Unravel › Java concepts › OOP and types

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At a glance

1. 1compareTo(other) returns a negative number if this comes first, zero if equal, positive if this comes afterInteger, String and LocalDate already have one.
2. 2Comparator.comparing(User::getName).thenComparing(User::getAge).reversed() builds a rule from small pieceslist.sort(cmp) and stream.sorted(cmp) use it.
3. 3TreeMap, TreeSet and PriorityQueue use the natural order or the comparator you give them to decide order and, for sets and maps, also what counts as a duplicate

• The rule must be consistent: if a < b and b < c then a < c, and compare(a,b) must be the opposite sign of compare(b,a)
• Never compare numbers with subtraction (a - b): large values overflow and flip the sign
• List.sort and Arrays.sort for objects use TimSort (merge sort with run detection, O(n log n), stable)

import java.util.*;public class Person implements Comparable<Person> {  final String name; final int age;  Person(String name, int age) { this.name = name; this.age = age; }  @Override public int compareTo(Person o) { return Integer.compare(age, o.age); }  static void sortByName(List<Person> people) {    people.sort(Comparator.comparing((Person p) -> p.name).thenComparing(p -> p.age).reversed());  }}

1. line 2 uses Comparable, the core of Comparable and Comparator
2. line 5 uses compareTo, the core of Comparable and Comparator
3. line 7 uses comparing, the core of Comparable and Comparator

How Unravel recognises it

When you paste code, Unravel looks for these signs of comparable and comparator. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: Comparable
• Method calls: comparing() comparingInt() comparingDouble() comparingLong() thenComparing() reversed() compareTo() Comparator.naturalOrder() Comparator.reverseOrder() nullsFirst() nullsLast()
• Method names you declare: compareTo() compare()

Part of a learning path

• Modern Java syntax · previous: Wildcards and bounded generics

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