Operator overloading concept in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is operator overloading in Kotlin?

Operator overloading allows you to provide custom implementations for standard operators (like +, -, *, etc.) for your own classes. This means you can define how operators behave when used with your objects.

beginner

How do you enable operator overloading for a function in Kotlin?

You mark the function with the keyword operator. For example, operator fun plus(other: MyClass): MyClass lets you use the + operator.

beginner

Which function name corresponds to the '+' operator in Kotlin operator overloading?

The function name is plus. Defining operator fun plus() lets you use the + operator with your class.

intermediate

Can you overload the assignment operator '=' in Kotlin?

No, Kotlin does not allow overloading the assignment operator '='. You can overload many operators, but '=' is not one of them.

intermediate

Example: How to overload the '*' operator for a class Vector to multiply by a scalar?

Define operator fun times(scalar: Double): Vector inside the Vector class. This lets you write val result = vector * 2.0.

Which keyword is used to enable operator overloading in Kotlin?

A overload

B fun

C operator

D override

The keyword operator is used before a function to enable operator overloading.

What function name should you use to overload the '+' operator?

A plus

B sum

C add

D append

The function name plus corresponds to the '+' operator in Kotlin.

Can you overload the '=' assignment operator in Kotlin?

A No, it is not allowed

B Only for data classes

C Yes, always

D Only with inline functions

Kotlin does not allow overloading the assignment operator '='.

Which of these operators can you overload in Kotlin?

A = (assignment)

B ?: (Elvis operator)

C && (logical AND)

D + (plus)

You can overload arithmetic operators like '+', but not logical operators like '&&' or assignment '='.

What is the correct way to overload the '*' operator for a class?

A operator fun multiply(other: ClassName): ClassName

B operator fun times(other: ClassName): ClassName

C fun times(other: ClassName): ClassName

D fun multiply(other: ClassName): ClassName

The '*' operator corresponds to the function named times and must be marked with operator.

Explain how operator overloading works in Kotlin and how you can define a custom '+' operator for your class.

Think about how you tell Kotlin what '+' means for your objects.

List some operators that can and cannot be overloaded in Kotlin and explain why assignment '=' cannot be overloaded.

Consider which operators affect basic language behavior.

Practice

1. What does operator overloading in Kotlin allow you to do?

easy

A. Disable operators for certain classes

B. Change the meaning of built-in types like Int or String

C. Create new operators not available in Kotlin

D. Use standard operators like + or * with your own classes

Solution

1. Step 1: Understand operator overloading purpose

   Operator overloading lets you define how operators like + work with your own classes.

2. Step 2: Check options meaning

   Only Use standard operators like + or * with your own classes correctly states this ability. Options A, B, and C are incorrect because Kotlin does not let you change built-in types, create new operators, or disable operators this way.

3. Final Answer:

   Use standard operators like + or * with your own classes -> Option D

4. Quick Check:

   Operator overloading = Use operators with custom classes [OK]

Hint: Operator overloading means using +, * with your own classes [OK]

Common Mistakes:

• Thinking you can create new operators
• Believing it changes built-in types
• Confusing disabling operators with overloading

2. Which of the following is the correct syntax to overload the plus operator in a Kotlin class?

easy

A. operator fun plus(other: MyClass): MyClass { ... }

B. fun plus(other: MyClass): MyClass { ... }

C. override fun plus(other: MyClass): MyClass { ... }

D. fun operator plus(other: MyClass): MyClass { ... }

Solution

1. Step 1: Recall operator function syntax

   In Kotlin, to overload an operator, the function must be marked with the keyword operator before the function declaration.

2. Step 2: Analyze options

   operator fun plus(other: MyClass): MyClass { ... } correctly uses operator fun plus(...). fun plus(other: MyClass): MyClass { ... } misses the operator keyword, C wrongly uses override, and D has incorrect keyword order.

3. Final Answer:

   operator fun plus(other: MyClass): MyClass { ... } -> Option A

4. Quick Check:

   Operator keyword before function = Correct syntax [OK]

Hint: Always put 'operator' before function to overload operators [OK]

Common Mistakes:

• Omitting the 'operator' keyword
• Using 'override' instead of 'operator'
• Placing 'operator' after 'fun'

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

data class Point(val x: Int, val y: Int) {
    operator fun plus(other: Point) = Point(x + other.x, y + other.y)
}

fun main() {
    val p1 = Point(1, 2)
    val p2 = Point(3, 4)
    val p3 = p1 + p2
    println(p3)
}

medium

A. Point(x=4, y=6)

B. p1 + p2

C. Compilation error

D. Point(4, 6)

Solution

1. Step 1: Understand operator plus function

   The plus operator adds the x and y values of two Point objects and returns a new Point with summed coordinates.

2. Step 2: Calculate the result of p1 + p2

   p1 has (1, 2), p2 has (3, 4). Adding gives (4, 6). Printing p3 calls data class's toString(), which outputs 'Point(x=4, y=6)'.

3. Final Answer:

   Point(x=4, y=6) -> Option A

4. Quick Check:

   p1 + p2 = Point(x=4, y=6) [OK]

Hint: Data class toString shows property names; sum coordinates [OK]

Common Mistakes:

• Expecting just coordinates without property names
• Thinking '+' prints expression instead of result
• Assuming compilation error without operator keyword

4. Identify the error in this Kotlin code that tries to overload the minus operator:

class Counter(var count: Int) {
    fun minus(other: Counter): Counter {
        return Counter(count - other.count)
    }
}

medium

A. Wrong return type; should be Int instead of Counter

B. Missing 'operator' keyword before the minus function

C. Function name should be 'operatorMinus' instead of 'minus'

D. Cannot overload minus operator for classes

Solution

1. Step 1: Check operator overloading syntax

   To overload an operator in Kotlin, the function must have the 'operator' keyword before its declaration.

2. Step 2: Analyze the code

   The minus function lacks the 'operator' keyword, so Kotlin won't treat it as operator overloading, causing errors when using '-'.

3. Final Answer:

   Missing 'operator' keyword before the minus function -> Option B

4. Quick Check:

   Operator functions need 'operator' keyword [OK]

Hint: Operator functions must start with 'operator' keyword [OK]

Common Mistakes:

• Forgetting the 'operator' keyword
• Changing function name incorrectly
• Assuming operator overloading is not allowed

5. Given this Kotlin class:

class Vector(val x: Int, val y: Int) {
    operator fun plus(other: Vector) = Vector(x + other.x, y + other.y)
    operator fun times(scalar: Int) = Vector(x * scalar, y * scalar)
}

What is the result of this expression?

val v1 = Vector(2, 3)
val v2 = Vector(1, 1)
val result = (v1 + v2) * 2
println("(${result.x}, ${result.y})")

hard

A. (3, 4)

B. (4, 6)

C. (6, 8)

D. Compilation error due to operator precedence

Solution

1. Step 1: Evaluate v1 + v2

   v1 is (2,3), v2 is (1,1). Adding gives Vector(3,4).

2. Step 2: Multiply the result by 2

   Vector(3,4) times 2 gives Vector(6,8).

3. Step 3: Print the result

   Printing prints (6, 8) as expected.

4. Final Answer:

   (6, 8) -> Option C

5. Quick Check:

   (v1 + v2) * 2 = (6, 8) [OK]

Hint: Evaluate plus first, then times; operator overloading respects precedence [OK]

Common Mistakes:

• Multiplying v2 by 2 instead of the sum
• Ignoring operator precedence
• Expecting compilation error