Java parallel stream用法

Posted 2020-10-09 huangzifu

先摘录关于collection并发的一段话：

One difficulty in implementing parallelism in applications that use collections is that collections are not thread-safe, which means that multiple threads cannot manipulate a collection without introducing thread interference or memory consistency errors. The Collections Framework provides synchronization wrappers, which add automatic synchronization to an arbitrary collection, making it thread-safe. However, synchronization introduces thread contention. You want to avoid thread contention because it prevents threads from running in parallel. Aggregate operations and parallel streams enable you to implement parallelism with non-thread-safe collections provided that you do not modify the collection while you are operating on it.

这段话能够说明为什么需要parallel stream.

下面看看如何使用parallel stream,有两种调用方式：

1.

double average = roster
    .parallelStream()
    .filter(p -> p.getGender() == Person.Sex.MALE)
    .mapToInt(Person::getAge)
    .average()
    .getAsDouble();

　　

2.

double average = roster
    .stream().parallel()
    .filter(p -> p.getGender() == Person.Sex.MALE)
    .mapToInt(Person::getAge)
    .average()
    .getAsDouble();

　　

还有个关于排序的问题，下面这段代码使用forEachOrdered是能保证顺序的，用一般的forEach是无法保证顺序的：

listOfIntegers
    .parallelStream()
    .forEachOrdered(e -> System.out.print(e + " "));