Java 8 – How to sort TreeSet in descending-order using Stream ?

September 28, 2021 SJ Collection, Java 8 0

In this article, we will discuss how to sort TreeSet elements in descending order using Java 8 Stream API

Before proceeding with this sorting examples understand below items,

1. Stream.sorted() method

  • This Stream method is an stateful intermediate operation which sorts elements present in the stream according to natural order
  • If elements present in the stream aren’t Comparable then java.lang.ClassCastException is thrown when final terminal operation is executed
  • For ordered streams, the sort is stable
  • For unordered streams, no stability guarantees are made
  • Method signature 1 :- Stream<T> sorted()
  • Method signature 2 :- Stream<T> sorted(Comparator<? super T> comparator)

1.1 Sorting TreeSet of Integer numbers

  • A TreeSet contains integer numbers in natural order ( or ascending order)
  • We are sorting these integer numbers in descending order using Java 8 Stream’s sorted() method

SortTreeSetOfIntegerInDescUsingStream.java

package net.bench.resources.sort.treeset;

import java.util.Comparator;
import java.util.TreeSet;
import java.util.Set;

public class SortTreeSetOfIntegerInDescUsingStream {

	public static void main(String[] args) {

		// 1. create TreeSet
		Set<Integer> numbers = new TreeSet<>();


		// 1.2 add integer numbers to TS
		numbers.add(181);
		numbers.add(219);
		numbers.add(529);
		numbers.add(761);
		numbers.add(431);
		numbers.add(671);
		numbers.add(351);


		// 1.3 original TreeSet in insertion order
		System.out.println("1. Original TreeSet<Integer> in ASC order :- \n");
		numbers.forEach(System.out::println);


		// 2. Reverse Order
		System.out.println("\n2. TreeSet<Integer> in DESC order :- \n");


		// 2.1 reverse order - Stream.sorted() and print
		numbers
		.stream() // get sequential stream
		.sorted(Comparator.reverseOrder()) // reverse order sorting
		.forEach(System.out::println); // print
	}
}

Output:

1. Original TreeSet<Integer> in ASC order :- 

181
219
351
431
529
671
761

2. TreeSet<Integer> in DESC order :- 

761
671
529
431
351
219
181

1.2 Sorting TreeSet of String elements

  • A TreeSet contains String elements in alphabetical order ( or ascending order)
  • We are sorting these String elements in reverse alphabetical order using Java 8 Stream’s sorted() method

SortTreeSetOfStringInDescUsingStream.java

package net.bench.resources.sort.treeset;

import java.util.Comparator;
import java.util.TreeSet;
import java.util.Set;

public class SortTreeSetOfStringInDescUsingStream {

	public static void main(String[] args) {

		// 1. create TreeSet
		Set<String> names = new TreeSet<>();


		// 1.1 add names to TS
		names.add("Saranya");
		names.add("Karthika");
		names.add("Amudha");
		names.add("Pushpa");
		names.add("Bhagyalakshmi");
		names.add("Meena");


		// 1.2 original TreeSet in insertion order
		System.out.println("1. Original TreeSet<String> "
				+ "in alphabetical order :- \n");
		names.forEach(System.out::println);


		// 2. Sorting Reverse Alphabetical order
		System.out.println("\n2. TreeSet<String> "
				+ "in reverse alphabetical order :- \n");


		// 2.1 sort using Stream - reverse alphabetical order
		names
		.stream() // get sequential stream
		.sorted(Comparator.reverseOrder()) // reverse alphabetical sorting
		.forEach(System.out::println); // print
	}
}

Output:

1. Original TreeSet<String> in alphabetical order :- 

Amudha
Bhagyalakshmi
Karthika
Meena
Pushpa
Saranya

2. TreeSet<String> in reverse alphabetical order :- 

Saranya
Pushpa
Meena
Karthika
Bhagyalakshmi
Amudha

1.3 Sorting TreeSet of Employee objects

  • A TreeSet contains Employee objects with attributes such as Id, name and their salary in ascending order of their Id‘s
  • We are sorting these Employee objects in descending order according to their salary using Java 8 Stream’s sorted() method

Employee.java

package net.bench.resources.sort.treeset;

public class Employee implements Comparable<Employee> {

	// member variables
	private int id;
	private String name;
	private long salary;

	// 3-arg parameterized constructor

	// getters and setters

	// toString()

	// compareTo
	@Override
	public int compareTo(Employee emp) {
		return Long.compare(this.getId(), emp.getId());
	}
}

SortLinkedHashSetOfEmployeesUsingStreamSorted.java

package net.bench.resources.sort.treeset;

import java.util.Comparator;
import java.util.TreeSet;
import java.util.Set;

public class SortTreeSetOfEmployeesInDescUsingStreamSorted {

	public static void main(String[] args) {

		// 1. create TreeSet
		Set<Employee> employees = new TreeSet<>();


		// 1.1 add Employees to TS
		employees.add(new Employee(101, "Saranya", 85000L));
		employees.add(new Employee(102, "ThenMozhi", 57000L));
		employees.add(new Employee(103, "Amudha", 49000L));
		employees.add(new Employee(104, "Pushpa", 24000L));
		employees.add(new Employee(105, "Bhagyalakshmi", 29000L));
		employees.add(new Employee(106, "Meena", 38000L));


		// 1.2 original TreeSet in insertion order
		System.out.println("1. Original TreeSet "
				+ "as per sorted-order of Id :- \n");
		employees.forEach(System.out::println);


		// 2. Sorting Employee - decreasing salary
		System.out.println("\n2. Sorted Employee TreeSet - "
				+ "according to decreasing salary :- \n");


		// 2.1 sort using Stream - decreasing-order of salary
		employees
		.stream()
		.sorted(Comparator.comparingLong(Employee::getSalary).reversed())
		.forEach(System.out::println);
	}
}

Output:

1. Original TreeSet as per sorted-order of Id :- 

Employee [id=101, name=Saranya, salary=85000]
Employee [id=102, name=ThenMozhi, salary=57000]
Employee [id=103, name=Amudha, salary=49000]
Employee [id=104, name=Pushpa, salary=24000]
Employee [id=105, name=Bhagyalakshmi, salary=29000]
Employee [id=106, name=Meena, salary=38000]

2. Sorted Employee TreeSet - according to decreasing salary :- 

Employee [id=101, name=Saranya, salary=85000]
Employee [id=102, name=ThenMozhi, salary=57000]
Employee [id=103, name=Amudha, salary=49000]
Employee [id=106, name=Meena, salary=38000]
Employee [id=105, name=Bhagyalakshmi, salary=29000]
Employee [id=104, name=Pushpa, salary=24000]

Related Articles:

References:

Happy Coding !!
Happy Learning !!

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