Java.Core.Is it possible to implement the equals() method of the MyClass class like this: class MyClass {public boolean equals(MyClass that) {return this == that;}}?

1️⃣ Why Is This Implementation Wrong?

❌ Incorrect equals() Implementation

class MyClass {
    public boolean equals(MyClass that) { // ❌ Wrong! Does not override Object.equals()
        return this == that;
    }
}

📌 Problems:

1. Does Not Override Object.equals(Object)
   • Java does not recognize this as an override because the method signature must be equals(Object that).
   • It is a separate method, not an actual override.
2. Breaks Polymorphism
   • If MyClass is used in collections like HashSet, Java will call Object.equals(), not this custom method.
3. Only Checks References (==)
   • This makes equals() useless because == already does this.
   • The whole point of overriding equals() is to compare content, not memory addresses.

2️⃣ How Java’s equals() Works

📜 Correct equals(Object) Signature

The correct method signature for overriding equals() in Java:

@Override
public boolean equals(Object obj) { ... }

• Java calls equals(Object), not equals(MyClass).
• Collections (HashSet, HashMap) and built-in methods (contains(), remove()) expect equals(Object), not equals(MyClass).

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3️⃣ How This Fails in a HashSet

❌ Wrong Implementation That Breaks HashSet

import java.util.HashSet;
import java.util.Set;

class MyClass {
    int value;

    public MyClass(int value) {
        this.value = value;
    }

    public boolean equals(MyClass that) { // ❌ Wrong! Does not override Object.equals()
        return this == that;
    }
}

public class Main {
    public static void main(String[] args) {
        Set<MyClass> set = new HashSet<>();
        MyClass obj1 = new MyClass(42);
        MyClass obj2 = new MyClass(42);

        set.add(obj1);
        System.out.println(set.contains(obj2)); // ❌ false (Expected: true)
    }
}

📌 What Went Wrong?

• contains(obj2) checks Object.equals(), but since equals(MyClass) is not an override, it calls Object.equals(), which defaults to ==.
• Since obj1 != obj2, it fails to find the object.

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4️⃣ Correct Way to Override equals()

✅ Fixed Version: Correctly Overriding equals(Object)

import java.util.Objects;

class MyClass {
    int value;

    public MyClass(int value) {
        this.value = value;
    }

    @Override
    public boolean equals(Object obj) { // ✅ Correct: Overrides Object.equals()
        if (this == obj) return true; // Check reference equality
        if (obj == null || getClass() != obj.getClass()) return false; // Check class
        MyClass myClass = (MyClass) obj;
        return this.value == myClass.value; // Compare content
    }

    @Override
    public int hashCode() {
        return Objects.hash(value); // ✅ Ensures compatibility with HashSet
    }
}

public class Main {
    public static void main(String[] args) {
        Set<MyClass> set = new HashSet<>();
        MyClass obj1 = new MyClass(42);
        MyClass obj2 = new MyClass(42);

        set.add(obj1);
        System.out.println(set.contains(obj2)); // ✅ true (Now works correctly)
    }
}

📌 Now equals() works properly:

• equals() compares content (value), not references.
• contains(obj2) correctly finds the object in the HashSet.

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📌 Summary: Why equals(MyClass that) Is Wrong

Issue	Why It Fails
Does Not Override Object.equals(Object)	Java calls equals(Object), but equals(MyClass) is a separate method.
Breaks Polymorphism	Collections like HashSet and HashMap rely on Object.equals(Object), so they won’t recognize this method.
Only Checks References (==)	This makes equals() redundant—== already does this.

✅ Best Practices

✔ Always override equals(Object) with the correct signature.
✔ Use @Override to ensure proper overriding.
✔ Compare meaningful fields instead of using ==.
✔ Override hashCode() whenever you override equals().

By following these rules, your Java objects will work correctly and efficiently in collections! 🚀