Java Comparator with SortedMap
March 24, 2019
Comparator can be used to control the order of keys of SortedMap data structures. The implementing classes of SortedMap are TreeMap and ConcurrentSkipListMap. We can pass Comparator instance to the constructor of TreeMap and ConcurrentSkipListMap classes to control the order of its keys. SortedMap provides comparator() method that returns the comparator used to order the keys in this map. If SortedMap uses natural ordering of key elements, then comparator() method returns null. Here on this page we will provide examples to use comparator with TreeMap and ConcurrentSkipListMap classes.
Comparator with TreeMap
TreeMap are sorted according to the natural ordering of its keys, or by a comparator provided at map creation time, depending on which constructor is used. We can instantiate TreeMap class by passing Comparator using following constructor.
TreeMap(Comparator<? super K> comparator)
TreeMap sorts keys to its natural ordering. For natural ordering, an element class needs to implement Comparable interface and override compareTo method.
To get the comparator used by our
TreeMap object, SortedMap provides comparator() method.
Now find the example using comparator with
TreeMap class to control the order of its keys.
TreeMapDemo.java
package com.concretepage;
import java.util.Comparator;
import java.util.TreeMap;
public class TreeMapDemo {
public static void main(String[] args) {
Student s1 = new Student("Shyam", 18);
Student s2 = new Student("Mohan", 20);
Student s3 = new Student("Ram", 22);
System.out.println("---TreeMap Order With Comparator---");
Comparator<Student> ageComparator = Comparator.comparing(Student::getAge);
TreeMap<Student, String> myTreeMap = new TreeMap<>(ageComparator);
myTreeMap.put(s1, "Varanasi");
myTreeMap.put(s2, "Mathura");
myTreeMap.put(s3, "Kashi");
myTreeMap.forEach((k, v) -> System.out.println(k + " - " + v));
//System.out.println("Comparator: "+ myTreeMap.comparator());
System.out.println("---TreeMap Natural Order (With Comparable)---");
myTreeMap = new TreeMap<>();
myTreeMap.put(s1, "Varanasi");
myTreeMap.put(s2, "Mathura");
myTreeMap.put(s3, "Kashi");
myTreeMap.forEach((k, v) -> System.out.println(k + " - " + v));
}
}
package com.concretepage;
public class Student implements Comparable<Student> {
private String name;
private int age;
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
@Override
public int compareTo(Student s) {
return name.compareTo(s.getName());
}
@Override
public String toString(){
return name + "-" + age;
}
}
---TreeMap Order With Comparator--- Shyam-18 - Varanasi Mohan-20 - Mathura Ram-22 - Kashi ---TreeMap Natural Order (With Comparable)--- Mohan-20 - Mathura Ram-22 - Kashi Shyam-18 - Varanasi
Comparator with ConcurrentSkipListMap
ConcurrentSkipListMap are sorted according to the natural ordering of its keys, or by a comparator provided at map creation time, depending on which constructor is used. We can instantiate ConcurrentSkipListMap class by passing Comparator using following constructor.
ConcurrentSkipListMap(Comparator<? super K> comparator)
ConcurrentSkipListMap sorts keys to its natural ordering. For natural ordering, an element class needs to implement Comparable interface and override compareTo method.
To get the comparator used by our
ConcurrentSkipListMap object, SortedMap provides comparator() method.
Now find the example using comparator with
ConcurrentSkipListMap class to control the order of its keys.
ConcurrentSkipListMapDemo.java
package com.concretepage;
import java.util.Comparator;
import java.util.concurrent.ConcurrentSkipListMap;
public class ConcurrentSkipListMapDemo {
public static void main(String[] args) {
Student s1 = new Student("Shyam", 18);
Student s2 = new Student("Mohan", 20);
Student s3 = new Student("Ram", 22);
System.out.println("---ConcurrentSkipListMap Order With Comparator---");
Comparator<Student> ageComparator = Comparator.comparing(Student::getAge);
ConcurrentSkipListMap<Student, String> myConcurrentSkipListMap = new ConcurrentSkipListMap<>(ageComparator);
myConcurrentSkipListMap.put(s1, "Varanasi");
myConcurrentSkipListMap.put(s2, "Mathura");
myConcurrentSkipListMap.put(s3, "Kashi");
myConcurrentSkipListMap.forEach((k, v) -> System.out.println(k + " - " + v));
//System.out.println("Comparator: "+ myConcurrentSkipListMap.comparator());
System.out.println("---ConcurrentSkipListMap Natural Order (With Comparable)---");
myConcurrentSkipListMap = new ConcurrentSkipListMap<>();
myConcurrentSkipListMap.put(s1, "Varanasi");
myConcurrentSkipListMap.put(s2, "Mathura");
myConcurrentSkipListMap.put(s3, "Kashi");
myConcurrentSkipListMap.forEach((k, v) -> System.out.println(k + " - " + v));
}
}
---ConcurrentSkipListMap Order With Comparator--- Shyam-18 - Varanasi Mohan-20 - Mathura Ram-22 - Kashi ---ConcurrentSkipListMap Natural Order (With Comparable)--- Mohan-20 - Mathura Ram-22 - Kashi Shyam-18 - Varanasi
References
Interface ComparatorInterface SortedMap
Class TreeMap
Class ConcurrentSkipListMap
Java Comparator.comparing
