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We have seen many examples in earlier lessons of a terminal operation producing a count, returning a boolean or optional or simply printing results to a console. But what if want the stream used by our terminal operation to output results to a collection which we can use afterwards, or maybe to aggregate or group and/or partition the stream?

Well we can do this using the overloaded collect() method of the Stream<E> interface that takes a Collector object as a parameter. However , before we look at using the collect() method in detail its worth looking at what we can pass to the collect() method and to do this we need to look at the Collector<T,A,R> interface. The Collector<T,A,R> interface provides methods for implementing a specific type of mutable reduction known as a collector.

We will also investigate the Collectors class in some detail, an implementation of the Collector<T,A,R> interface which has some very useful static methods, including ones to create collections, find aggregates and group and/or partition a stream.

Collection creation and aggregations will be looked at in the Collecting & Aggregating Streams lesson.

Grouping & Partitioning our streams will be discussed in the Grouping & Partitioning Streams lesson.

Collector<T,A,R> Interface Top

The Collector<T,A,R> interface consists of five implementation methods and an of static metod for Collector creation.

You can implement the Collector<T,A,R> interface for a specific purpose or more generally use the static methods of the Collectors implementation class.

Type Parameter Pnemonics:

1. T - Generic type of input elements to reduction operation
2. A - Mutable accumulation type of reduction operation (object accumulated on during collection)
3. R - Result type of reduction operation

The following table lists the implementation methods of the Collector<T,A,R> interface:

The following code shows an example implementation of the Collector<T,A,R> interface:

package com.server2client;

import java.util.*;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

import static java.util.stream.Collector.Characteristics.*;

public class CollectorImpl<T> implements Collector<T, Set<T>, Set<T>> {

    @Override
    public Supplier<Set<T>> supplier() {
        return HashSet::new;
    }

    @Override
    public BiConsumer<Set<T>, T> accumulator() {
        return Set::add;
    }

    @Override
    public BinaryOperator<Set<T>> combiner() {
        return (setA, setB) -> {
            setA.addAll(setB);
            return setA;
        };
    }

    @Override
    public Function<Set<T>, Set<T>> finisher() {
        return Function.identity();
    }

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

Lets go though the code!

The supplier() method creates and returns an empty instance of an accumulator used during collection; in our case an empty Set<E> object.

The accumulator() method adds the current item to the accumulator used during collection and returns the accumulator object; in our case a Set<E> object.

The combiner() method merges partial results from the subparts of the stream and returns the combined results; in our case a Set<E> object.

The finisher() returns the final transformation from the intermediate accumulation type A to the final result type R; in our case both are of type Set<T>.

The characteristics() method returns an immutable set of Characteristics which define the behaviour of this Collector.

The Characteristics class is an enumeration containing the following items:

1. CONCURRENT - The accumulator()) function can be called from multiple threads for the same result container. If not also marked as UNORDERED, then it should only be evaluated concurrently if applied to an unordered data source.
2. UNORDERED - The results of the reduction aren't affected by traversal and accumulation order.
3. IDENTITY_FINISH - Indicates the finisher() function is the same as the identity function and can be left out.

Collectors Class Top

The Collectors class has lots of static method implementations of Collector for collecting a stream into a list, set or map and even into a collection implementation, for aggregating, grouping, partitioning as well as utility methods.

The following table lists all the Collectors class static methods available:

Related Quiz

Streams Quiz 9 - Stream Collectors Quiz

Lesson 9 Complete

In this lesson we looked at the Collector<T,A,R> interface and the Collectors class.

What's Next?

In the next lesson we look at grouping and partitioning.

• <<    Reduction Operations
• Collecting & Aggregating Streams     >>