Java8中Lambda表达式使用和Stream API详解

前言

Java8 的新特性：Lambda表达式、强大的 Stream API、全新时间日期 API、ConcurrentHashMap、MetaSpace。总得来说，Java8 的新特性使 Java 的运行速度更快、代码更少、便于并行、最大化减少空指针异常。

0x00. 前置数据

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()));
}

0x01. 提取对象中的一列

/**
* 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);
}

###println: args1----
21
22
23
24
25
26
###println: args2----
21
22
23
24
25
26

/**
  * 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()));
}

People{name='K.O6', age=26, birthday=Wed May 15 22:20:22 CST 2019}

0x03. 列表中对象数值型列数据求和

/**
  * 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);
}

###sum1: 141
    ###sum2: 141

0x04. 取出集合符合条件的第一个元素

/**
  * 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()));
}

###name1: K.O5
###name2: K.O5

0x05. 对集合中对象字符列按规则拼接

/**
  * 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);
}

K.O1,K.O2,K.O3,K.O4,K.O5,K.O6

0x06. 将集合元素提取，转为Map

/**
  * 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;
}

K.O2:22
K.O3:23
K.O1:21
K.O6:26
K.O4:24
K.O5:25
--------
K.O2:People{name='K.O2', age=22, birthday=Wed May 15 22:42:39 CST 2019}
K.O3:People{name='K.O3', age=23, birthday=Wed May 15 22:42:39 CST 2019}
K.O1:People{name='K.O1', age=21, birthday=Wed May 15 22:42:39 CST 2019}
K.O6:People{name='K.O6', age=26, birthday=Wed May 15 22:42:39 CST 2019}
K.O4:People{name='K.O4', age=24, birthday=Wed May 15 22:42:39 CST 2019}
K.O5:People{name='K.O5', age=25, birthday=Wed May 15 22:42:39 CST 2019}

0x07. 按集合某一属性进行分组

/**
  * 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()));
}

K.O2:1
K.O3:1
K.O1:2
K.O6:1
K.O4:1
K.O5:1

0x08. 求集合对象数值列平均数

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

23.5

0x09. 对集合按某一列排序

/**
  * 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()));
}

###排序前---
21
23
24
25
22
26
###排序后---
21
22
23
24
25
26

未完待续

<源码地址：https://github.com/cos2a/learning-repo/tree/master/core-java8>

总结

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