Lambda: Mutable state can be shared through a captured array or AtomicInteger

Because a lambda cannot capture a reassignable local variable, code that needs shared mutable state captures a one-element array or an AtomicInteger instead, whose reference is effectively final even though its contents change.

Code

int[] counter = {0};
Runnable increment = () -> counter[0]++;
increment.run();
increment.run();
increment.run();
System.out.println("Counter via array: " + counter[0]);

AtomicInteger atomic = new AtomicInteger(0);
Runnable incrementAtomic = atomic::incrementAndGet;
incrementAtomic.run();
incrementAtomic.run();
System.out.println("Counter via AtomicInteger: " + atomic.get());

Output

Counter via array: 3
Counter via AtomicInteger: 2