Why Does Collections.sort in Java 8 Sometimes Fail to Sort JPA Returned Lists?

Why does Collections.sort in Java 8 sometimes fail to sort lists returned from JPA queries?

List<MyEntity> entities = em.createQuery("SELECT e FROM MyEntity e", MyEntity.class).getResultList(); 
Collections.sort(entities, Comparator.comparing(MyEntity::getField));

The issue with the Collections.sort method sometimes failing to sort a list returned from JPA queries stems from the way the data is handled and the nature of the list returned.

Collections.sort(entities, new Comparator<MyEntity>() {
    @Override
    public int compare(MyEntity o1, MyEntity o2) {
        return o1.getField().compareTo(o2.getField());
    }
});

Causes

• The list returned by JPA queries is not a simple list, but it may be wrapped in a proxy object like a Hibernate proxy.
• The elements being sorted may not implement the Comparable interface, or their natural ordering might not be what is expected.
• If the list has duplicate elements and the comparison is not consistent, the sort may produce unexpected results.

Solutions

• Ensure that the entities you are trying to sort implement the Comparable interface properly, overriding the compareTo method if needed.
• Use a custom Comparator when calling Collections.sort(), specifying the field you want to sort by explicitly.
• Consider using a sorting mechanism within the database query itself (using ORDER BY clause) to ensure the data is sorted before it's retrieved.

Mistake: Assuming the list is mutable and can be changed in place.

Solution: Ensure you are working with a mutable List, like ArrayList, instead of immutable lists.

Mistake: Not explicitly handling null values in the list.

Solution: Implement checks within your Comparator to handle null cases appropriately to avoid NullPointerExceptions.