Java8 Stream流如何操作集合，一文带你了解！

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<String> ageList=userList.stream().map(User::getAge).collect(Collectors.toList());
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-- 取出集合中对象的指定值-并去重
List<String> ageDistinct=userList.stream().map(User::getAge).collect(Collectors.toList())
.stream().distinct().collect(Collectors.toList());
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-- 取出对象中字段等于指定值的 数据
List<User> age12List=userList.stream().filter(user ->user.getAge().equals("12")).collect(Collectors.toList());
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-- 以对象中的指定值分组
Map<String,List<User>> ageMap=userList.stream().collect(Collectors.groupingBy(User::getAge));
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-- 以指定字段排序
List<User> sortAscUserList = userList.stream().sorted(Comparator.comparing(User::getAge)).collect(Collectors.toList());
List<User> sortDescUserList = userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());
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-- 取出对象指定字段的最大值
int maxAge = userList.stream().mapToInt(user -> Integer.valueOf(user.getAge())).max().getAsInt();
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-- List集合中Map对象
List<Map<String,String>> userList=new ArrayList<>();
Map<String,String> user1=new HashMap<>();
user1.put("name","A");user1.put("age","12");userList.add(user1);
Map<String,String> user2=new HashMap<>();
user2.put("name","B");user2.put("age","13");userList.add(user2);
Map<String,String> user3=new HashMap<>();
user3.put("name","B");user3.put("age","12");userList.add(user3);

List<String> ageList=userList.stream().map(user->user.get("age")).collect(Collectors.toList());
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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<String> list= Arrays.asList("abc","ab","","","fg",null);
List<String> notNullList = list.stream().filter(str -> StringUtils.isNotBlank(str)).collect(Collectors.toList());
System.out.println(notNullList);
--输出[abc, ab, fg]

List<String> containsList = list.stream().filter(str -> StringUtils.isNotBlank(str) && str.contains("a")).collect(Collectors.toList());
System.out.println(containsList);
--输出[abc, ab]
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-- distinct
List<String> list= Arrays.asList("abc","ab","ab");
List<String> distinctList = list.stream().distinct().collect(Collectors.toList());
System.out.println(distinctList);
--输出[abc, ab]
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-- limit
List<String> list= Arrays.asList("abc","ab","ab");
List<String> limitList = list.stream().limit(2).collect(Collectors.toList());
System.out.println(limitList);
--输出[abc, ab]
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-- skip
List<String> list= Arrays.asList("abc","ab","ab");
List<String> limitList = list.stream().skip(2).collect(Collectors.toList());
System.out.println(limitList);
-- 输出 [ab]
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-- sort
List<String> list= Arrays.asList("1","3","2");
List<String> sortList = list.stream().sorted().collect(Collectors.toList());
System.out.println(sortList);
--输出[1, 2, 3]
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-- map
List<String> list= Arrays.asList("abc","ab","ab");
List<String> mapList = list.stream().map(str->"case_"+str).collect(Collectors.toList());
System.out.println(mapList);
--输出[case_abc, case_ab, case_ab]
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3.2 终止操作符

-- anyMatch
List<String> list= Arrays.asList("abc","ab","","","fg",null);
boolean flag = list.stream().anyMatch(str -> str == null);
System.out.println(flag);
-- 输出 true
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-- allMatch
List<String> list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().allMatch(str -> str != null));
-- 输出 true
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-- noneMatch
List<String> list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().noneMatch(str -> str == null));
-- 输出 true
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-- findFirst
List<String> list= Arrays.asList("abc","ab","","","fg");
System.out.println(list.stream().findFirst().get());
-- 输出 abc
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-- collect
List<String> list= Arrays.asList("abc","ab","","","fg");
List<String> collectList = list.stream().collect(Collectors.toList());
System.out.println(collectList);
-- 输出 [abc, ab, , , fg]
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-- count
List<String> 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
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