Java8中Lambda表达式使用和Stream API详解

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 peoples = null;

@BeforeEach void before () {

peoples = new ArrayList<>();

peoples.add(new People("K.O1", 21, new Date()));

peoples.add(new People("http://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 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()));

http://}

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 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 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}

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> 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>

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