Using Comparator in HashMap

In this tutorial I will show you how to use Comparator to sort values in HashMap.

Create below Book class that overrides equals and hashCode methods

package com.roytuts.map;

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 + "]";
    }

}

Create below main class which will test the user defined objects as keys in Map. We will also use Comparator to sort the values in HashMap.

package com.roytuts.map;

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()));
	}

}

Run the above class to check out the 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

Thanks for reading.

Soumitra Roy Sarkar

I am a professional Web developer, Enterprise Application developer, Software Engineer and Blogger. Connect me on Roy Tutorials Twitter Facebook  Google Plus Linkedin Or Email Me

Leave a Reply

Your email address will not be published. Required fields are marked *