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.O2", 22, 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.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 260x02. 通过字段中条件过滤集合列表
/** * 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: 1410x04. 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.O50x05. 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.O60x06. 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.O5:10x08. 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); } 23.50x09. 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>