Source #

Collection #

A stream can be created out of a Java Collection with the instance method stream.

Example.

List<Unit> units = getUnits();
units.stream();

If the collection has a predictable order of iteration (e.g. if the collection is a List), then the elements will appear in this order in the stream.

To create a stream out of the entries of a map, we can use the instance method Map.entrySet that we have seen earlier. It returns a Set (which is a collection).

Example.

Map<String, Integer> map = getMap();
map.entrySet().stream();

Array #

A stream can be created out of a Java array with the static method Arrays.stream.

Example.

Unit[] units = getUnits();
Arrays.stream(units);

Generate a stream with indices #

An IntStream is a stream of int (see the dedicated section on streams with primitive numeric types). An IntStream can for instance be created with the static method IntStream.range, as follows:

Example.

// Contains [0, 2]
List<Unit> evenIntegers = IntStream.range(0, 4)             // produce the stream (0, 1, 2, 3)
                             .filter(i -> i % 2 == 0)       // retain even numbers
                             .toList();

This can be used to iterate over a list or array while keeping track of the elements’ positions.

Example.

Unit[] units = getUnits();
List<Unit> evenUnits = IntStream.range(0, units.length)
                          .filter(i -> i % 2 == 0)
                          .mapToObj(i -> units[i])
                          .toList();

However, for this usage, external libraries offer more natural options. For instance, the StreamEx library can create a map from index to entry in an array, and generate a stream out of the entries of this map (analogous to the set of entries produced by Map.entrySet).

Unit[] units = getUnits();
List<Unit> evenUnits = EntryStream.of(units)
                            .filterKeyValue((index, unit) -> index % 2 == 0)
                            .values()
                            .toList();
}

To use StreamEx in a Maven project, declare this dependency:

<dependencies>
  ...
  <dependency>
    <groupId>one.util</groupId>
    <artifactId>streamex</artifactId>
    <version>0.8.2</version>
    <scope>runtime</scope>
  </dependency>
  ...
</dependencies>

or in a Gradle project:

implementation group: 'one.util', name: 'streamex', version: '0.8.2'