java8 Stream API之reduce使用说明

  @Test
  public void testSum() {
    List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);

    // 没有起始值时返回为Optional类型
    Optional<Integer> sumOptional = integers.stream().reduce(Integer::sum);
    System.out.println(sumOptional.get());

    // 可以给一个起始种子值
    Integer sumReduce = integers.stream().reduce(0, Integer::sum);
    System.out.println(sumReduce);

    //直接用sum方法
    Integer sum = integers.stream().mapToInt(i -> i).sum();
    System.out.println(sum);
  }

@Test
  public void testConcat() {
    //构造字符串流
    List<String> strs = Arrays.asList("H", "E", "L", "L", "O");

    // reduce
    String concatReduce = strs.stream().reduce("", String::concat);
    System.out.println(concatReduce);
  }

  @Test
  public void testMin() {

    //min reduce
    Stream<Integer> integerStream = Stream.of(1, 2, 3, 4, 5);
    Integer minReduce = integerStream.reduce(Integer.MAX_VALUE, Integer::min);
    System.out.println(minReduce);


    // min
    Stream<Integer> integerStream1 = Stream.of(1, 2, 3, 4, 5);
    OptionalInt min = integerStream1.mapToInt(i -> i).min();
    System.out.println(min.getAsInt());
  }

  @Test
  public void testMax() {

    //max reduce
    Stream<Integer> integerStream = Stream.of(1, 2, 3, 4, 5);
    Integer maxReduce = integerStream.reduce(Integer.MIN_VALUE, Integer::max);
    System.out.println(maxReduce);


    // max
    Stream<Integer> integerStream1 = Stream.of(1, 2, 3, 4, 5);
    OptionalInt max = integerStream1.mapToInt(i -> i).max();
    System.out.println(max.getAsInt());
  }