|
##?概念
>?Stream是Java8?API的新成员，它允许以声明性方式处理数据集合?。
##?特点
>?（1）**代码简洁**：函数式编程写出的代码简洁且意图明确，使用*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行程序从未如此简单，你需要的全部就是调用一下方法。?
##?流程
```
1）第一步：把集合转换为流stream
2）第二步：操作stream流
stream流在管道中经过中间操作（intermediate?operation）的处理，最后由最终操作(terminal?operation)得到前面处理的结果
```
##?操作符
>?两种：中间操作符、终止操作符
###?中间操作符
|?流方法???|?含义?????????????????????????????????????????????????????????|?示例?????????????????????????????????????????????????????????|
|?--------?|?------------------------------------------------------------?|?------------------------------------------------------------?|
|?filter???|?用于通过设置的条件过滤出元素?????????????????????????????????|?List<String>?strings?=?Arrays.asList("abc",?"",?"bc",?"efg",?"abcd","",?"jkl");List<String>?filtered?=?strings.stream().filter(string?->?!string.isEmpty()).collect(Collectors.toList());?|
|?distinct?|?返回一个元素各异（根据流所生成元素的hashCode和equals方法实现）的流。?|?List<Integer>?numbers?=?Arrays.asList(1,?2,?1,?3,?3,?2,?4);numbers.stream().filter(i?->?i?%?2?==?0).distinct().forEach(System.out::println);?|
|?limit????|?会返回一个不超过给定长度的流。???????????????????????????????|?List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");List<String>?limited?=?strings.stream().limit(3).collect(Collectors.toList());?|
|?skip?????|?返回一个扔掉了前n个元素的流。????????????????????????????????|?List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");List<String>?skiped?=?strings.stream().skip(3).collect(Collectors.toList());?|
|?map??????|?接受一个函数作为参数。这个函数会被应用到每个元素上，并将其映射成一个新的元素（使用映射一词，是因为它和转换类似，但其中的细微差别在于它是“创建一个新版本”而不是去“修改”）。?|?List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");List<String>?mapped?=?strings.stream().map(str->str+"-itcast").collect(Collectors.toList());?|
|?flatMap??|?使用flatMap方法的效果是，各个数组并不是分别映射成一个流，而是映射成流的内容。所有使用map(Arrays::stream)时生成的单个流都被合并起来，即扁平化为一个流。?|?List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");Stream<Character>?flatMap?=?strings.stream().flatMap(Java8StreamTest::getCharacterByString);?|
|?sorted???|?返回排序后的流???????????????????????????????????????????????|?List<String>?strings1?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");List<String>?sorted1?=?strings1.stream().sorted().collect(Collectors.toList());?|
>?示例代码：
>
>?1）filter
>
>?```java
>?/**
>??*?功能描述:根据条件过滤集合数据
>??*?@return?:?void
>??*/
>?@Test
>?public?void?filter(){
>?????List<String>?strings?=?Arrays.asList("abc",?"",?"bc",?"efg",?"abcd","",?"jkl");
>?????List<String>?filtered?=?strings.stream().filter(string?->?!string.isEmpty()).collect(Collectors.toList());
>?????out.println(filtered);
>?}
>?```
>
>?2）distinct
>
>?```java
>?/**
>??*?功能描述:去除集合中重复数据
>??*?@return?:?void
>??*/
>?@Test
>?public?void?distinct(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
>?????List<String>?distincted?=?strings.stream().distinct().collect(Collectors.toList());
>?????out.println(distincted);
>?}
>?```
>
>?3）limit
>
>?```java
>?/**
>??*?功能描述:指定获取集合前x条数据，重新构造一个新的集合
>??*?@return?:?void
>??*/
>?@Test
>?public?void?limit(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
>?????List<String>?limited?=?strings.stream().limit(3).collect(Collectors.toList());
>?????out.println(limited);
>?}
>?```
>
>?4）skip
>
>?```java
>?/**
>??*?功能描述:排除集合前x条数据，把后面的数据重新构造一个新的集合
>??*?@return?:?void
>??*/
>?@Test
>?????public?void?skip(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
>?????List<String>?skiped?=?strings.stream().skip(3).collect(Collectors.toList());
>?????out.println(skiped);
>?}
>?```
>
>?5）map
>
>?```java
>?/**
>??*?功能描述:对集合中所有元素统一处理
>??*?@return?:?void
>??*/
>?@Test
>?public?void?map(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
>?????List<String>?mapped?=?strings.stream().map(str->str+"-itcast").collect(Collectors.toList());
>?????out.println(mapped);
>?}
>?```
>
>?6）flatMap
>
>?```java
>?/**
>??*?功能描述:对集合中所有元素统一处理
>??*?@return?:?void
>??*/
>?@Test
>?public?void?flatMap(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
>?????Stream<String>?stringStream?=?strings.stream().map(x?->?x);
>?????Stream<String>?stringStream1?=?strings.stream().flatMap(x?->?Arrays.asList(x.split("?")).stream());
>?}
>?```
>
>?7）sorted
>
>?```java
>?/**
>??*?功能描述?:?对集合进行排序
>??*?@return?:?void
>??*/
>?@Test
>?public?void?sorted(){
>?????List<String>?strings1?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????List<String>?strings2?=?Arrays.asList("张三",?"李四",?"王五",?"赵柳",?"张哥","李哥",?"王哥");
>?????List<Integer>?strings3?=?Arrays.asList(10,?2,?30,?22,?1,0,?-9);
>?????List<String>?sorted1?=?strings1.stream().sorted().collect(Collectors.toList());
>?????List<String>?sorted2?=?strings2.stream().sorted(Collections.reverseOrder(Collator.getInstance(Locale.CHINA))).collect(Collectors.toList());
>?????List<Integer>?sorted3?=?strings3.stream().sorted().collect(Collectors.toList());
>?????out.println(sorted1);
>?????out.println(sorted2);
>?????out.println(sorted3);
>?}
>?```
>
>??Map、flatMap区别
```
map：对流中每一个元素进行处理
flatMap：流扁平化，让你把一个流中的“每个值”都换成另一个流，然后把所有的流连接起来成为一个流?
