java8中3個參數(shù)的reduce方法怎么理解？

例如這個練習題，使用reduce和lambda表達式來實現(xiàn)map。
不明白的是reduce第三個參數(shù)的意義，感覺多此一舉

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Stream;

public class MapUsingReduce {
   
        public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper) {
            return stream.reduce(new ArrayList<O>(), (acc, x) -> {
                // We are copying data from acc to new list instance. It is very inefficient,
                // but contract of Stream.reduce method requires that accumulator function does
                // not mutate its arguments.
                // Stream.collect method could be used to implement more efficient mutable reduction,
                // but this exercise asks to use reduce method.
                List<O> newAcc = new ArrayList<>(acc);
                newAcc.add(mapper.apply(x));
                return newAcc;
            }, (List<O> left, List<O> right) -> {
                // We are copying left to new list to avoid mutating it. 
                List<O> newLeft = new ArrayList<>(left);
                newLeft.addAll(right);
                return newLeft;
            });
        }
    
    }

下面是文檔的說明，仍然不明白。。。

<U> U reduce(U identity,
              BiFunction<U,? super T,U> accumulator,
              BinaryOperator<U> combiner) 

Performs a reduction on the elements of this stream, using the provided identity, accumulation and combining functions. This is equivalent to:

  U result = identity;
  for (T element : this stream)
      result = accumulator.apply(result, element)
  return result;

but is not constrained to execute sequentially.

The identity value must be an identity for the combiner function. This means that for all u, combiner(identity, u) is equal to u. Additionally, the combiner function must be compatible with the accumulator function; for all u and t, the following must hold:

combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)

望指教，謝謝！