http://blog.jooq.org/2014/03/28/java-8-friday-optional-will-remain-an-option-in-java/

OptionalTest.java

package foo;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Optional;

public class OptionalTest {

    static class Foo<T> {

        Collection<? extends Optional<? extends T>> source;
        Collection<? super Optional<? extends T>> target;

        void copy() {
            target.addAll(source);
        }
    }

    public static void main(String[] args) {

        Foo<Number> foo = new Foo<>();

        List<Optional<? extends Integer>> a = new ArrayList<>();
        a.add(Optional.of(1));
        a.add(Optional.<Integer>empty());
        a.add(Optional.of(3));

        List<Optional<? extends Object>> b = new ArrayList<>();
        b.add(Optional.of("A"));
        b.add(Optional.empty());

        foo.source = a;
        foo.target = b;
        foo.copy();

        System.out.println(b);
        // [Optional[A], Optional.empty, Optional[1], Optional.empty, Optional[3]]
    }
}