Understanding the Comparator Interface
The Comparator interface in Java is used when you want to customize the sorting order of objects, rather than relying on their natural ordering (as defined by the Comparable interface). It allows you to create multiple ways to sort objects without modifying the class itself.
Key Points:
- The
Comparableinterface defines natural ordering using thecompareTo()method, but sometimes you need custom ordering, such as:- Sorting by a different field.
- Sorting in descending order.
- Sorting based on multiple fields (e.g., first by price, then by name).
- The
Comparatorinterface has a method calledcompare()that allows us to define custom comparison logic.
Comparator Interface: The compare() Method
The Comparator interface requires us to implement the compare() method, which takes two objects as parameters and returns an integer:
compare(Object obj1, Object obj2):- Returns positive (+ve) if
obj1should be placed afterobj2. - Returns negative (-ve) if
obj1should be placed beforeobj2. - Returns
0if both objects are considered equal.
- Returns positive (+ve) if
Steps to Implement Comparator
- Create a class that implements the
Comparator<T>interface. - Override the
compare()method, defining how two objects should be compared based on your custom criteria (e.g., descending order by price). - Use this comparator to sort your objects by passing it to
Collections.sort()or using it in data structures likeTreeSet.
Code Example
x
import java.util.Comparator;import java.util.*;class Product { private String name; private double price; // Constructor public Product(String name, double price) { this.name = name; this.price = price; } // Getters public String getName() { return name; } public double getPrice() { return price; } // For better output public String toString() { return "Product{name='" + name + "', price=" + price + "}"; }}// Custom Comparator// Comparator to sort by price in descending orderclass PriceComparator implements Comparator<Product> { public int compare(Product p1, Product p2) { // Compare by price in descending order return Double.compare(p2.getPrice(), p1.getPrice()); }}public class ProductSortingExample { public static void main(String[] args) { // Create a list of Product objects List<Product> productList = new ArrayList<>(); productList.add(new Product("Laptop", 800.00)); productList.add(new Product("Smartphone", 500.00)); productList.add(new Product("Headphones", 50.00)); productList.add(new Product("Monitor", 300.00)); // Before sorting System.out.println("Before Sorting by Price:"); for (Product p : productList) { System.out.println(p); } // Sorting products by price in descending order using Comparator Collections.sort(productList, new PriceComparator()); // After sorting System.out.println("\nAfter Sorting by Price (Descending):"); for (Product p : productList) { System.out.println(p); } }}Output
Before Sorting by Price:
Product{name='Laptop', price=800.0}
Product{name='Smartphone', price=500.0}
Product{name='Headphones', price=50.0}
Product{name='Monitor', price=300.0}
After Sorting by Price (Descending):
Product{name='Laptop', price=800.0}
Product{name='Smartphone', price=500.0}
Product{name='Monitor', price=300.0}
Product{name='Headphones', price=50.0}
Explanation of compare() Method:
- We compare the two products,
p1andp2, by their price usingDouble.compare(). Double.compare(p2.getPrice(), p1.getPrice()):- This compares the prices in reverse order, so products with higher prices will appear first.
- This is how we achieve descending order.
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