Java Comparator

August 28, 2015 | 1 Minute Read

[Java] Sort a List

In the recent work, I have to retrieved a list of data with the following data structure.

  class Entity{
      int id; //Primary key
      type fields;
  }

However, the data retrieved are not in sequential order sorted by the ID. In order to maintain an ascending ordering by ID to show in the UI, a comparator is introduced to help.

Let’s say we have a list Entity and want to sort the list by its id. The following Java codes show the step.

    List<Entity> entityList = getEntityList();

    Collections.sort(entityList, new comparator(){
          @Override
          public int compare(Entity o1, Entity o2){
              return (o1.getId() - o2.getId());
          }
    })

For a descending order, just change the position o1 and o2.

Parameters: o1 - the first object to be compared. o2 - the second object to be compared.

Returns: a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Reference

https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html