Equality (== structural vs === referential) in Kotlin - Hands-On Comparison

Equality (== structural vs === referential)

📖 Scenario: Imagine you have two boxes that look the same. You want to check if the boxes have the same things inside or if they are actually the same box.

🎯 Goal: You will learn how to check if two variables in Kotlin have the same content (structural equality) or if they point to the exact same object (referential equality).

📋 What You'll Learn

Create two variables holding strings with the same text

Create a variable holding a different string

Create a variable to store the result of structural equality check

Create a variable to store the result of referential equality check

Print the results of both equality checks

💡 Why This Matters

🌍 Real World

Understanding equality helps when you want to compare data in apps, like checking if two user inputs are the same or if two objects represent the same thing.

💼 Career

Many programming jobs require knowing how to compare data correctly to avoid bugs, especially when working with objects and data structures.

1

Create two string variables with the same text

Create two variables called text1 and text2 and set both to the string "hello".

Kotlin

# Your code here

Hint

Use val to create variables and assign the string "hello" to both.

2

Create a different string variable

Create a variable called text3 and set it to the string "world".

Kotlin

val text1 = "hello"
val text2 = "hello"
# Your code here

Hint

Create text3 with the value "world".

3

Check structural and referential equality

Create a variable called structuralEqual that stores the result of text1 == text2 and a variable called referentialEqual that stores the result of text1 === text2.

Kotlin

val text1 = "hello"
val text2 = "hello"
val text3 = "world"
# Your code here

Hint

Use == for structural equality and === for referential equality.

4

Print the results of equality checks

Print the values of structuralEqual and referentialEqual using two separate println statements.

Kotlin

val text1 = "hello"
val text2 = "hello"
val text3 = "world"
val structuralEqual = text1 == text2
val referentialEqual = text1 === text2
# Your code here

Hint

Use println(structuralEqual) and println(referentialEqual) to show the results.

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