Java8 Stream流如何操作集合，一文带你了解！

• 一、基本介绍
• 二、常用集合操作
• 三、操作符
• • 3.1 中间操作符
  • 3.2 终止操作符

一、基本介绍

• 概念：Stream是Java8 API的新成员，它允许以声明性的方式处理数据集合。
• 特点：1.代码简明：函数式编程写出的代码简洁且意图明确，是用stream接口让你从此告别for循环。2.多核友好：Java函数式编程使得编写并运行程序更简单，需要的都是调用方法
• 流程：1.把集合转换为stream 2.操作stream:stream流在管道中经过中间操作符的处理，最后由最终操作得到前面处理的结果

二、常用集合操作

-- 取出集合中对象的指定值-不去重
List ageList=userList.stream().map(User::getAge).collect(Collectors.toList());

-- 取出集合中对象的指定值-并去重
List ageDistinct=userList.stream().map(User::getAge).collect(Collectors.toList())
.stream().distinct().collect(Collectors.toList());

-- 取出对象中字段等于指定值的 数据
List age12List=userList.stream().filter(user ->user.getAge().equals("12")).collect(Collectors.toList());

-- 以对象中的指定值分组
Map> ageMap=userList.stream().collect(Collectors.groupingBy(User::getAge));

-- 以指定字段排序
List sortAscUserList = userList.stream().sorted(Comparator.comparing(User::getAge)).collect(Collectors.toList());
List sortDescUserList = userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());

-- 取出对象指定字段的最大值
int maxAge = userList.stream().mapToInt(user -> Integer.valueOf(user.getAge())).max().getAsInt();

-- List集合中Map对象
List> userList=new ArrayList<>();
Map user1=new HashMap<>();
user1.put("name","A");user1.put("age","12");userList.add(user1);
Map user2=new HashMap<>();
user2.put("name","B");user2.put("age","13");userList.add(user2);
Map user3=new HashMap<>();
user3.put("name","B");user3.put("age","12");userList.add(user3);List ageList=userList.stream().map(user->user.get("age")).collect(Collectors.toList());

三、操作符

Java Stream的操作符可以分为中间操作符合终止操作符两种

3.1 中间操作符

-- filter 
List list= Arrays.asList("abc","ab","","","fg",null);
List notNullList = list.stream().filter(str -> StringUtils.isNotBlank(str)).collect(Collectors.toList());
System.out.println(notNullList);
--输出[abc, ab, fg]List containsList = list.stream().filter(str -> StringUtils.isNotBlank(str) && str.contains("a")).collect(Collectors.toList());
System.out.println(containsList);
--输出[abc, ab]

-- distinct
List list= Arrays.asList("abc","ab","ab");
List distinctList = list.stream().distinct().collect(Collectors.toList());
System.out.println(distinctList);
--输出[abc, ab]

-- limit
List list= Arrays.asList("abc","ab","ab");
List limitList = list.stream().limit(2).collect(Collectors.toList());
System.out.println(limitList);
--输出[abc, ab]

-- skip
List list= Arrays.asList("abc","ab","ab");
List limitList = list.stream().skip(2).collect(Collectors.toList());
System.out.println(limitList);
-- 输出 [ab]

-- sort
List list= Arrays.asList("1","3","2");
List sortList = list.stream().sorted().collect(Collectors.toList());
System.out.println(sortList);
--输出[1, 2, 3]

-- map
List list= Arrays.asList("abc","ab","ab");
List mapList = list.stream().map(str->"case_"+str).collect(Collectors.toList());
System.out.println(mapList);
--输出[case_abc, case_ab, case_ab]

3.2 终止操作符

-- anyMatch
List list= Arrays.asList("abc","ab","","","fg",null);
boolean flag = list.stream().anyMatch(str -> str == null);
System.out.println(flag);
-- 输出 true

-- allMatch
List list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().allMatch(str -> str != null));
-- 输出 true

-- noneMatch
List list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().noneMatch(str -> str == null));
-- 输出 true

-- findFirst
List list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().findFirst().get());
-- 输出 abc

-- collect
List list= Arrays.asList("abc","ab","","","fg");
List collectList = list.stream().collect(Collectors.toList());
System.out.println(collectList);
-- 输出 [abc, ab, , , fg]

-- count
List list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().count());
-- 输出5
System.out.println(list.stream().filter(str-> StringUtils.isNotBlank(str)).count());
-- 输出3