Custom Collector - Implementing GroupingBy

Determine the type#

To customize the collector, you need to implement the Collector interface, and first determine the type:

The type of the elements to be collected
The type of the accumulator/accumulate
The type of the final result

Suppose you want to implement such a collector:

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

The types are:

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

Implement the components of the collector#

The collector has 4 important components, which are all functions:

supplier
accumulator
combiner
finisher

supplier#

The supplier is used to create a container.

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

The accumulator is the accumulator, which is equivalent to the second parameter in reduce, used to add the next content to the previous result.

 @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);
        };
    }

Check if there is a list in the map before adding the next element.

The key is obtained by the classifier passed in, and the key is obtained through the classifier.

combiner#

Equivalent to parameter 3 of reduce, used to merge the generated containers.

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

Just put the latter into the former and return.

finisher#

Describes the final result.

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

Additional characteristics#

Describes the form of the returned data.

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

Related explanations:

    /**
     * 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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