Stream流

Stream是java8新加的流接口
获取流的方法：
1、所有的Collection集合都可以通过stream默认方法获取流
2、Stream接口的静态方法of可以获取数组对应的流

1、Collection中的方法 stream（）

//list--->stream
ArrayList list = new ArrayList<>();
Stream stream = list.stream();

//set--->stream
HashSet set = new HashSet<>();
Stream stream1 = set.stream();

//map--->stream
HashMap map = new HashMap<>();
Set keys = map.keySet();
Stream stream2 = keys.stream();//key的stream

Collection values = map.values();//value的值是非唯一的，所以用的是Collection
Stream stream3 = values.stream();//values的stream

2、Stream静态方法
静态方法只能接口自己调用，子类都不用调用

Stream stream =Stream.of("a","b","c");//of(可变参数)，所以可以写多个参数

1、终结方法：返回值类型不再是stream接口自身类型的方法，终结方法只能调用一次
比如：count forEach
2、非终结方法：返回类型选仍旧是Stream接口自身类型的方法，非终结方法可以多次调用
filter limit skip concat

常用方法：
1、forEach：逐一处理
该方法并不保证元素的逐一消费动作在流中被有序执行
forEach(Consumer action)

        stream.forEach((String s)->{
            System.out.println(s);
        });

2、count:统计个数

long count=stream.count();
System.out.println(count);

3、skip:跳过前几个
skip(long n)

Stream stream =Stream.of("张三","李四","王五");
stream.skip(2).forEach((String s)-> System.out.println(s));//跳过前2个，返回王五

4、limit:取用前几个
limit(long maxSize)

 Stream stream =Stream.of("张三","李四","王五");
 stream.limit(2).forEach((String s)-> System.out.println(s));//取前两个，返回张三 李四

5、filter:过滤
filter(Predicate predicate)

Stream stream =Stream.of("张三","李四","王五","张六");
Stream stringStream = stream.filter((String s) -> {
    return s.contains("张"); //过滤出包含”张“字的名字，所以返回张三 张六
});
stringStream.forEach(s -> System.out.println(s));

6、distinct:去重

stream.distinct().forEach(s-> System.out.println(s));

7、map：映射
map(Function mapper)

Stream stream =Stream.of("10","20","30","40");
Stream integerStream=stream.map((String s)->{
    return Integer.parseInt(s);
});
integerStream.forEach((Integer i)-> {
    System.out.println(i);
}); //打印 10 20 30 40

Stream stream =Stream.of("张三","李四","王五");
Stream personStream = stream.map((String name)->{
    return new Person(name);
});

8、concat：合并
concat（Stream a，Stream b）将stream对象a和b合并为一个新的stream流对象

Stream< String> stream1 =Stream.of("aa","bb","cc");
Stream< String> stream2 =Stream.of("张三","李四","王五");
Stream< String> stringStream = Stream.concat(stream1,stream2); //将两个对象合并起来

1、转化为List集合
2、转化为数组

Stream stream =Stream.of("aa","bb","cc");
//stream---->List
List list=stream.collect(Collectors.toList());
System.out.println(list);
//stream--->Set
Set set = stream.collect(Collectors.toSet());
System.out.println(set);

//stream--->ArrayList,toCollection 单独某一个集合
ArrayList arrayList = stream.collect(Collectors.toCollection(ArrayList::new));
//stream--->HashSet,toCollection 单独某一个集合
HashSet hashSet = stream.collect(Collectors.toCollection(HashSet::new));

//stream--->object[]数组
Object[] objects = stream.toArray();

//stream--->String[]
String[] strings=stream.toArray(String[]::new);