1.The collect is declared by the Interface of Stream.The param is Collector Interface.

<R, A> R collect(Collector<? super T, A, R> collector);

2.The Collector Interface mainly contains 4 functions about:

　　(1)creation of a new result container ({@link #supplier()})

　　(2)incorporating a new data element into a result container ({@link #accumulator()})

　　(3)combining two result containers into one ({@link #combiner()})

　　(4)performing an optional final transform on the container ({@link #finisher()}

3.The Collectors Class has a inner class which implements above Collector.So you can invoke collect by this assistant class (Collectors).The Collectors class implement some common operations.

　　ex:

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);
    }

"collect" method