java stream Interface Collector＜T,A,R＞

左手的ㄟ右手 2022-11-04 08:04 129阅读 0赞

`Interface Collector<T,A,R>`  
T:输入的类型  
A:中间存放数据的容器  
R:输出的类型

Collector通过下面四个方法协同工作以完成汇聚操作：  
supplier： 创建新的结果容器  
accumulator：将输入元素合并到结果容器中  
combiner：合并两个结果容器（并行流使用，将多个线程产生的结果容器合并）  
finisher：将结果容器转换成最终的表示

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NsYXJvamE_size_16_color_FFFFFF_t_70_pic_center]

@Override
        @SuppressWarnings("unchecked")
        public final <R, A> R collect(Collector<? super P_OUT, A, R> collector) { 
            A container;
            if (isParallel()
                    && (collector.characteristics().contains(Collector.Characteristics.CONCURRENT))
                    && (!isOrdered() || collector.characteristics().contains(Collector.Characteristics.UNORDERED))) { 
                container = collector.supplier().get();
                BiConsumer<A, ? super P_OUT> accumulator = collector.accumulator();
                forEach(u -> accumulator.accept(container, u));
            }
            else { 
                container = evaluate(ReduceOps.makeRef(collector));
            }
            return collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)
                   ? (R) container
                   : collector.finisher().apply(container);
        }

public static <T, I> TerminalOp<T, I>
        makeRef(Collector<? super T, I, ?> collector) { 
            Supplier<I> supplier = Objects.requireNonNull(collector).supplier();
            BiConsumer<I, ? super T> accumulator = collector.accumulator();
            BinaryOperator<I> combiner = collector.combiner();
            class ReducingSink extends Box<I>
                    implements AccumulatingSink<T, I, ReducingSink> { 
                @Override
                public void begin(long size) { 
                    state = supplier.get();
                }
    
                @Override
                public void accept(T t) { 
                    accumulator.accept(state, t);
                }
    
                @Override
                public void combine(ReducingSink other) { 
                    state = combiner.apply(state, other.state);
                }
            }
            return new ReduceOp<T, I, ReducingSink>(StreamShape.REFERENCE) { 
                @Override
                public ReducingSink makeSink() { 
                    return new ReducingSink();
                }
    
                @Override
                public int getOpFlags() { 
                    return collector.characteristics().contains(Collector.Characteristics.UNORDERED)
                           ? StreamOpFlag.NOT_ORDERED
                           : 0;
                }
            };
        }

参考:  
[java8新特性(四) Collector（收集器）][java8_ Collector]  
[Java8中重要的收集器collector][Java8_collector]  
[JAVA8学习9-自定义收集器（Characteristics 使用说明）][JAVA8_9-_Characteristics]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NsYXJvamE_size_16_color_FFFFFF_t_70_pic_center]: /images/20221023/3121ce9261bc4f059fd3d32dde8aa961.png
[java8_ Collector]: https://blog.csdn.net/xiliunian/article/details/88773718
[Java8_collector]: http://www.voidcn.com/article/p-uflflvaw-bad.html
[JAVA8_9-_Characteristics]: https://blog.csdn.net/z_yemu/article/details/89401321