Equality (== structural vs === referential) in Kotlin - When to Use Which

The Big Idea

Ever wondered why two things that look the same might still be different in your program?

The Scenario

Imagine you have two boxes with the same toys inside. You want to check if the boxes are the same. Manually, you might look inside each box and compare every toy one by one.

The Problem

This manual checking is slow and easy to mess up. Sometimes you might think two boxes are different just because they are different boxes, even if the toys inside are exactly the same. Or you might miss a toy and say they are the same when they are not.

The Solution

Kotlin gives you two ways to compare: == checks if the contents (toys) are the same, and === checks if the boxes themselves are the exact same box. This makes comparing clear and easy.

Before vs After

✗ Before

if (box1 === box2) { println("Same box") } else { println("Different boxes") }

✓ After

if (box1 == box2) { println("Boxes have same toys") } else { println("Boxes have different toys") }

What It Enables

This lets you quickly and correctly check if two things are truly equal in content or just the same object, avoiding confusion and bugs.

Real Life Example

When checking if two user profiles have the same information, you want to compare their details (==), not if they are the exact same profile object in memory (===).

Key Takeaways

== checks if contents are equal (structural equality).

=== checks if two references point to the same object (referential equality).

Using the right equality avoids mistakes and makes your code clearer.

Practice

1. In Kotlin, what does the operator == check when comparing two variables?

easy

A. If the two variables are the exact same object (referential equality)

B. If the two variables have the same content (structural equality)

C. If the two variables are of the same type

D. If the two variables are both null

Solution

1. Step 1: Understand the meaning of == in Kotlin

   The == operator checks if two variables have the same content, meaning their values are equal.

2. Step 2: Differentiate from ===

   The === operator checks if two variables point to the exact same object in memory, which is different from ==.

3. Final Answer:

   If the two variables have the same content (structural equality) -> Option B

4. Quick Check:

   == means structural equality [OK]

Hint: Remember: == compares values, not object identity [OK]

Common Mistakes:

• Confusing == with ===
• Thinking == checks object identity
• Assuming == checks type equality

2. Which of the following Kotlin expressions correctly checks if two variables refer to the exact same object?

easy

A. a === b

B. a != b

C. a.equals(b)

D. a == b

Solution

1. Step 1: Identify the operator for referential equality

   In Kotlin, === checks if two variables point to the exact same object in memory.

2. Step 2: Confirm other options

   == and .equals() check structural equality, and != checks inequality, so they don't check referential equality.

3. Final Answer:

   a === b -> Option A

4. Quick Check:

   === means referential equality [OK]

Hint: Use === to check if two variables are the same object [OK]

Common Mistakes:

• Using == instead of === for object identity
• Confusing .equals() with ===
• Assuming != checks referential equality

3. What will be the output of the following Kotlin code?

val a = "hello"
val b = String(charArrayOf('h', 'e', 'l', 'l', 'o'))
println(a == b)
println(a === b)

medium

A. true\nfalse

B. false\nfalse

C. true\ntrue

D. false\ntrue

Solution

1. Step 1: Evaluate a == b

   Both strings contain the same characters "hello", so a == b is true because it checks structural equality.

2. Step 2: Evaluate a === b

   They are two different String objects in memory, so a === b is false because it checks referential equality.

3. Final Answer:

   true\nfalse -> Option A

4. Quick Check:

   == true, === false [OK]

Hint: == checks content, === checks if same object [OK]

Common Mistakes:

• Assuming === is true because contents match
• Confusing == with ===
• Thinking different objects with same content are ===

4. Consider this Kotlin code snippet:

val x = listOf(1, 2, 3)
val y = listOf(1, 2, 3)
if (x === y) {
    println("Same object")
} else {
    println("Different objects")
}

What is the output and why?

medium

A. Same object

B. Compilation error

C. Different objects

D. Runtime exception

Solution

1. Step 1: Understand listOf creates new objects

   Each call to listOf(1, 2, 3) creates a new list object, so x and y are different objects.

2. Step 2: Check referential equality with ===

   Since x and y are different objects, x === y is false, so the else branch runs.

3. Final Answer:

   Different objects -> Option C

4. Quick Check:

   === false for different list objects [OK]

Hint: listOf creates new objects; === checks identity [OK]

Common Mistakes:

• Assuming lists with same content are ===
• Confusing structural and referential equality
• Expecting compilation or runtime errors

5. You want to check if two variables a and b in Kotlin either have the same content or are the exact same object. Which code snippet correctly prints "Equal" if either condition is true, and "Not Equal" otherwise?

hard

A. if (a === b) {
println("Equal")
} else if (a == b) {
println("Not Equal")
} else {
println("Equal")
}

B. if (a === b && a == b) {
println("Equal")
} else {
println("Not Equal")
}

C. if (a == b) {
println("Not Equal")
} else if (a === b) {
println("Equal")
} else {
println("Not Equal")
}

D. if (a == b || a === b) {
println("Equal")
} else {
println("Not Equal")
}

Solution

1. Step 1: Understand the condition needed

   We want to print "Equal" if either a == b (same content) OR a === b (same object) is true.

2. Step 2: Check each option's logic

   if (a == b || a === b) {
       println("Equal")
   } else {
       println("Not Equal")
   }

   uses logical OR (||) correctly.

   if (a === b && a == b) {
       println("Equal")
   } else {
       println("Not Equal")
   }

   requires both to be true (AND), which is too strict.

   if (a == b) {
       println("Not Equal")
   } else if (a === b) {
       println("Equal")
   } else {
       println("Not Equal")
   }

   incorrectly prints "Not Equal" when a == b is true.

   if (a === b) {
       println("Equal")
   } else if (a == b) {
       println("Not Equal")
   } else {
       println("Equal")
   }

   prints "Not Equal" incorrectly when a == b is true but a === b is false.

3. Final Answer:

   if (a == b || a === b) -> Option D

4. Quick Check:

   Use || to combine == and === checks [OK]

Hint: Use || to check either structural or referential equality [OK]

Common Mistakes:

• Using && instead of ||
• Confusing order of conditions
• Printing wrong messages for conditions