总结：map是对一级元素进行操作，flatmap是对二级元素操作。
```
`本质区别`：map返回一个值；flatmap返回一个流，多个值。
`应用场景`：map对集合中每个元素加工,返回加工后结果；flatmap对集合中每个元素加工后，做扁平化处理后（拆分层级，放到同一层）然后返回
```java
/**
?*?方法一
?*?功能描述:??通过使用map、flatMap把字符串转换为字符输出对比区别
?*?@return?:?void
?*/
@Test
public?void?flatMap2Map(){
????List<String>?strings?=?Arrays.asList("abc",?"abc",?"bc",?"efg",?"abcd","jkl",?"jkl");
????final?Stream<Character>?flatMap?=?strings.stream().flatMap(Java8StreamTest::getCharacterByString);
????flatMap.forEach(System.out::println);
????//----------------------------------------------
????final?Stream<Stream<Character>>?mapStream?=?strings.stream().map(Java8StreamTest::getCharacterByString);
????//mapStream.forEach(System.out::println);
????out.println("------------------------------------------------");
????mapStream.forEach(stream->?{stream.forEach(character->{System.out.println(character);});});
}
```
>?公共方法（字符串转换为字符流）
```java
/**
*?功能描述:字符串转换为字符流
*?@param?str
*?@return?:?java.util.stream.Stream<java.lang.Character>
*/
public?static?Stream<Character>?getCharacterByString(String?str)?{
????List<Character>?characterList?=?new?ArrayList<>();
????for?(Character?character?:?str.toCharArray())?{
????????characterList.add(character);
????}
????return?characterList.stream();
}
```
###?终止操作符
|?流方法????|?含义?????????????????????????????????????|?示例?????????????????????????????????????????????????????????|
|?---------?|?----------------------------------------?|?------------------------------------------------------------?|
|?anyMatch??|?检查是否至少匹配一个元素，返回boolean。??|?List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");boolean?b?=?strings.stream().anyMatch(s?->?s?==?"abc");?|
|?allMatch??|?检查是否匹配所有元素，返回boolean。??????|?List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");boolean?b?=?strings.stream().allMatch(s?->?s?==?"abc");?|
|?noneMatch?|?检查是否没有匹配所有元素，返回boolean。??|?List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");boolean?b?=?strings.stream().noneMatch(s?->?s?==?"abc");?|
|?findAny???|?将返回当前流中的任意元素。???????????????|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");Optional<String>?any?=?strings.stream().findAny();?|
|?findFirst?|?返回第一个元素???????????????????????????|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");Optional<String>?first?=?strings.stream().findFirst();?|
|?forEach???|?遍历流???????????????????????????????????|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");strings.stream().forEach(s?->?out.println(s));?|
|?collect???|?收集器，将流转换为其他形式。?????????????|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");Set<String>?set?=?strings.stream().collect(Collectors.toSet());List<String>?list?=?strings.stream().collect(Collectors.toList());Map<String,?String>?map?=?strings.stream().collect(Collectors.toMap(v?->v.concat("_name"),?v1?->?v1,?(v1,?v2)?->?v1));?|
|?reduce????|?可以将流中元素反复结合起来，得到一个值。?|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");Optional<String>?reduce?=?strings.stream().reduce((acc,item)?->?{return?acc+item;});if(reduce.isPresent())out.println(reduce.get());?|
|?count?????|?返回流中元素总数。???????????????????????|?List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");long?count?=?strings.stream().count();?|
>?示例代码
>
>?1）anyMatch
>
>?```java
>?/**
>??*?功能描述?:?判断集合中是否至少存在一个元素满足条件
>??*?@return?:?void
>??*/
>?@Test
>?public?void?anyMatch(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????boolean?b?=?strings.stream().anyMatch(s?->?s?==?"abc");
>?????out.println(b);
>?}
>?```
>
>?2）allMatch
>
>?```java
>?/**
