Java8中Lambda表达式使用和Stream API知识点详解

private List<People> peoples = null;

@BeforeEach void before () {
  peoples = new ArrayList<>();
  peoples.add(new People("K.O1", 21, new Date()));
  peoples.add(new People("K.O3", 23, new Date()));
  peoples.add(new People("K.O4", 24, new Date()));
  peoples.add(new People("K.O5", 25, new Date()));
  peoples.add(new People("K.O2", 22, new Date()));
  peoples.add(new People("K.O6", 26, new Date()));
}

/**
* 提取1列
*/
@Test void whenExtractColumnSuccess () {
  //第一种写法
  List<Integer> ages1 = peoples.stream().map(people -> people.getAge()).collect(Collectors.toList());
  System.out.println("###println: args1----");
  ages1.forEach(System.out::println);

  //简单一点的写法
  List<Integer> ages2 = peoples.stream().map(People::getAge).collect(Collectors.toList());
  System.out.println("###println: args2----");
  ages1.forEach(System.out::println);
}

/**
  * 只要年纪大于25岁的人
  */
@Test void whenFilterAgeGT25Success () {
  List<People> peoples1 = peoples.stream().filter(x -> x.getAge() > 25).collect(Collectors.toList());
  peoples1.forEach(x -> System.out.println(x.toString()));
}

/**
  * 求和全部年纪
  */
@Test void sumAllPeopleAgeSuccess () {
  Integer sum1 = peoples.stream().collect(Collectors.summingInt(People::getAge));
  System.out.println("###sum1: " + sum1);
  Integer sum2 = peoples.stream().mapToInt(People::getAge).sum();
  System.out.println("###sum2: " + sum2);
}

/**
  * 取出年纪为25岁的人
  */
@Test void extractAgeEQ25Success () {
  Optional<People> optionalPeople = peoples.stream().filter(x -> x.getAge() == 25).findFirst();
  if (optionalPeople.isPresent()) System.out.println("###name1: " + optionalPeople.get().getName());

  //简写
  peoples.stream().filter(x -> x.getAge() == 25).findFirst().ifPresent(x -> System.out.println("###name2: " + x.getName()));
}

/**
  * 逗号拼接全部名字
  */
@Test void printAllNameSuccess () {
  String names = peoples.stream().map(People::getName).collect(Collectors.joining(","));
  System.out.println(names);
}

/**
  * 将集合转成(name, age) 的map
  */
@Test void list2MapSuccess () {
  Map<String, Integer> map1 = peoples.stream().collect(Collectors.toMap(People::getName, People::getAge));
  map1.forEach((k, v) -> System.out.println(k + ":" + v));

  System.out.println("--------");

  //(name object)
  Map<String, People> map2 = peoples.stream().collect(Collectors.toMap(People::getName, People::getThis));
  map2.forEach((k, v) -> System.out.println(k + ":" + v.toString()));
}

//People中自己实现的方法
public People getThis () {
  return this;
}

/**
  * 按名字分组
  */
@Test void listGroupByNameSuccess() {
  //添加一个元素方便看效果
  peoples.add(new People("K.O1", 29, new Date()));
  Map<String, List<People>> map = peoples.stream().collect(Collectors.groupingBy(People::getName));

  map.forEach((k, v) -> System.out.println(k + ":" + v.size()));
}

/**
  * 求人平均年龄
  */
@Test void averagingAgeSuccess () {
  Double avgAge = peoples.stream().collect(Collectors.averagingInt(People::getAge));
  System.out.println(avgAge);
}

/**
  * 按年龄排序
  */
@Test void sortByAgeSuccess () {
  System.out.println("###排序前---");
  peoples.forEach(x -> System.out.println(x.getAge()));

  peoples.sort((x, y) -> {
    if (x.getAge() > y.getAge()) {
      return 1;
    } else if (x.getAge() == y.getAge()) {
      return 0;
    }
    return -1;
  });

  System.out.println("###排序后---");
  peoples.forEach(x -> System.out.println(x.getAge()));
}