Collectors的作用

1.將數(shù)據(jù)流縮減為一個(gè)單一值:單一的值可以是集合碟刺、基本類型或者自定義對象扳炬。
2.將一個(gè)數(shù)據(jù)流中的元素進(jìn)行分組：這將產(chǎn)生一個(gè)Map<Object, List<T>>
3.分割一個(gè)流中的元素：可以將一個(gè)流分割成兩個(gè)流

Collectors的功能

Stream中有兩個(gè)個(gè)方法collect和collectingAndThen用于對流中的數(shù)據(jù)進(jìn)行處理，可以對流中的數(shù)據(jù)進(jìn)行聚合操作屯仗，如：

(1)數(shù)據(jù)轉(zhuǎn)成集合類型: toList、toSet晃酒、ToMap故源、toCollection
(2)數(shù)據(jù)(字符串)使用分隔符拼接在一起: joining
(3)數(shù)據(jù)求 最大值 maxBy、最小值 minBy账千、求和 summingInt侥蒙、求平均值 averagingDouble
(4)數(shù)據(jù)進(jìn)行映射處理: mapping
(5)數(shù)據(jù)分組: groupingBy、partitioningBy
(6)數(shù)據(jù)累計(jì)計(jì)算: reducing

Collectors的主要方法

數(shù)據(jù)轉(zhuǎn)集合類型

數(shù)據(jù)(字符串)拼接

數(shù)據(jù)統(tǒng)計(jì)

數(shù)據(jù)映射

數(shù)據(jù)分組

數(shù)據(jù)累計(jì)計(jì)算

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Collectors 方法演示

Collectors是終端操作（terminal）

數(shù)據(jù)轉(zhuǎn)集合類型

List<Integer> buildList = Arrays.asList(1,1,2,2,3);

//[1],[2],[3]
List<Integer> list = buildList.stream().distinct().collect(Collectors.toList());
//[3]
Set<Integer> set = buildList.stream().filter(s->s>2).collect(Collectors.toSet());
//"1"=[100],[100];"2"=[200],[200];"3"=[300]
Map collect = buildList.stream().map(s -> s * 100).collect(Collectors.toMap(key->key/100,values->values));
//[1],[2],[3]
TreeSet treeSet = buildList.stream().sorted().collect(Collectors.toCollection(TreeSet::new));

數(shù)據(jù)(字符串)拼接

List<String> buildList = Arrays.asList("1","2","3");

//1這是拼接的2這是拼接的3
String joining = buildList.stream().collect(Collectors.joining("這是拼接的"));

數(shù)據(jù)統(tǒng)計(jì)

List<Integer> buildList = Arrays.asList(11,22,33,44,55);

//最大值：55
Integer maxValue = buildList.stream().collect(Collectors.collectingAndThen(Collectors.maxBy((a, b) -> a - b), Optional::get));
//最小值：11
Integer minValue = buildList.stream().collect(Collectors.collectingAndThen(Collectors.minBy((a, b) -> a - b), Optional::get));
// 求和 165
Integer sumValue = buildList.stream().collect(Collectors.summingInt(item -> item));
// 平均值 33.0
Double avg = buildList.stream().collect(Collectors.averagingDouble(x -> x));

數(shù)據(jù)分組

List<Integer> buildList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
/**
* 奇數(shù)偶數(shù)分組
*/
//groupingBy
Map<Boolean, List<Integer>> groupingBy = buildList.stream().collect(Collectors.groupingBy(item -> item % 2 == 0));
//partitioningBy
Map<Boolean, List<Integer>> partitioningBy = buildList.stream().collect(Collectors.partitioningBy(item -> item % 2 == 0));
//根據(jù)字段分組 (查看**JAVA8新特性--集合流操作Stream筆記**中的案例)

數(shù)據(jù)累計(jì)計(jì)算

// sum: 是每次累計(jì)計(jì)算的結(jié)果具帮，b是Function的結(jié)果
System.out.println(Stream.of(1, 3, 4).collect(Collectors.reducing(0, x -> x + 1, (sum, b) -> {
System.out.println(sum + "-" + b);
return sum + b;
})));

// 下面代碼是對reducing函數(shù)功能實(shí)現(xiàn)的描述博肋，用于理解reducing的功能
int sum = 0;
List<Integer> list3 = Arrays.asList(1, 3, 4);
for (Integer item : list3) {
    int b = item + 1;
    System.out.println(sum + "-" + b);
    sum = sum + b;
}
System.out.println(sum);

// 注意reducing可以用于更復(fù)雜的累計(jì)計(jì)算低斋，加減乘除或者更復(fù)雜的操作
// result = 2 * 4 * 5 = 40
System.out.println(Stream.of(1, 3, 4).collect(Collectors.reducing(1, x -> x + 1, (result, b) -> {
    System.out.println(result + "-" + b);
    return result * b;
})));