在实践中的Java并发中，Brian Goetz

这是一个计算阶乘的示例：

public static void main(String[] args) throws Exception {    //create a memoizer that performs factorials    final Memoizer<Integer, Integer> memo = new Memoizer<Integer, Integer> (new Computable<Integer, Integer>() {        @Override        public Integer compute(Integer a) { int result = 1 ; for(int i = 1 ; i < a ; i++){     result = result*i; } return result;        }    });    //now call the memoizer    System.out.println(memo.compute(10));    //call it with 10 threads concurrently    ExecutorService exec = Executors.newFixedThreadPool(10);    ExecutorCompletionService<Integer> compService = new ExecutorCompletionService<Integer>(exec);    for(int i = 0 ; i < 15 ; i++){        compService.submit(new Callable<Integer>(){ @Override public Integer call() throws Exception {     return memo.compute(5); }        });    }    exec.shutdown();    for(int i = 0 ; i < 15 ; i++){        System.out.println(compService.take().get());    }}

因此，如果两个线程尝试在完全相同的时间计算相同的阶乘，则由于

putIfAbsent