Java Comparator 101

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In Java programming, the Comparator interface plays a crucial role in sorting objects in custom orders. Understanding its usage is essential for any Java developer. In this blog post, we’ll delve into the basics of Comparator, its implementation, and how it can be used to solve common interview problems. By the end, you’ll have a solid understanding of how to leverage Comparator effectively in your Java projects.

1. What is a Comparator?

• A Comparator in Java is an interface used to impose a total ordering on some collection of objects. It allows custom comparison logic for sorting objects.

2. Implementation:

• To implement a Comparator, you need to create a class that implements the Comparator interface and override its compare method.
• The compare method takes two objects of the same type and returns a negative integer, zero, or a positive integer based on whether the first object is less than, equal to, or greater than the second object.

3. Arrays.sort() with Comparator:

• The Arrays.sort() method in Java is used to sort arrays of objects. When you want to sort objects based on a custom ordering defined by a Comparator, you can pass the Comparator as a second argument to the Arrays.sort() method.

Three Simple Comparator Problems:

1. Sorting Strings by Length:

• Given an array of strings, sort them based on their lengths.

   import java.util.Arrays;
   import java.util.Comparator;

   public class StringLengthComparator implements Comparator<String> {
       @Override
       public int compare(String a, String b) {
           return a.length() - b.length();
       }

       public static void main(String[] args) {
           String[] strings = {"apple", "banana", "orange", "kiwi", "grape"};
           Arrays.sort(strings, new StringLengthComparator());
           System.out.println(Arrays.toString(strings));
       }
   }

Output:

   [kiwi, grape, apple, banana, orange]

2. Sorting Integers by Absolute Value:

• Given an array of integers, sort them based on their absolute values.

   import java.util.Arrays;
   import java.util.Comparator;

   public class AbsoluteValueComparator implements Comparator<Integer> {
       @Override
       public int compare(Integer a, Integer b) {
           return Math.abs(a) - Math.abs(b);
       }

       public static void main(String[] args) {
           Integer[] integers = {-4, 3, -2, 0, 1, -5};
           Arrays.sort(integers, new AbsoluteValueComparator());
           System.out.println(Arrays.toString(integers));
       }
   }

Output:

   [0, 1, -2, 3, -4, -5]

3. Sorting Custom Objects:

• Suppose you have a class Person with attributes name and age, and you want to sort an array of Person objects based on their ages.

   import java.util.Arrays;
   import java.util.Comparator;

   class Person {
       String name;
       int age;

       Person(String name, int age) {
           this.name = name;
           this.age = age;
       }
   }

   public class PersonAgeComparator implements Comparator<Person> {
       @Override
       public int compare(Person a, Person b) {
           return a.age - b.age;
       }

       public static void main(String[] args) {
           Person[] people = {
               new Person("Alice", 25),
               new Person("Bob", 20),
               new Person("Charlie", 30)
           };
           Arrays.sort(people, new PersonAgeComparator());
           for (Person person : people) {
               System.out.println(person.name + " - " + person.age);
           }
       }
   }

Output:

   Bob - 20
   Alice - 25
   Charlie - 30

These examples demonstrate the basics of implementing a Comparator and using it with Arrays.sort() for various types of objects.

Who Am I?

• I’m Aswin Barath, a Software Engineering Nerd who loves building Web Applications and sharing my knowledge through Blogging 
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