You may write your own comparator
public class ExampleComparator implements Comparator<String> {
public int compare(String obj1, String obj2) {
if (obj1 == obj2) {
return 0;
}
if (obj1 == null) {
return -1;
}
if (obj2 == null) {
return 1;
}
return obj1.compareTo(obj2);
}
}