SynchronizedList gets null values while swapping

Question

I'm trying to write a "swap-list", think double buffer but for objects instead of raw bytes. Basically I'm doing this to cut down on contention, so one task can do a lot of removals while the swapped list is added to.

public class SwapList<T> {
    List<T> A;
    List<T> B;
    volatile boolean swap;
    public SwapList() {
        A = Collections.synchronizedList( new ArrayList<T>());
        B = Collections.synchronizedList( new ArrayList<T>());
        swap = false;
    }
    public void swap() {
        swap = !swap;
    }
    public List<T> getA() {
        return swap ? A : B;
    }
    public List<T> getB() {
        return swap ? B : A;
    }
    public int size() {
        return A.size() + B.size();
    }
    public void clear() {
        A.clear();
        B.clear();
    }
}

Several threads could be like this, only one of them would call swap(), and get the old A by calling getB()

// declared statically in same package
public static SwapList<String> list = new SwapList<>();

// one thread does this 
int i = 1000;
while( i-- > 0 ) {
    list.getA().add("A");


// another thread does this
int i = 1000;
list.swap();
while( i-- > 0 ) {
    list.getB().add("B");

Both threads are started like this, but I've tried other things like, an "executor" service.

Thread b = new Thread() {
    @Override
    public void run() {
        int i = lim;  // lim is some int 
        list.swap();  // only the second one will call swap
        while( i-- > 0 ) {
            list.getB().add("B");
        }
    }
};

// later
a.start();
b.start();
a.join();
b.join();
System.out.println( list.getA() );

However I often get an unexpected ["A","A","A",null,"A","A",...]
Where could the null values be coming from?