Question

The issue is to sort a huge collection of POJO objects

class Entity{
    String key1;
    String key2;
    String key3;
    String key4;
}

in alphabetical order by all of fields subsequently. That means that first we are sorting by key1, then by key2 and etc. Any of that key can be null. The question is the simplest way to do that.

Answer 1

A good way to do append all keys together and then use a Comparator<Entity> –

// all the keys of an entity are appended and than compared with other entity
int compare(Entity e1, Entity e2){
  return appendAndHandleNull(e1.key1, e1.key2, e1.key3).compareTo(appendAndHandleNull(e2.key1, e2.key2, e2.key3));
}



/** 
 * method to get all keys of an entity in appended form
 */
private static final String appendAndHandleNull(String list...){
   StringBuilder result = new StringBuilder ();
   for(String s : list){
      result.append(s!=null?s:"").append(" ");//note: a space is appended after each key
   }
   return result.toString();
}

What i am doing here is….. all the keys of an entity are appended together in the order in which comparison need to be done, and then compared with the other entity.

you might also need to trim each value before appending.

EDITED:
It is also necessary to seperate each key by space for code to work properly.

Code fixed above. Thanks @Thilo for pointing it.