Comparator and Comparable In Java + Difference Between Comparator and Comparable + Java 8

 Hi All,

Greeting of the day!

Today we will learn how to use comparator and comparable in sorting. Also, we will go through the differences between them.

So let's start,

Comparator and comparable both are interfaces used for comparison.

We will take 1 example of sorting a list of custom objects using different properties.

package com.demo; public class Product{ private int productId; private String productName; private int productPrice; //Add Constructors,Getter,Setter & ToString }

Now to use a Comparable interface for comparing and sorting we need to override

  public int compareTo(T o)

• Compares this object with the specified object for order. 
• compareTo method can returns  
• negative integer  =if this object is less than the specified object
• zero    =if this object is equal to specified object
• positive integer  =if this object is greater than the specified object

Below is how we can override the compareTo method.

Sorting using an integer in descending order

@Override public int compareTo(Product product) { return product.productId-this.productId; }

Sorting using an integer in ascending order 

@Override public int compareTo(Product product) { return this.productId-product.productId; }

Sorting using String in ascending order

@Override public int compareTo(Product product) { return this.productName.compareTo(product.productName); }

Sorting using String in descending order

@Override public int compareTo(Product product) { return product.productName.compareTo(this.productName); }

A comparable interface is implemented at the class level so we can sort objects using a single property.

So let's sort the List of Products by productId using Comparable

package com.demo; public class Product implements Comparable<Product>{ private int productId; private String productName; private int productPrice; @Override public int compareTo(Product product) { return this.productId-product.productId; } public int getProductId() { return productId; } public void setProductId(int productId) { this.productId = productId; } public String getProductName() { return productName; } public void setProductName(String productName) { this.productName = productName; } public int getProductPrice() { return productPrice; } public void setProductPrice(int productPrice) { this.productPrice = productPrice; } public Product(int productId, String productName, int productPrice) { super(); this.productId = productId; this.productName = productName; this.productPrice = productPrice; } public Product() { } @Override public String toString() { return "Product [productId=" + productId + ", productName=" + productName + ", productPrice=" + productPrice + "]"; } }

Now let's create a list of products and try to sort it.

package com.demo; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class Test { public static void main(String[] args) { List<Product> products = new ArrayList<Product>(); Product p1 = new Product(3, "Washing Machine", 20000); Product p2 = new Product(1, "Mobile", 10000); Product p3 = new Product(2, "Air Conditioner", 35000); products.add(p1); products.add(p2); products.add(p3); Collections.sort(products); System.out.println(products); } }

Below will be our output.

productId=1, productName=Mobile, productPrice=10000], productId=2, productName=Air Conditioner, productPrice=35000], productId=3, productName=Washing Machine, productPrice=20000]]

Now let's explore the Comparator interface.

Note - Before executing below examples remove implements Comparable<Product> and 

        @Override

public int compareTo(Product product) {

return this.productId-product.productId;

}

from Product.java

Using the Comparator interface we can sort lists by multiple properties and with Comparable we can sort by a single property it's the main difference between Comparator and Comparable.

Comparable is added at class level, Comparator is independent of class its another difference between them. 

We need to implement the below method for using the Comparator interface.

    

int compare(T o1, T o2)

 

Note this method compares its two-argument for ordering and returns below values

• negative integer -if the first argument is less than the second.
• zero-if first argument is equal to the second.
• positive integer -if the first argument is greater than the second.

We need to create a separate class that will implement a Comparator interface for sorting based on different properties.

Example- PriceSorter.java to sort the list of products in ascending order by price

package com.demo; import java.util.Comparator; public class PriceSorter implements Comparator<Product>{ @Override public int compare(Product product1, Product product2) { return product1.getProductId()-product2.getProductId(); } }

 Example 2 NameSorter to sort the list of products in descending order by name.

package com.demo; import java.util.Comparator; public class NameSorter implements Comparator<Product>{ @Override public int compare(Product product1, Product product2) { return product2.getProductName().compareTo(product1.getProductName()); } }

We need to pass an instance of NameSorter / PriceSorter as follows for sorting.

package com.demo; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class Test { public static void main(String[] args) { List<Product> products = new ArrayList<Product>(); Product p1 = new Product(3, "Washing Machine", 20000); Product p2 = new Product(1, "Mobile", 10000); Product p3 = new Product(2, "Air Conditioner", 35000); products.add(p1); products.add(p2); products.add(p3); Collections.sort(products,new NameSorter()); System.out.println(products); } }

package com.demo; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class Test { public static void main(String[] args) { List<Product> products = new ArrayList<Product>(); Product p1 = new Product(3, "Washing Machine", 20000); Product p2 = new Product(1, "Mobile", 10000); Product p3 = new Product(2, "Air Conditioner", 35000); products.add(p1); products.add(p2); products.add(p3); Collections.sort(products,new PriceSorter()); System.out.println(products); } }

Note we can either use a comparator or a comparable.

Now, let's sort List using Java 8.

package com.demo; import java.util.ArrayList; import java.util.Comparator; import java.util.List; import java.util.stream.Collectors; public class Test { public static void main(String[] args) { List<Product> products = new ArrayList<Product>(); Product p1 = new Product(3, "Washing Machine", 20000); Product p2 = new Product(1, "Mobile", 10000); Product p3 = new Product(2, "Air Conditioner", 35000); products.add(p1); products.add(p2); products.add(p3); List<Product> sortedListByProductId= products.stream(). sorted(Comparator.comparingInt(Product::getProductId)). collect(Collectors.toList()); System.out.println("Sorting by product price "); sortedListByProductId.forEach(System.out::println); System.out.println("Sorting by product name "); List<Product> sortedListByProductName= products.stream(). sorted(Comparator.comparing(Product::getProductName)). collect(Collectors.toList()); sortedListByProductName.forEach(System.out::println); } }

Here is output

Sorting by product price Product [productId=1, productName=Mobile, productPrice=10000] Product [productId=2, productName=Air Conditioner, productPrice=35000] Product [productId=3, productName=Washing Machine, productPrice=20000] Sorting by product name Product [productId=2, productName=Air Conditioner, productPrice=35000] Product [productId=1, productName=Mobile, productPrice=10000] Product [productId=3, productName=Washing Machine, productPrice=20000]

Let me know if you have any questions.

Other articles you can refer

Java Interview Programs - Count No Of Occurrences Of Element In List + Find Duplicate Elements In List + Java 8

Sorting HashSet In Ascending & Descending order + Java 8

Sorting HashMap By Key And Value + Java 8

Sorting Array List In Ascending & Descending Order + Java 8 + Sorting Array List Of Custom Object

Statement + Prepared Statement + Callable Statement In Java

For spring visit Spring Tutorial

Thank you!

Happy Learning !!