廖雪峰Java16函数式编程-2Stream-7其他操作

1. 排序

Stream<T> sorted(); //按元素默认大小排序（必须实现Comparable接口）
Stream<T> sorted(Comparator<? super T> cp); //按指定Comparator比较的结果排序

    public static void main(String[] args){
        Stream<String> s = Stream.of("hello","JAVA","python","RUBY","PHP");
        s.sorted().forEach(str-> System.out.print(str+"\t"));
        System.out.println();
        Stream<Integer> s1 = Stream.of(1,9,5,3,7);
        s1.sorted(Comparator.reverseOrder()).forEach(str-> System.out.print(str+"\t"));
    }

## 2.去除重复元素
Stream distinct(); //返回去除重复元素的Stream
```#java
Stream s = Stream.of("hello","JAVA","hello","RUBY","JAVA");
s.distinct().forEach(str-> System.out.print(str+"\t"));
```

3. 截取

Stream<T> limit(long); //截取Stream前long个元素
Stream<T> skip(long); //跳过Stream前long个元素

        Stream<String> s = Stream.of("hello","JAVA","hello","RUBY","JAVA");
        s.limit(3).forEach(str-> System.out.print(str+"\t"));
        System.out.println();
        Stream<Integer> s1 = Stream.of(1,9,5,3,7);
        s1.skip(3).forEach(str-> System.out.print(str+"\t"));

4. concat合并

        Stream<String> s1 = Stream.of("php","JAVA","python");
        Stream<String> s2 = Stream.of("android", "ios");
        Stream<String> s = Stream.concat(s1,s2);
        s.forEach(str-> System.out.print(str+"\t"));

## 5. flatMap把列表的元素取出，构成一个新的Stream
把元素映射为Stream，然后合并成一个新的Stream

        List<String> s1 = new ArrayList<>();
        s1.add( "php");
        s1.add("JAVA");
        s1.add("python");
        List<String> s2 = Arrays.asList("android", "ios");
        Stream<List> l = Stream.of(s1,s2);
        Stream<String> s = l.flatMap(list -> list.stream());
        s.forEach(str-> System.out.print(str+"\t"));

6. parallel把一个Stream转换为可以并行处理的Stream

Stream的元素是串行处理的，也就是只能用单线程来处理，如果要提高效率，比如进行并行排序，只需使用parallel()就可以把一个Stream变成一个并行处理的Stream，通过这个方法调用，后续的操作会尽量的并行处理。

        Stream<String> s = Stream.of("php","JAVA","python","android", "ios");
        String[] result = s.parallel() //变成一个可以并行处理的Stream
            .sorted(). //可以进行并行排序
            toArray(String[]::new);
        System.out.println(Arrays.toString(result));

## 7. Stream的聚合方法
Optional reduce(BinaryOperater bo)
long count()
T max(Comparator cp)//查找最大元素
T min(Comparator cp) //查找最下元素
针对IntStream, longStream, DoubleStream:
```#java
sum() //求和
average() //求平均值
```
```#java
Integer[] nums = {1,2,3,4,5,6};
long max = Stream.of(nums).max((i1,i2)->i1.compareTo(i2)).get();
long min = Stream.of(nums).min((i1,i2)->i1.compareTo(i2)).get();
System.out.println("最大值："+max+"\t最小值："+min);
IntStream ins1 = IntStream.builder().add(1).add(2).add(3).add(4).add(5).add(6).build();
IntStream ins2 = IntStream.builder().add(1).add(2).add(3).add(4).add(5).add(6).build();
Double avg = ins1.average().getAsDouble();
long sum = ins2.sum();
System.out.println("求和："+sum+"\t平均值："+avg);
```

## 8. 测试Stream的元素是否满足
boolean allMatch(Predicate) //所有元素均满足测试条件
boolean anyMatch(Predicate) //至少一个元素满足测试条件
```#java
Integer[] nums = {1,2,3,4,5,6};
boolean max = Stream.of(nums).allMatch(x->x>0);
boolean min = Stream.of(nums).anyMatch(x->x>5);
System.out.println("元素全部>0："+max+"\t存在元素>5："+min);
```

## 9. forEach循环处理Stream的元素
void forEach(Consumer action)
## 10. 把Stream转换为其他类型
Object[] toArray() //转换为Object数组
A[] toArray(IntFunction) //转换为A[]数组
R collect(Collector Collector) //转换为List/Set等集合类型
```#java
Stream s1 = Stream.of("php","JAVA","python","android", "ios");
String[] arr = s1.toArray(String[]::new);
Stream s2 = Stream.of("php","JAVA","python","android", "ios");
List list = s2.collect(Collectors.toList());
System.out.println(Arrays.toString(arr));
System.out.println(list.toString());
```

## 11. 总结
java.util.stream.Stream题哦那个了许多实用的功能
* 转换操作：map，filter，sorted，distinct
* 合并操作：concat，flatMap
* 并行处理：parallel
* 聚合操作：reduce，count，max，min，sum，average
* 其他操作：allMatch，anyMatch，forEach
参考
[https://blog.csdn.net/qq_28410283/article/details/80783505](https://blog.csdn.net/qq_28410283/article/details/80783505)
[https://www.cnblogs.com/a-du/p/8289537.html](https://www.cnblogs.com/a-du/p/8289537.html)