

Stream.of(1, 2, 3, 4, 5)
      .dropWhile(n -> n < 4)
      .forEach(System.out::println);

4. Iterating Streams

In Java 10, the iterate method has been overloaded to take a predicate (condition). This allows you to create a stream that grows values on demand basis until the predicate returns false.

Stream.iterate(0, i -> i < 10, i -> i + 1)
      .forEach(System.out::println);

5. Optional to Stream

Java 10 introduced a new method stream in the Optional class. This method returns a sequential Stream containing only that element, otherwise returns an empty Stream.

Optional.of("Hello")
        .stream()
        .map(String::toUpperCase)
        .forEach(System.out::println);

6. Collecting to a Map

You can collect a stream of elements into a map. This is useful when you want to transform your data into a key-value pair.

Map<Integer, String> map = Stream.of("Java", "Python", "C++")
                                 .collect(Collectors.toMap(String::length, Function.identity()));

7. Flat Mapping Optionals

The flatMap method can be used with Optional::stream to avoid empty values. This is useful when you want to ignore the empty Optional values in your stream.

Stream.of(Optional.of("Java"), Optional.empty(), Optional.of("Python"))
      .flatMap(Optional::stream)
      .forEach(System.out::println);

8. Grouping by with downstream collector

You can group elements of the stream by a classifier function and store results in a Map. This is useful when you want to group your data based on a certain condition or value.

Map<Integer, List<String>> map = Stream.of("Java", "Python", "C++")
                                       .collect(Collectors.groupingBy(String::length));

9. Reducing Stream

The reduce method combines elements of a stream into a single result. This is useful when you want to aggregate your data.

Optional<String> reduced = Stream.of("J", "a", "v", "a")
                                 .reduce((s1, s2) -> s1 + s2);

10. Parallel Stream

The parallel method allows you to process elements of a stream in parallel. This is useful when you want to leverage multi-core processors to process your data faster.

Stream.of("Java", "Python", "C++")
      .parallel()
      .forEach(System.out::println);

These are just a few of the many powerful features available in the Stream API.