java - 并行化简操作

Question

是否可以通过简单地将流更改为并行流，将如下常见的“最大”操作作为归约操作分布在多个内核上？
不同线程的结果之间如何进行最终协调(没有显式组合器)？

List<Employee> emps = new ArrayList<>();
emps.add(new Employee("Roy1",32));
emps.add(new Employee("Roy2",12));
emps.add(new Employee("Roy3",22));
emps.add(new Employee("Roy4",42));
emps.add(new Employee("Roy5",52));

Integer maxSal= emps.parallelStream().mapToInt(e -> e.getSalary()).reduce((a,b)->Math.max(a, b)).getAsInt();

System.out.println("Maximum in parallel " + maxSal);

Answer 1

是的，reduce 可以并行化。但是，这需要您传递关联运算符。摘自java.util.stream JavaDoc:

Associativity

An operator or function op is associative if the following holds:

(a op b) op c == a op (b op c)

The importance of this to parallel evaluation can be seen if we expand this to four terms:

a op b op c op d == (a op b) op (c op d)

So we can evaluate (a op b) in parallel with (c op d), and then invoke op on the results. Examples of associative operations include numeric addition, min, and max, and string concatenation.