java8的collectors怎么使用

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java8中提供了对集合进行转化，然后进行归集的的操作。这就不得不提Collectors这个类了，真的很强大，我们看示例吧。

KeyAndValue a1 = new KeyAndValue().setName("kevin").setValue("lee");KeyAndValue a2 = new KeyAndValue().setName("kevin").setValue("lee");KeyAndValue a3 = new KeyAndValue().setName("kevin1").setValue("lee");KeyAndValue a4 = new KeyAndValue().setName("kevin1").setValue("123123");KeyAndValue a5 = new KeyAndValue().setName("kevin2").setValue("lee5");KeyAndValue a6 = new KeyAndValue().setName("kevin3").setValue("lee8");List list = Arrays.asList(a1, a2, a3, a4, a5, a6);

基础的toList、toSet、toMap就不讲了，简单易用，一看就会，我说说稍微复杂的。

collectingAndThen：使用collector进行归集，然后对归结的结果进行加工

List collect = list.stream().collect(Collectors.collectingAndThen(Collectors.mapping(a -> a.getValue(), Collectors.toList()), v -> {    v.sort(Comparator.comparing(String::length).reversed());return v;}));System.out.println(collect);//[123123, lee5, lee8, lee, lee, lee]

groupingBy 分组操作：基于一个key值，对元素进行分组

Map> collect = list.stream().collect(Collectors.groupingBy(a -> a.getName()));System.out.println(JSON.toJSONString(collect));// {"kevin":[{"name":"kevin","value":"lee"},{"name":"kevin","value":"lee"}],"kevin2":[{"name":"kevin2","value":"lee5"}],"kevin1":[{"name":"kevin1","value":"lee"},{"name":"kevin1","value":"123123"}],"kevin3":[{"name":"kevin3","value":"lee8"}]}Map> collect1 = list.stream().collect(Collectors.groupingBy(KeyAndValue::getName, Collectors.mapping(b -> b.getValue(), Collectors.toSet())));System.out.println(JSON.toJSONString(collect1));// {"kevin":["lee"],"kevin2":["lee5"],"kevin1":["123123","lee"],"kevin3":["lee8"]}Map> collect3 = list.stream().collect(Collectors.groupingBy(KeyAndValue::getName, Collectors.toSet()));System.out.println(JSON.toJSONString(collect3));// {"kevin":[{"name":"kevin","value":"lee"}],"kevin2":[{"name":"kevin2","value":"lee5"}],"kevin1":[{"name":"kevin1","value":"lee"},{"name":"kevin1","value":"123123"}],"kevin3":[{"name":"kevin3","value":"lee8"}]}Map> collect4 = list.stream().collect(Collectors.groupingBy(KeyAndValue::getName,TreeMap::new, Collectors.toSet()));System.out.println(JSON.toJSONString(collect4));// {"kevin":[{"name":"kevin","value":"lee"}],"kevin1":[{"name":"kevin1","value":"lee"},{"name":"kevin1","value":"123123"}],"kevin2":[{"name":"kevin2","value":"lee5"}],"kevin3":[{"name":"kevin3","value":"lee8"}]}

join 操作：对元素进行拼接，有三个方法。

String joinResult = list.stream().map(KeyAndValue::getName).collect(Collectors.joining());System.out.println(joinResult);//kevinkevinkevin1kevin1kevin2kevin3String joinResult1 = list.stream().map(KeyAndValue::getName).collect(Collectors.joining(","));System.out.println(joinResult1);// kevin,kevin,kevin1,kevin1,kevin2,kevin3String joinResult2 = list.stream().map(KeyAndValue::getName).collect(Collectors.joining(",","(",")"));System.out.println(joinResult2);// (kevin,kevin,kevin1,kevin1,kevin2,kevin3)

partitioningBy操作：基于断言对元素按true和false进行分组

Map> partitioningBy = list.stream()        .collect(Collectors.partitioningBy(k -> k.getName().equals("kevin")));System.out.println(partitioningBy);//{false=[KeyAndValue(name=kevin1, value=lee), KeyAndValue(name=kevin1, value=123123), KeyAndValue(name=kevin2, value=lee5), KeyAndValue(name=kevin3, value=lee8)], true=[KeyAndValue(name=kevin, value=lee), KeyAndValue(name=kevin, value=lee)]}Map> partitioningBy1 = list.stream()        .collect(Collectors.partitioningBy(k -> k.getName().equals("kevin"), Collectors.mapping(v -> v.getValue(), Collectors.toList())));System.out.println(partitioningBy1);// {false=[lee, 123123, lee5, lee8], true=[lee, lee]}

min/minBy , max/maxBy 用法一致：使用一个比较器获得最大值或最小值

Optional min = list.stream().map(KeyAndValue::getName).min(Comparator.comparing(v->v.length()));System.out.println(min.get());//kevinOptional min1 = list.stream().min(Comparator.comparing(v->v.getValue().length()));System.out.println(min1.get());// KeyAndValue(name=kevin, value=lee)Optional min2 = list.stream().map(KeyAndValue::getName).collect(Collectors.minBy(Comparator.comparing(String::length)));System.out.println(min2.get());//kevinOptional min3 = list.stream().collect(Collectors.minBy(Comparator.comparing(v->v.getValue().length())));System.out.println(min3.get());// KeyAndValue(name=kevin, value=lee)

mapping操作:使用一个function对元素进行加工，然后使用collector进行收集

List mapping = list.stream().collect(Collectors.mapping(v -> v.getValue().concat("agc"), Collectors.toList()));System.out.println(mapping);// 类似 Stream 先进行map操作，再进行collectList mapping_ = list.stream().map(v -> v.getValue().concat("agc")).collect(Collectors.toList());System.out.println(mapping_);// [leeagc, leeagc, leeagc, 123123agc, lee5agc, lee8agc]

toCollection:将元素按照顺序复制到另一个收集器（集合）中,类似于Collectors.toList()

    ArrayList collect1 = list.stream().collect(Collectors.toCollection(ArrayList::new));    System.out.println(collect1);//    [KeyAndValue(name=kevin, value=lee), KeyAndValue(name=kevin, value=lee), KeyAndValue(name=kevin1, value=lee), KeyAndValue(name=kevin1, value=123123), KeyAndValue(name=kevin2, value=lee5), KeyAndValue(name=kevin3, value=lee8)]

根据某个属性进行去重

TreeSet collect = list.stream().collect(Collectors.toCollection(() -> new TreeSet(Comparator.comparing(KeyAndValue::getName))));System.out.println(collect);// [KeyAndValue(name=kevin, value=lee), KeyAndValue(name=kevin1, value=lee), KeyAndValue(name=kevin2, value=lee5), KeyAndValue(name=kevin3, value=lee8)]

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