Java.Core.Can different objects (ref0 != ref1) have ref0.equals(ref1) == true?

🚀 Can Different Objects (ref0 != ref1) Have ref0.equals(ref1) == true?

Yes! Different objects (with different memory addresses) can still be considered equal if equals() is overridden to compare their contents rather than their references.

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1️⃣ Default Behavior of equals() in Object

By default, equals() in Object behaves like ==, meaning:

public boolean equals(Object obj) {
    return (this == obj);
}

• If equals() is not overridden, two references must point to the same object for equals() to return true.
• Different objects (ref0 != ref1) will return false by default.

❌ Example: Default equals() in Object (Comparing Memory References)

class Person {
    String name;

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

public class Main {
    public static void main(String[] args) {
        Person p1 = new Person("Alice");
        Person p2 = new Person("Alice");

        System.out.println(p1 == p2);      // ❌ false (Different memory addresses)
        System.out.println(p1.equals(p2)); // ❌ false (Default equals() is same as ==)
    }
}

📌 Why does equals() return false here?

• Since Person does not override equals(), it behaves like == (compares memory references).

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2️⃣ Overriding equals() to Compare Object Contents

To allow different objects to be considered equal, we override equals() to compare field values instead of memory addresses.

✅ Example: Overriding equals() for Logical Equality

import java.util.Objects;

class Person {
    String name;

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

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;  // Reflexivity: same reference → true
        if (obj == null || getClass() != obj.getClass()) return false;
        Person person = (Person) obj;
        return Objects.equals(name, person.name); // Compare contents
    }
}

public class Main {
    public static void main(String[] args) {
        Person p1 = new Person("Alice");
        Person p2 = new Person("Alice"); // Different object, same content

        System.out.println(p1 == p2);      // ❌ false (Different objects in memory)
        System.out.println(p1.equals(p2)); // ✅ true (Same name, so equals() is true)
    }
}

📌 Now equals() returns true even though p1 and p2 are different objects!

• p1 != p2 (different memory locations).
• p1.equals(p2) == true because we compare name, not references.

3️⃣ Common Cases Where equals() Returns true for Different Objects

✅ String

Java’s String class overrides equals() to compare characters, not memory references.

String s1 = new String("hello");
String s2 = new String("hello");

System.out.println(s1 == s2);      // ❌ false (Different objects)
System.out.println(s1.equals(s2)); // ✅ true (Same character sequence)

📌 Why?

• == compares memory (false).
• equals() compares content (true).

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✅ Wrapper Classes (Integer, Double, etc.)

Java’s wrapper classes (Integer, Double, etc.) override equals() to compare values.

Integer i1 = new Integer(42);
Integer i2 = new Integer(42);

System.out.println(i1 == i2);      // ❌ false (Different objects)
System.out.println(i1.equals(i2)); // ✅ true (Same value)

📌 Why?

• Integer overrides equals() to compare values, not references.

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✅ Custom Classes with Overridden equals()

Example with Point class:

class Point {
    int x, y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Point point = (Point) obj;
        return this.x == point.x && this.y == point.y;
    }
}

public class Main {
    public static void main(String[] args) {
        Point p1 = new Point(3, 4);
        Point p2 = new Point(3, 4);

        System.out.println(p1 == p2);      // ❌ false (Different memory locations)
        System.out.println(p1.equals(p2)); // ✅ true (Same coordinates)
    }
}

📌 Summary: When ref0.equals(ref1) == true Even If ref0 != ref1

Case	ref0 != ref1	ref0.equals(ref1)
Default Object.equals()	✅ Yes (Different objects)	❌ false (Default behavior uses ==)
Overridden equals() in Custom Classes	✅ Yes	✅ true (If content is the same)
String ("hello")	✅ Yes	✅ true (Compares characters)
Integer (new Integer(42))	✅ Yes	✅ true (Compares values)

✅ Key Takeaways

✔ If equals() is not overridden, it behaves like == (compares references).
✔ If equals() is overridden, it can return true for different objects if their contents match.
✔ Java’s built-in classes like String and Integer already override equals().
✔ Custom classes should override equals() when logical equality matters.

By following these principles, your Java objects will behave correctly and efficiently in collections like HashSet and HashMap! 🚀