Comparing things Comparable & Comparator.

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Presentation transcript:

Comparing things Comparable & Comparator

Maybe we are jumping the gun a bit here but please live with it for a while. We are wanting to implement a set of “things” of the same class obviously  A set disallows duplicates. So, when we add something to the set we must make sure it is not already in the set. Consequently we need to be able to compare “things”. There are, broadly, 2 ways to do this

preliminaries We are going to use two classes from java.util TreeSet HashSet

TreeSet

HashSet

Comparable

Comparable We want to create a set of people (class Person) where a Person has a first and second name

Comparable

Comparable

Comparable

Comparable It’s comparable

Comparable x.compareTo(y)

Comparable

Comparable Duplicate not added

Comparable Same again but Species rather than Person

Comparable Duplicate avoided

Comparator

Comparator We now look at an alternative, the Comparator, an object that is used to make comparisons Our example uses cars

Comparator

Comparator

Comparator

Comparator

Comparator

Comparator Duplicate omitted

Comparator

Comparator Why do we get duplicates?

HashSet v TreeSet

HashSet doesn’t allow us to use Comparable or Comparator Why?