Jan Amoyo

on software development (possibly other stuff)

Examples of Streams and Lamdas in Java 8

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Below is a code showcasing Streams and Lamdas in Java 8 (written as a JUnit test allow execution of individual methods).
@RunWith(BlockJUnit4ClassRunner.class)
public class Java8Showcase {

  @FunctionalInterface
  public static interface Concat {
    String concat(String str1, String str2);
  }

  @Test
  public void functional_interfaces() {
    // Traditional way of declaring an
    // interface implementation
    @SuppressWarnings("unused")
    Concat _concat = new Concat() {
      @Override
      public String concat(String str1, String str2) {
        return str1.concat(str2);
      }
    };

    Concat contact = (str1, str2) -> str1.concat(str2);
    System.out.println(contact.concat("hello", " world"));
  }

  @Test
  public void function_reference() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    // nums.stream().reduce((a, b) -> Math.max(a, b))
    Integer max = nums.stream().reduce(Math::max).get();

    System.out.println(max);
  }

  @Test
  public void streams_filter() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    List<Integer> evens = nums.stream().filter((n) -> n % 2 == 0)
      .collect(Collectors.toList());

    System.out.println(evens);
  }

  @Test
  public void streams_map() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    List<Integer> x2 = nums.stream().map((n) -> n * 2)
      .collect(Collectors.toList());

    System.out.println(x2);
  }

  @Test
  public void streams_reduce() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Integer sum = nums.stream().reduce((a, b) -> a + b).get();

    System.out.println(sum);
  }

  @Test
  public void streams_fluent() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Integer sum = nums.stream()
        .filter((n) -> n % 2 == 0) // filter the even numbers
        .map((n) -> n * 2)         // multiply each number by 2
        .reduce((a, b) -> a + b)   // add each number
        .get();

    System.out.println(sum);
  }

  @Test
  public void streams_parallel() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Integer sum = nums.parallelStream().reduce((a, b) -> a + b).get();

    System.out.println(sum);
  }

  @Test
  public void streams_forEach() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    nums.stream().forEach(System.out::println);
  }

  @Test
  public void streams_collectors() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Double ave = nums.stream().collect(Collectors.averagingInt((i) -> i));

    System.out.println(ave);
  }

  @Test
  public void streams_collectors_grouping1() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Map<Boolean, List<Integer>> oddEven = nums.stream()
      .collect(Collectors.groupingBy((i) -> i % 2 == 0));

    System.out.println(oddEven);
  }

  @Test
  public void streams_collectors_grouping2() {
    List<Integer> nums = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    Map<Boolean, Integer> oddEvenSum = nums.stream()
      .collect(Collectors.groupingBy((i) -> i % 2 == 0, Collectors.summingInt((Integer i) -> i)));

    System.out.println(oddEvenSum);
  }
}

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