Is it possible in java make something like Comparator but for implementing custom equals() and hashCode()

Question

I have an array of objects and I want to concatenate it with another array of objects, except that objects that have same id\'s. That objects are used in many places in the

Answer 1

You will not succeed doing your de-duplicating concatenation with a Comparator. Presumably you are looking to do something like this:

List<Object> list = new ArrayList<Object>();
list.addAll( a );
list.addAll( b );
Collections.sort( list, new MyCustomComparator() );

The problem is that Comparator needs to compare not just for equals/not-equals, but also for relative order. Given objects x and y that are not equal, you have to answer if one is greater than the other. You will not be able to do that, since you aren't actually trying to compare the objects. If you don't give a consistent answer, you will send the sorting algorithm into an infinite loop.

I do have a solution for you. Java has a class called LinkedHashSet, whose virtue is that it doesn't allow duplicates to be inserted, but maintains insertion order. Rather than implementing a comparator, implement a wrapper class to hold the actual object and implement hashCode/equals.

Answer 2

90% of the time when a user wants an equivalence relation there is already a more straightforward solution. You want to de-duplicate a bunch of things based on ids only? Can you just put them all into a Map with the ids as keys, then get the values() collection of that?