>??*?功能描述?:?判断集合中是否所有元素都满足条件
>??*?@return?:?void
>??*/
>?@Test
>?public?void?allMatch(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????boolean?b?=?strings.stream().allMatch(s?->?s?==?"abc");
>?????out.println(b);
>?}
>?```
>
>?3）noneMatch
>
>?```java
>?/**
>??*?功能描述?:?判断集合中是否所有元素都不满足条件
>??*?@return?:?void
>??*/
>?@Test
>?public?void?noneMatch(){
>?????List<String>?strings?=?Arrays.asList("abc",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????boolean?b?=?strings.stream().noneMatch(s?->?s?==?"abc");
>?????out.println(b);
>?}
>?```
>
>?4）findAny
>
>?```java
>?/**
>??*?功能描述?:?返回当前流中任意元素
>??*?@return?:?void
>??*/
>?@Test
>?public?void?findAny(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????Optional<String>?any?=?strings.stream().findAny();
>?????if(any.isPresent())?out.println(any.get());
>?}
>?```
>
>?5）findFirst
>
>?```java
>?/**
>??*?功能描述?:?返回当前流中第一个元素
>??*?@return?:?void
>??*/
>?@Test
>?public?void?findFirst(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????Optional<String>?first?=?strings.stream().findFirst();
>?????if(first.isPresent())?out.println(first.get());
>?}
>?```
>
>?6）forEach?java?
>
>?```java
>?/**
>??*?功能描述?:?遍历流
>??*?@return?:?void
>??*/
>?@Test
>?public?void?foreach(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????strings.stream().forEach(s?->?out.println(s));
>?}
>?```
>
>?7）collect
>
>?```java
>?/**
>??*?功能描述?:?流转换为其他形式
>??*?@return?:?void
>??*/
>?@Test
>?public?void?collect(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????Set<String>?set?=?strings.stream().collect(Collectors.toSet());
>?????List<String>?list?=?strings.stream().collect(Collectors.toList());
>?????Map<String,?String>?map?=?strings.stream().collect(Collectors.toMap(v?->v.concat("_name"),?v1?->?v1,?(v1,?v2)?->?v1));
>?????out.println(set);
>?????out.println(list);
>?????out.println(map);
>?}
>?```
>
>?8）reduce
>
>?```java
>?/**
>??*?功能描述?:?将流中元素反复结合起来，得到一个值
>??*?@return?:?void
>??*/
>?@Test
>?public?void?reduce(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????//reduce方法一
>?????Optional<String>?reduce1?=?strings.stream().reduce((acc,item)?->?{return?acc+item;});
>?????//reduce方法二
>?????String?reduce2?=?strings.stream().reduce("itcast",?(acc,?item)?->?{
>???????return?acc?+?item;
>?????});
>?????//reduce方法三
>?????ArrayList<String>?reduce3?=?strings.stream().reduce(
>?????????new?ArrayList<String>(),
>???????????new?BiFunction<ArrayList<String>,?String,?ArrayList<String>>()?{
>???????????@Override
>???????????public?ArrayList<String>?apply(ArrayList<String>?acc,?String?item)?{
>???????????????acc.add(item);
>???????????????return?acc;
>???????????}
>???????},?
>?????????new?BinaryOperator<ArrayList<String>>()?{
>???????????????@Override
>???????????public?ArrayList<String>?apply(ArrayList<String>?acc,?ArrayList<String>?item)?{
>???????????return?acc;
>???????????}
>???????}
>?????);
>?????if(reduce1.isPresent())out.println(reduce1.get());
>?????out.println(reduce2);
>?????out.println(reduce3);
>?}
>?```
>
>?9）count
>
>?```java
>?/**
>?*?功能描述?:?返回流中元素总数
>?*?@return?:?void
>?*/
>?@Test
>?public?void?count(){
>?????List<String>?strings?=?Arrays.asList("cv",?"abd",?"aba",?"efg",?"abcd","jkl",?"jkl");
>?????long?count?=?strings.stream().count();
>?????out.println(count);
>?}
>?```
注意：文章中因排序部分用到外部比较器，需要导入外部jar包
```xml
<!--apache集合操作工具包-->
<dependency>
????<groupId>org.apache.commons</groupId>
????<artifactId>commons-collections4</artifactId>
????<version>4.4</version>
</dependency>
```
|