摘要：前置數據提取對象中的一列提取列第一種寫法簡單一點的寫法通過字段中條件過濾集合列表只要年紀大于歲的人列表中對象數值型列數據求和求和全部年紀取出集合符合條件的第一個元素取出年紀為歲的人簡寫對集合中對象

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 () {
    //第一種寫法
    List ages1 = peoples.stream().map(people -> people.getAge()).collect(Collectors.toList());
    System.out.println("###println: args1----");
    ages1.forEach(System.out::println);

    //簡單一點的寫法
    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

/**
    * 只要年紀大于25歲的人
    */
@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}

/**
    * 求和全部年紀
    */
@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

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

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

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

/**
    * 逗號拼接全部名字
    */
@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

/**
    * 將集合轉成(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中自己實現的方法
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}

/**
    * 按名字分組
    */
@Test void listGroupByNameSuccess() {
    //添加一個元素方便看效果
    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

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

23.5

/**
    * 按年齡排序
    */
@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

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