Lambda Schedule to println from a ArrayList

Question

I am trying to create a timer to print out from an ArrayList at a schedule. But I am receiving this error:

Cannot resolve method 'schedule(<lambda expression>,int,java.util.concurret.TimeUnit)'

Why am I receiving this error and what can I do about it?

List<String> finalQuestionsList = questionsList;
timer.schedule(()-> finalQuestionsList.forEach(write::println),120, TimeUnit.SECONDS);

Check out the different signature of the method `schedule` and make sure to comply with one of them: https://docs.oracle.com/javase/7/docs/api/java/util/Timer.html — Nir Alfasi, Apr 07 '17 at 17:00
`TimerTask` is not a functional interface, but you shouldn’t use that class anyway. Use a `ScheduledThreadPoolExecutor`… — Holger, Apr 07 '17 at 17:52

holi-java · Accepted Answer · 2017-04-08T08:45:33.303

JLS asserts type of a lambda expression:

A lambda expression is compatible in an assignment context, invocation context, or casting context with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.

TimerTask is not a Function Interface, so you can't assign a lambda expression to a TimerTask. but you can adaptee a Runnable to a TimerTask with an adapter method task.
Timer.schedule methods can't take a TimeUnit, but you can using TimeUnit.toMills() to convert a duration to milliseconds.

I can't see any write variable in your code snippet, so I use System.out instead.

List<String> finalQuestionsList = questionsList;
timer.schedule(task(() -> finalQuestionsList.forEach(System.out::println))
         , 120, TimeUnit.SECONDS.toMillis(1));

private TimerTask task(Runnable task) {
    return new TimerTask() {
      @Override
      public void run() {
        task.run();
      }
    };
}

Lambda Schedule to println from a ArrayList

1 Answers1