Java8-Lambda表达式使用与Stream API

摘要：前置数据提取对象中的一列提取列第一种写法简单一点的写法通过字段中条件过滤集合列表只要年纪大于岁的人列表中对象数值型列数据求和求和全部年纪取出集合符合条件的第一个元素取出年纪为岁的人简写对集合中对象

private List 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 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 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 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}

/**
    * 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

/**
    * 取出年纪为25岁的人
    */
@Test void extractAgeEQ25Success () {
    Optional 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

/**
    * 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

/**
    * 将集合转成(name, age) 的map
    */
@Test void list2MapSuccess () {
    Map 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 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}

/**
    * 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> 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

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

23.5

/**
    * 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

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