collector收集器和joining函数

• collect()⽅法的作⽤

⼀个终端操作, ⽤于对流中的数据进⾏归集操作，collect⽅法接受的参数是⼀个Collector
有两个重载⽅法，在Stream接⼝⾥⾯

//重载⽅法⼀
 <R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T>

accumulator, BiConsumer<R, R>combiner);
 //重载⽅法⼆
 <R, A> R collect(Collector<? super T, A, R> collector);

• 常见收集器

Collectors.toList()
Collectors.toMap()
Collectors.toSet()

• Collectors.toList()详情

    public static <T>
    Collector<T, ?, List<T>> toList() {
        return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
                                   (left, right) -> { left.addAll(right); return left; },
                                   CH_ID);
    }

ArrayList::new，创建⼀个ArrayList作为累加器
List::add，对流中元素的操作就是直接添加到累加器中
reduce操作, 对⼦任务归集结果addAll，后⼀个⼦任务的结果直接全部添加到前⼀个⼦任务结果中
CH_ID 是⼀个unmodifiableSet集合

• Collectors.toCollection() ：⽤⾃定义的实现Collection的数据结构收集

Collectors.toCollection(LinkedList::new)
Collectors.toCollection(CopyOnWriteArrayList::new)
Collectors.toCollection(TreeSet::new)

• 案例

public class Main {

    public static void main(String[] args) throws Exception {
        // 新建集合
        List<String> list = Arrays.asList("sdfsdf","xxxx","bbb","bbb");
        // 转为ArrayList
        List<String> results = list.stream().collect(Collectors.toList());
        // 转为set
        list.stream().collect(Collectors.toSet());
        // 转为LinkedList
        List<String> result = list.stream().collect(Collectors.toCollection(LinkedList::new));
        // 转为CopyOnWriteArrayList
        List<String> result1 = list.stream().collect(Collectors.toCollection(CopyOnWriteArrayList::new));

        Set<String> stringSet = list.stream().collect(Collectors.toCollection(TreeSet::new));
        System.out.println(result);
        System.out.println(stringSet);
    }
    
}

• 拼接函数 Collectors.joining

//3种重载⽅法
Collectors.joining()
Collectors.joining("param")
Collectors.joining("param1", "param2", "param3")

• 案例

public class Main {

    public static void main(String[] args) throws Exception {
        // 直接拼接
        List<String> list = Arrays.asList("springboot教程","springcloud教程","java教程","架构教程");
        String result1 = list.stream().collect(Collectors.joining());
        System.out.println(result1);

        // 使用自定义的分隔符
        String result2 = list.stream().collect(Collectors.joining("||"));
        System.out.println(result2);

        // 使用前后缀
        String result3 = list.stream().collect(Collectors.joining("||","[","]"));
        System.out.println(result3);

        // 使用前后缀
        String result = Stream.of("springboot","mysql","html5","css3").collect(Collectors.joining(",","[","]"));
        System.out.println(result);
    }

}