Sorting TreeMap Using Comparator

TreeMap are sorted naturally using their Keys. If you have your objects as keys then Sorting TreeMap using Comparator is a good way to decide the map order.
Lets see an example on how to use Comparator in a TreeMap

1. User Defined Object – Employee.java

package com.kscodes.collections;

public class Employee {

private String firstName;

private String lastName;

private double salary;

public Employee(String firstName, String lastName, double salary) {

super();

this.firstName = firstName;

this.lastName = lastName;

this.salary = salary;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public double getSalary() {

return salary;

}

public void setSalary(double salary) {

this.salary = salary;

}

@Override

public String toString() {

return "Employee [firstName=" + firstName + ", lastName=" + lastName + ", salary=" + salary + "]";

}

2. Comparator – Sorting by Salary

package com.kscodes.collections;

import java.util.Comparator;

public class EmpComparator implements Comparator<Employee> {

@Override

public int compare(Employee o1, Employee o2) {

if (o1.getSalary() > o2.getSalary()) {

return 1;

} else if (o1.getSalary() < o2.getSalary()) {

return -1;

} else {

return 0;

}

3. Code to use Comparator

package com.kscodes.collections;

import java.util.Iterator;

import java.util.Map;

import java.util.Set;

import java.util.TreeMap;

import java.util.Map.Entry;

public class TreeMapComparatorExample {

public static void main(String args[]) {

TreeMap<Employee, String> empTreeMap = new TreeMap<>(new EmpComparator());

empTreeMap.put(new Employee("Steve", "Smith", 2500), "0001");

empTreeMap.put(new Employee("John", "Doe", 1500), "0002");

empTreeMap.put(new Employee("Alex", "Rock", 4500), "0003");

empTreeMap.put(new Employee("Tim", "Bax", 1000), "0004");

Set<Entry<Employee, String>> set = empTreeMap.entrySet();

Iterator<Entry<Employee, String>> iterator = set.iterator();

while (iterator.hasNext()) {

Map.Entry mapEntry = (Map.Entry) iterator.next();

System.out.print((Employee) mapEntry.getKey() + " : " + mapEntry.getValue());

System.out.println("");

}

Output

After executing the above code, we get the following into the console. You can see that the TreeMap was sorted based on the employee salary

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KSCodes

Sorting TreeMap Using Comparator

1. User Defined Object – Employee.java

2. Comparator – Sorting by Salary

3. Code to use Comparator

Output

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