🧠 Why do we need `equals()` and `hashCode()`?¶

equals() → decides logical equality (are two objects “the same”?)
hashCode() → decides which bucket the object goes into

👉 Hash-based collections use both:

Find bucket using hashCode()
Compare using equals()

📦 Example: Without overriding (Problem)¶

class Person {
    String name;
    int age;
}

Person p1 = new Person("Swapnil", 25);
Person p2 = new Person("Swapnil", 25);

System.out.println(p1.equals(p2)); // ❌ false

👉 Why? Default equals() from Object checks memory address, not values.

✅ Correct Implementation¶

import java.util.Objects;

class Person {
    String name;
    int age;

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true; // same reference
        if (o == null || getClass() != o.getClass()) return false;

        Person person = (Person) o;
        return age == person.age &&
               Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

🔍 Step-by-Step Explanation¶

🧩 equals()¶

if (this == o) return true;

👉 Same object in memory → obviously equal

if (o == null || getClass() != o.getClass()) return false;

👉 Prevent null comparison and type mismatch

return age == person.age &&
       Objects.equals(name, person.name);

👉 Compare actual fields (business logic)

🧮 hashCode()¶

return Objects.hash(name, age);

👉 Generates a number based on fields used in equals()

⚠️ Rule:

If two objects are equal → their hashCode must be equal

⚠️ Important Contract (Interview Gold)¶

🔑 Rule 1¶

If equals() is true → hashCode() must be same

🔑 Rule 2¶

If hashCode() is same → equals() may or may not be true

💥 What happens if you break it?¶

Set<Person> set = new HashSet<>();

set.add(new Person("Swapnil", 25));
set.add(new Person("Swapnil", 25));

👉 If not overridden properly:

You may get duplicate entries 😱

🎯 Real Interview Answer¶

“equals() is used for logical comparison, while hashCode() is used by hash-based collections to determine bucket location. Both must be overridden together to maintain consistency and avoid unexpected behavior in collections like HashMap and HashSet.”

🚀 Memory Trick¶

Think of it like a library 📚:

hashCode() → which shelf (fast lookup)
equals() → which exact book (final check)

🧠 Why do we need equals() and hashCode()?¶