Equality (== structural vs === referential) in Kotlin - Visual Side-by-Side Comparison

Concept Flow - Equality (== structural vs === referential)

Start

↓

Compare with == ?

↓

Check structural

↓

Return true

True↓

Return true

No↓

Return false

First, Kotlin checks structural equality with ==, then referential equality with === if needed, returning true or false accordingly.

Execution Sample

Kotlin

val a = "hello"
val b = String("hello".toCharArray())
println(a == b)
println(a === b)

Compares two strings a and b using structural (==) and referential (===) equality.

Execution Table

Step	Expression	Evaluation	Result
1	a == b	Compare contents of a and b	true
2	a === b	Compare references of a and b	false

💡 Both comparisons done; structural equality true, referential equality false.

Variable Tracker

Variable	Start	After 1	After 2	Final
a	"hello"	"hello"	"hello"	"hello"
b	new String("hello".toCharArray())	new String("hello".toCharArray())	new String("hello".toCharArray())	new String("hello".toCharArray())

Key Moments - 2 Insights

Why does 'a == b' return true but 'a === b' return false?

Because '==' checks if the text inside the strings is the same (structural equality), which it is, but '===' checks if both variables point to the exact same object in memory (referential equality), which they do not. See execution_table rows 1 and 2.

Can two different objects be structurally equal?

Yes, two separate objects can have the same content, so '==' returns true even if '===' returns false. This is shown by variables a and b in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of 'a == b' at step 1?

A null

B true

C false

D error

💡 Hint

Check the 'Result' column in execution_table row 1.

At which step does the referential equality check happen?

A Step 2

B Step 1

C Before Step 1

D After Step 2

💡 Hint

Refer to the 'Expression' column in execution_table to find 'a === b'.

If 'b' was assigned as 'val b = a', what would 'a === b' return?

A false

B null

C true

D error

💡 Hint

Refer to variable_tracker and understand that assigning 'b = a' makes both variables point to the same object.

Concept Snapshot

In Kotlin:
- '==' checks structural equality (content).
- '===' checks referential equality (same object).
- Use '==' to compare values.
- Use '===' to check if two variables point to the same object.
- Structural equality can be true even if referential equality is false.

Full Transcript

This example shows how Kotlin compares two strings using '==' and '===' operators. The '==' operator checks if the contents of the strings are the same, which returns true. The '===' operator checks if both variables point to the exact same object in memory, which returns false because they are two different objects with the same content. This helps understand the difference between structural and referential equality in Kotlin.

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