OK task 1

FibTest.java

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;

import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class FibTest {

    private static final Logger LOG = LoggerFactory.getLogger(FibTest.class);

    @Test
    public void test() {
        Fib fib = new Fib();
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            res.add(fib.next());
        }
        LOG.info("res: {}", res);
    }

    static class Fib {

        private AtomicReference<Val> ref;

        public Fib() {
            ref = new AtomicReference<>(new Val(1, 1));
        }

        public int next() {
            Val prev;
            Val next;
            do {
                prev = this.ref.get();
                next = prev.next();
            } while (!ref.compareAndSet(prev, next));
            return this.ref.get().b;
        }

    }

    static class Val {

        private final int a;
        private final int b;

        public Val(int a, int b) {
            this.a = a;
            this.b = b;
        }

        public Val next() {
            return new Val(b, a + b);
        }

    }

}

Надо потокобезопасно выдавать последовательность Фибоначи (начиная со второго элемента): 1, 2, 3, 5, 8, ...