1. COMP 103 Comperator and Comparable Thomas Kuehne 2015-T2 Lecture 9
School of Engineering and Computer Science, Victoria University of Wellington
2015-T2 Lecture 9

2. RECAP  TODAY RECAP TODAY Declaring and Creating Collections2
RECAP
Declaring and Creating Collections
Different Collections: List, Set, Stack, Queue, Map, …
Using collections: adding, removing, getting, setting,….
Collections of collections
Iterators
TODAY
Ways to compare objects of the same type

3. Bringing Order to Collections3
Ordered collections may be sorted
the original order of addition may not be useful
different orderings may serve different purposes
public class Product {
String name;
int price;
int rating; : }
Products in an online shop should be sortable according to different criteria

4. Custom Sorting Orders Looking for products by name? Product p1, p2;4
Looking for products by name?
Galaxy S6
$789
iPhone 6
$959
Xperia Z3
$668
Product p1, p2;
: p1.name < p.2 name :
Does this work?

5. Custom Sorting Orders Looking for products by rating? Product p1, p2;5
Looking for products by rating?
Xperia Z3
$668
Galaxy S6
$789
iPhone 6
$959
Product p1, p2;
: p1.rating > p.2 rating :

6. Custom Sorting Orders Looking for products by price? Product p1, p2;6
Looking for products by price?
Galaxy S6
$789
iPhone 6
$959
Xperia Z3
$668
Product p1, p2;
: p1.price > p2.price :

7. Sorting Collections The good news p1 p2
works on any List (with comparable elements) 7
The good news
java.util.Collections provides a sort method for Lists
But how to provide the sorting criterion?
products
p1.name < p2.name
Sorting Algorithm
p1.rating > p2.rating
p1 p2
p1.price > p2.price
products sorted

8. Using a Sorting Criterion8
List
Sorting Algorithm
Comparator -2 -4 3
Three potential outcomes of a comparison
p1 = p2  result = 0
p1 > p2  result > 0
p1 < p2  result < 0

9. return p1.price – p2.price;
Supplying the Sorting Criterion 9
Approach 1: Passing in a method (lambda)
Collections.sort(phones,
(p1, p2) –> { } );
return p1.price – p2.price;
Annotation Type FunctionalInterface -> if there is only one abstract method, you can provide a lambda
= 0  p1 = p2 > 0  p1 > p2
< 0  p1 < p1
products are provided by sorting algorithm

10. object that responds to one compare(…) message
Supplying the Sorting Criterion 10
Approach 2: Passing in an object (comparator)
object that responds to one compare(…) message
Collections.sort(phones, );
priceComparator

11. Supplying the Sorting Criterion11
Approach 2: Passing in an object (comparator)
object that responds to one compare(…) message
Collections.sort(phones, );
new PriceComparator()
class PriceComparator implements Comparator<Product> {
public int compare(Product p1, Product p2) {
return p1.price - p2.price; }
Where is a good place for this class?

12. Defining Sorting Criteria12
The key to achieving different sorting orders is to define different comparators
PriceComparator, RatingComparator, NameComparator, …
Comparator is a Java interface
stipulates “int compare(T o1, T o2)” method
Comparators have further usages
priority queues, supporting sorted structures, …

13. Comparator Alternative13
do we always have to supply or even define a comparator?
can we not just use “Collections.sort(phones);”?
how about “if (todayDate > deadlineDate) {…}”?
can we have default, “natural” orderings?
e.g., lexicographical ordering for String
Interface Comparable to the rescue!
natural ordering for products, dates, strings, …
stipulates “int compareTo(T o)” method

14. Comparator Alternative14
“Berta”.compareTo(“Celia”)  -1
iPhone.compareTo(xperia)  -291
today.compareTo(new Date(2015, 8, 2))  2
say “It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y))”
Method compareTo(…) yields
0, if the receiver and the argument are equal
<0, if the receiver is smaller than the argument
>0, if the receiver is bigger than the argument
(cf. API documentation regarding “equals”…)

15. Making Products Comparable15
public class Product implements Comparable<Product> {
String name; // product name
int price; // end price
int rating; // star rating :
public int compareTo(Product p) {
return this.price - p.price;
} }
Natural ordering for products is defined to be by price

16. Multiple Criteria with Priority?16
how to support multi-criterion comparison?
e.g., first star rating then price
easy to support by
first comparing using the highest priority criterion
… and in the case of a tie, use the next
etc.

17. Comparator vs Comparable17
A comparator object defines one of many ways to compare two objects to each other
think “operator” as in “o1 operator o2” (e.g., vs 2 * 3)
A comparable object is one that can be compared to another object of the same kind
think “can be comparedTo” as in o1 <natural order o2 (o1 and o2 are comparable)

18. Comparator vs Comparable18
Comparator is an interface that allows clients to use custom comparison criteria
int compare(o1, o2)  -ve, 0, +ve
Comparable is an interface that allows objects to be compared according to their natural ordering
int o1.compareTo(o2)  -ve, 0, +ve
A default comparator may just refer to compareTo()