Is lesson me hum seekhenge:

• equals() method
• hashCode() method
• equals() override
• hashCode() override
• Hash collections me kaise kaam karta hai
• Kyun pehle hashCode check hota hai phir equals()

equals() method do objects ko compare karta hai.

Default behavior:

• reference comparison karta hai
• yani memory address compare hota hai

public boolean equals(Object obj)

class Student {
    String name;
}

public class EqualsExample {
    public static void main(String[] args) {

        Student s1 = new Student();
        Student s2 = new Student();

        System.out.println(s1.equals(s2));
    }
}

Output

false

Kyun?

• Dono objects different memory location par hain.

hashCode() method object ka unique integer code return karta hai.

Ye code mainly use hota hai:

• HashMap
• HashSet
• Hashtable

public int hashCode()

public class HashCodeExample {

    public static void main(String[] args) {

        String s1 = "Java";
        String s2 = "Java";

        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());
    }
}

Output

2301506
2301506

Kyunki content same hai.

Agar hum chahte hain ki objects content se compare ho, to equals override karna padta hai.

class Student {

    String name;
    int age;

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

    @Override
    public boolean equals(Object obj){

        Student s = (Student) obj;

        if(this.name.equals(s.name) && this.age == s.age){
            return true;
        }
        return false;
    }
}

public class EqualsOverrideExample {

    public static void main(String[] args){

        Student s1 = new Student("Sujit",22);
        Student s2 = new Student("Sujit",22);

        System.out.println(s1.equals(s2));
    }
}

Output

true

Ab content match hone par true aayega.

Java rule:

Agar equals override karte ho to hashCode bhi override karna chahiye

class Student {

    String name;
    int age;

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

    @Override
    public boolean equals(Object obj){

        Student s = (Student) obj;

        return this.name.equals(s.name) && this.age == s.age;
    }

    @Override
    public int hashCode(){

        return name.hashCode() + age;
    }
}

public class HashCodeOverrideExample {

    public static void main(String[] args){

        Student s1 = new Student("Sujit",22);
        Student s2 = new Student("Sujit",22);

        System.out.println(s1.hashCode());
        System.out.println(s2.hashCode());

        System.out.println(s1.equals(s2));
    }
}

Output

same hashcodes
true

Hash collections (HashMap / HashSet) me performance improve karne ke liye:

Java hashCode() call karta hai

hash = key.hashCode()

Isse decide hota hai object kis bucket me jayega.

Agar same bucket me multiple objects ho gaye (collision):

Tab Java equals() call karta hai

if(obj1.equals(obj2))

Tab actual comparison hota hai.

Suppose HashSet me do objects add karte hain:

Student("Sujit",22)
Student("Sujit",22)

Process:

1️⃣ hashCode() call
2️⃣ same bucket mila
3️⃣ equals() call
4️⃣ equals true → duplicate reject

Java contract:

if two objects are equal
then their hashCode must be same

Example:

s1.equals(s2) = true
→ s1.hashCode() == s2.hashCode()

But reverse always true nahi hota.

1️⃣ equals() aur == me difference kya hai?
2️⃣ hashCode() ka purpose kya hai?
3️⃣ equals override karte waqt hashCode override kyun zaruri hai?
4️⃣ HashMap ka working mechanism kya hai?
5️⃣ Collision kya hota hai?
6️⃣ Why hashCode first then equals?

Is lesson me humne seekha:

✔ equals() method
✔ hashCode() method
✔ equals override
✔ hashCode override
✔ Hash collections working
✔ Why first hashCode then equals

Java collections framework me correct equals + hashCode implementation bahut important hai, warna HashMap / HashSet incorrect behavior de sakte hain.

Next lesson me hum seekhenge:

• Java Polymorphism
• Method Overloading
• Method Overriding
• Runtime vs Compile-time polymorphism