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ListExample.java
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readme.md

☕ Java Lesson – Custom Comparator (Sorting in Java)

Is lesson me hum seekhenge:

  • Comparator kya hota hai
  • Custom comparator kyun use karte hain
  • Comparator vs Comparable
  • Custom sorting kaise karte hain
  • Real examples

1️⃣ Comparator Kya Hota Hai?

Comparator ek interface hai:

jo custom sorting logic define karne ke liye use hota hai

Package:

java.util.Comparator

2️⃣ Comparator Ka Use Kyun?

Default sorting:

ascending order me hoti hai

Agar aapko chahiye:

descending
custom logic (age, name, marks)

to Comparator use karte hain.

3️⃣ Comparator Interface Method

int compare(Object o1, Object o2)

Return:

negative → o1 < o2
zero     → equal
positive → o1 > o2

4️⃣ Basic Example (Descending Order)

import java.util.*;

class Test {

    public static void main(String[] args){

        ArrayList<Integer> list = new ArrayList<>();

        list.add(10);
        list.add(30);
        list.add(20);

        Collections.sort(list, (a, b) -> b - a);

        System.out.println(list);
    }
}

Output:

[30, 20, 10]

5️⃣ Custom Object Sorting

class Student {

    String name;
    int marks;

    Student(String name, int marks){
        this.name = name;
        this.marks = marks;
    }
}

6️⃣ Sort by Marks

import java.util.*;

class Test {

    public static void main(String[] args){

        ArrayList<Student> list = new ArrayList<>();

        list.add(new Student("Sujit", 85));
        list.add(new Student("Rahul", 90));
        list.add(new Student("Amit", 80));

        Collections.sort(list, (s1, s2) -> s2.marks - s1.marks);

        for(Student s : list){
            System.out.println(s.name + " " + s.marks);
        }
    }
}

7️⃣ Sort by Name

Collections.sort(list, (s1, s2) -> s1.name.compareTo(s2.name));

8️⃣ Comparator vs Comparable

Feature Comparator Comparable
Package java.util java.lang
Method compare() compareTo()
Use external sorting internal sorting

9️⃣ Multiple Level Sorting

Collections.sort(list, (s1, s2) -> {

    if(s1.marks == s2.marks){
        return s1.name.compareTo(s2.name);
    }

    return s2.marks - s1.marks;
});

🔟 Using Comparator Class (Old Way)

Collections.sort(list, new Comparator<Student>() {

    public int compare(Student s1, Student s2){
        return s1.marks - s2.marks;
    }
});

1️⃣1️⃣ Built-in Comparators

Collections.sort(list, Comparator.reverseOrder());
Collections.sort(list, Comparator.naturalOrder());

1️⃣2️⃣ Real-Life Example

Collections.sort(list, (a, b) -> a.length() - b.length());

String length ke basis par sorting.

1️⃣3️⃣ Important Points

✔ Comparator external sorting ke liye use hota hai
✔ Multiple sorting conditions possible hain
✔ Lambda expressions se code short ho jata hai

🎯 Interview Questions

  1. Comparator kya hota hai?
  2. Comparator aur Comparable me difference?
  3. compare() method ka return kya hota hai?
  4. Custom sorting kaise implement karte hain?

🧾 Lesson Summary

Is lesson me humne seekha:

✔ Comparator interface
✔ Custom sorting logic
✔ Lambda-based sorting
✔ Comparator vs Comparable
✔ Multi-level sorting

Comparator Java me flexible aur powerful sorting mechanism provide karta hai.