Using Java Comparator in HashMap to Sort Elements

In this example I am going to show you an example on using Comparator in HashMap will show you how to use Comparator to sort values in HashMap. I will use custom object as a key in the HashMap. The object which is used as an object as a key in HashMap must override hashCode() and equals() methods.

Prerequisites

Knowledge of Java

Related Posts:

• Comparator interface in Java
• Custom object as a key in HashMap

Creating POJO Class

I will create a simple POJO class Book that will act as a key in HashMap. As the instance of this class is used as a key, so I am overriding hashCode() and equals() method in the Book class.

I will check both title and author attributes for the equality check for the two Book objects.

If you are using Eclipse then you can generate these two methods – hashCode() and equals() – from the option.

Shortcut keys in Windows system in Eclipse are Alt + Shift + S. On popup window click on Generate hashCode() and equals().

package com.roytuts.java.comparartor.in.hashmap;

public class Book {

	private String title;
	private String author;

	public Book(String title, String author) {
		this.title = title;
		this.author = author;
	}

	public String getTitle() {
		return title;
	}

	public String getAuthor() {
		return author;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((author == null) ? 0 : author.hashCode());
		result = prime * result + ((title == null) ? 0 : title.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Book other = (Book) obj;
		if (author == null) {
			if (other.author != null)
				return false;
		} else if (!author.equals(other.author))
			return false;
		if (title == null) {
			if (other.title != null)
				return false;
		} else if (!title.equals(other.title))
			return false;
		return true;
	}

	@Override
	public String toString() {
		return "Book [title=" + title + ", author=" + author + "]";
	}

}

Main Class

Create below main class which will test the user defined objects as keys in HashMap.

I will also use Comparator to sort the elements in HashMap. I will use comparator() method of Comparator interface in pre-Java 8 style and lambda expression.

I am using here Collections.sort() method to sort our list of Book objects.

package com.roytuts.java.comparartor.in.hashmap;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class HashMapComparator {

	public static void main(String[] args) {
		Map<Book, String> bookMap = new HashMap<>();

		bookMap.put(new Book("Java", "James Gosling"), "Java");
		bookMap.put(new Book("C++", "Bjourn Stroustup"), "C++");
		bookMap.put(new Book("C", "Denish Ritche"), "C");
		bookMap.put(new Book("Databse", "C J Date"), "Database");

		System.out.println("Unsorted HashMap Values");
		System.out.println("-----------------------");

		bookMap.forEach((k, v) -> System.out.println(k + " => " + v));

		System.out.println();

		Set<Entry<Book, String>> set = bookMap.entrySet();
		List<Entry<Book, String>> list = new ArrayList<Entry<Book, String>>(set);

		Collections.sort(list, new Comparator<Map.Entry<Book, String>>() {
			@Override
			public int compare(Entry<Book, String> b1, Entry<Book, String> b2) {
				return (b1.getValue()).compareTo(b2.getValue());
			}
		});

		System.out.println("Sorted HashMap Values");
		System.out.println("---------------------");

		list.forEach(item -> System.out.println(item.getKey() + " => " + item.getValue()));

		Collections.sort(list, (b1, b2) -> b1.getValue().compareTo(b2.getValue()));
		
		System.out.println();
		
		System.out.println("Sorted HashMap Values (Lambda Expression)");
		System.out.println("-----------------------------------------");

		list.forEach(item -> System.out.println(item.getKey() + " => " + item.getValue()));
	}

}

Using the lambda expression you can replace the below code snippets in the above class.

The Comparator interface is a functional interface and it has only one method and annotated with @FunctionalInterface so that no further new method can be added and lambda expression can be used easily.

Collections.sort(list, new Comparator<Map.Entry<Book, String>>() {
	@Override
	public int compare(Entry<Book, String> b1, Entry<Book, String> b2) {
		return (b1.getValue()).compareTo(b2.getValue());
	}
});

by

Collections.sort(list, (b1, b2) -> b1.getValue()).compareTo(b2.getValue());

Testing the Program

Run the above main class and you will get the below output:

Unsorted HashMap Values
-----------------------
Book [title=C++, author=Bjourn Stroustup] => C++
Book [title=Java, author=James Gosling] => Java
Book [title=C, author=Denish Ritche] => C
Book [title=Databse, author=C J Date] => Database

Sorted HashMap Values
---------------------
Book [title=C, author=Denish Ritche] => C
Book [title=C++, author=Bjourn Stroustup] => C++
Book [title=Databse, author=C J Date] => Database
Book [title=Java, author=James Gosling] => Java

Sorted HashMap Values (Lambda Expression)
-----------------------------------------
Book [title=C, author=Denish Ritche] => C
Book [title=C++, author=Bjourn Stroustup] => C++
Book [title=Databse, author=C J Date] => Database
Book [title=Java, author=James Gosling] => Java

Hope you got an idea how to use Comparator in HashMap to sort the objects.

Source Code

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