OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide: Exam 1Z0-809

A stateful lambda expression is one whose result depends on any state that might change during the execution of a pipeline. On the other hand, a stateless lambda expression is one whose result does not depend on any state that might change during the execution of a pipeline.

Source: OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide: Exam 1Z0-809by Jeanne Boyarsky,‎ Scott Selikoff

    List < Integer > data = Collections.synchronizedList(new ArrayList < > ());

            Arrays.asList(1, 2, 3, 4, 5, 6, 7).parallelStream()


                   .map(i -> {
                    data.add(i);
                    return i;
                }) // AVOID STATEFUL LAMBDA EXPRESSIONS!
                .forEachOrdered(i -> System.out.print(i+" "));


            System.out.println();
            for (int e: data) {
                System.out.print(e + " ");

Possible Output:

1 2 3 4 5 6 7 
1 7 5 2 3 4 6 

It strongly recommended that you avoid stateful operations when using parallel streams, so as to remove any potential data side effects. In fact, they should generally be avoided in serial streams wherever possible, since they prevent your streams from taking advantage of parallelization.