定制收集器Collector-實現GroupingBy

確定類型#

定制 collector 要實現 Collector 接口，首先要確定類型

待收集元素的類型
累加器 /accumulate 的類型
最終結果的類型

假設要實現這樣一個收集器:

public class GroupingBy<T,K> implements Collector<T,Map<K,List<T>>,Map<K,List<T>>>

類型分別為:

T
Map<K,List>
Map<K,List>

實現收集器的組件#

收集器有 4 個重要的組件，他們都是函數

supplier
accumulator
combiner
finisher

supplier#

supplier 用於創建容器.

@Override
    public Supplier<Map<K, List<T>>> supplier() {
        return ()-> new HashMap<>();
    }

accumulator 是疊加器，相當於 reduce 裡面的第二個參數，用於將下一個內容加入到前面的結果.

 @Override
    public BiConsumer<Map<K, List<T>>, T> accumulator() {
        return (accumulator,ele)->{
            K key = this.classifier.apply(ele);
            List<T> tList = accumulator.get(key);
            if (tList == null){
                tList = new ArrayList<>();
            }
            tList.add(ele);
            accumulator.put(key,tList);
        };
    }

在添加下一個元素之前判斷 map 中有無 list

關鍵的一點是 key 的獲取。由傳進來的一個 classifier 完成，通過 classifier 獲得 key.

combiner#

相當於 reduce 的參數 3，用於將產生的各個容器合併起來

@Override
    public BinaryOperator<Map<K, List<T>>> combiner() {
        return (l,r)->{
            l.putAll(r);
            return l;
        };
    }

直接把後一個裝到前一個並返回就行

finisher#

描述返回最終的結果.

@Override
    public Function<Map<K, List<T>>, Map<K, List<T>>> finisher() {
        return accumulator->accumulator;
    }

額外 characteristics#

描述數據的返回形式

@Override
    public Set<Characteristics> characteristics() {
        return Collections.unmodifiableSet(EnumSet.of(Characteristics.IDENTITY_FINISH));
    }

相關解釋:

    /**
     * Characteristics indicating properties of a {@code Collector}, which can
     * be used to optimize reduction implementations.
     */
    enum Characteristics {
        /**
         * Indicates that this collector is <em>concurrent</em>, meaning that
         * the result container can support the accumulator function being
         * called concurrently with the same result container from multiple
         * threads.
         *
         * <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED},
         * then it should only be evaluated concurrently if applied to an
         * unordered data source.
         */
        CONCURRENT,

        /**
         * Indicates that the collection operation does not commit to preserving
         * the encounter order of input elements.  (This might be true if the
         * result container has no intrinsic order, such as a {@link Set}.)
         */
        UNORDERED,

        /**
         * Indicates that the finisher function is the identity function and
         * can be elided.  If set, it must be the case that an unchecked cast
         * from A to R will succeed.
         */
        IDENTITY_FINISH
    }

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