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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