#vulnerable code 
                                @Override
    @SuppressWarnings("unchecked")
    public boolean retainAll(Collection<?> c) {
        this.clear();
        Iterator<? extends Model> iterator = (Iterator<? extends Model>) c
                .iterator();
        boolean success = true;
        while (iterator.hasNext()) {
            T element = (T) iterator.next();
            success &= internalAdd(element, false);
        }
        refreshStorage(true);
        return success;
    } 
                            #location 4 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    @SuppressWarnings("unchecked")
    public boolean retainAll(Collection<?> c) {
        this.clear();
        Iterator<? extends Model> iterator = (Iterator<? extends Model>) c
                .iterator();
        boolean success = true;
        while (iterator.hasNext()) {
            T element = (T) iterator.next();
            success &= internalAdd(element);
        }
        return success;
    }