Java Stream

入門

Java Stream,一個號稱能夠極大提高程序員開發效率的神器， 一個學會了都不想寫for循環的好東西。

本質上是一種函數式編程思惟，是將要處理的元素集合看做一種流， 流在管道中傳輸，進行各類操做。

這是官網給出的Stream的特性:

Sequence of elements − A stream provides a set of elements of specific type in a sequential manner. A stream gets/computes elements on demand. It never stores the elements.

Source − Stream takes Collections, Arrays, or I/O resources as input source.

Aggregate operations − Stream supports aggregate operations like filter, map, limit, reduce, find, match, and so on.

Pipelining − Most of the stream operations return stream itself so that their result can be pipelined. These operations are called intermediate operations and their function is to take input, process them, and return output to the target. collect() method is a terminal operation which is normally present at the end of the pipelining operation to mark the end of the stream.

Automatic iterations − Stream operations do the iterations internally over the source elements provided, in contrast to Collections where explicit iteration is required.
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概念

中間操做

（intermediate operation）

調用中間操做只會生成一個標記了該操做的新stream,能夠繼續執行

中間操做主要包括有：

• concat()
• distinct()
• filter()
• flatMap()
• limit()
• map()
• peek()
• skip()
• sorted()
• parallel()
• sequential()
• unordered()

最終操做

(terminal operation)

元素流在管道中通過中間操做的處理，最後由最終操做獲得前面處理的結果。

最終操做主要有：

• allMatch()
• anyMatch()
• collect()
• count()
• findAny()
• findFirst()
• forEach()
• forEachOrdered()
• max()
• min()
• noneMatch()
• reduce()
• toArray()

使用舉例

查找最大最小值

List<T> list = new LinkedList<>();
     ...
     //最小值
    Optional<T> min = 
        list.stream()
            .min(Comparator.comparing(T::getScore));
    if(min.isPresent()){
        T tMin = min.get();
    }
        
    //最大值    
    Optional<T> max =      
        list.stream()
            .max(Comparator.comparing(T::getScore));
    if(max.isPresent()){
        T tMax = t.get();
    }
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遍歷打印

List<T> list = new LinkedList<>();
     ...
     list.stream()
         .forEach(
                i ->System.out.println(i));
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分頁

List<T> list = new LinkedList<>();
     ...
    List<T> pagedList = list.stream()
                            .skip(offset)
                            .limit(limit)
                            .collect(Collectors.toList())；
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List 轉 Map

下標index爲key

List<T> list = new LinkedList<>();
Map<Long, Integer> map = IntStream.range(0, list.size())
                                  .boxed()
                                  .collect(Collectors.toMap(i -> i,i -> list.get(i), (p1,p2)->p1));

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提取List中某字段

List<T> list = new LinkedList<>();
    ...
    List<Integer> newList = list.stream()
                                .map(T::getField)
                                .collect(Collectors.toList());
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List<List> to List

List<List<T>> list = new LinkedList<>();
    ...
    List<Long> collectIds = list.stream()
                                .flatMap(List::stream)
                                .collect(Collectors.toList());
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