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.