Parameters with default values in Kotlin

Introduction

Parameters with default values let you give a starting value to a function's input. This way, you can call the function without always giving all inputs.

When you want a function to work even if some inputs are missing.

When you want to avoid writing many function versions for different input counts.

When you want to keep your code simple and clear by using common default settings.

When you want to make your functions flexible for different situations.

Syntax

Kotlin

fun functionName(param1: Type = defaultValue, param2: Type = defaultValue) {
    // function body
}

Default values are set using the equals sign (=) after the parameter type.

If you skip a parameter with a default value when calling the function, Kotlin uses the default.

Examples

This function says hello. If you don't give a name, it uses "Friend".

Kotlin

fun greet(name: String = "Friend") {
    println("Hello, $name!")
}

This function multiplies two numbers. If you skip the second number, it multiplies by 2.

Kotlin

fun multiply(a: Int, b: Int = 2): Int {
    return a * b
}

This function prints name and age. Both have default values.

Kotlin

fun printInfo(name: String = "Unknown", age: Int = 0) {
    println("Name: $name, Age: $age")
}

Sample Program

This program shows how the greet function works with and without a parameter.

Kotlin

fun greet(name: String = "Friend") {
    println("Hello, $name!")
}

fun main() {
    greet("Alice")  // Uses given name
    greet()          // Uses default name
}

Important Notes

You can mix parameters with and without default values, but parameters without defaults must come first.

Using default values helps avoid writing many overloaded functions.

Summary

Default parameter values let functions have optional inputs.

If you skip a parameter, Kotlin uses its default value.

This makes your code simpler and easier to use.

Practice

1. What happens when you call a Kotlin function with a parameter that has a default value but you don't provide an argument for it?
fun greet(name: String = "Guest") { println("Hello, $name!") }

easy

A. The function uses the default value for the missing parameter.

B. The function throws an error because the argument is missing.

C. The function ignores the parameter and prints nothing.

D. The function requires you to always provide the argument.

Solution

1. Step 1: Understand default parameter behavior

   In Kotlin, if a parameter has a default value, the function can be called without providing that argument.

2. Step 2: Apply to the example function

   Calling greet() will use the default value "Guest" for name, printing "Hello, Guest!".

3. Final Answer:

   The function uses the default value for the missing parameter. -> Option A

4. Quick Check:

   Default parameter means optional argument [OK]

Hint: If argument missing, default value is used automatically [OK]

Common Mistakes:

• Thinking missing argument causes error
• Assuming function ignores missing parameters
• Believing all parameters must be provided

2. Which of the following is the correct way to declare a Kotlin function with a default parameter value?

easy

A. fun add(a: Int, b: Int default 5) { return a + b }

B. fun add(a: Int = 5, b: Int) { return a + b }

C. fun add(a: Int, b: Int) = a + b = 5

D. fun add(a: Int, b: Int = 5) { return a + b }

Solution

1. Step 1: Check Kotlin syntax for default parameters

   In Kotlin, default values are assigned using = after the parameter type, like b: Int = 5.

2. Step 2: Validate each option

   fun add(a: Int, b: Int = 5) { return a + b } correctly uses b: Int = 5. fun add(a: Int = 5, b: Int) { return a + b } places default before non-default parameter, which is allowed but can cause ambiguity if called without named arguments. Options A and C have invalid syntax.

3. Final Answer:

   fun add(a: Int, b: Int = 5) { return a + b } -> Option D

4. Quick Check:

   Default value syntax = parameter: Type = value [OK]

Hint: Use '=' after type to set default value [OK]

Common Mistakes:

• Placing default value before type
• Using 'default' keyword instead of '='
• Assigning default outside parameter list

3. What is the output of the following Kotlin code?

fun multiply(x: Int, y: Int = 2) = x * y

println(multiply(4))

medium

A. 8

B. 4

C. Error: Missing argument for y

D. 2

Solution

1. Step 1: Identify default parameter usage

   The function multiply has a default value of 2 for y. Calling multiply(4) means x=4 and y=2.

2. Step 2: Calculate the result

   Multiply 4 by 2 to get 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   Default y=2 used, 4*2=8 [OK]

Hint: If argument missing, default value is used [OK]

Common Mistakes:

• Expecting error for missing argument
• Using only first argument as output
• Confusing default value with zero

4. Identify the error in this Kotlin function declaration:

fun greet(name: String = "User", age: Int) {
    println("Hello, $name! You are $age years old.")
}

medium

A. Cannot use default values with String parameters.

B. Default parameter must be last or followed by named arguments.

C. Function must return a value if default parameters are used.

D. No error, function is correct.

Solution

1. Step 1: Check parameter order rules

   In Kotlin, parameters with default values should be placed after parameters without default values, or you must use named arguments when calling.

2. Step 2: Analyze given function

   Here, name has a default but comes before age which has no default. This causes a problem when calling without named arguments.

3. Final Answer:

   Default parameter must be last or followed by named arguments. -> Option B

4. Quick Check:

   Default parameters after non-default or use named args [OK]

Hint: Place default parameters after non-default ones [OK]

Common Mistakes:

• Putting default parameters before required ones
• Assuming all parameters can have defaults anywhere
• Ignoring named argument calls

5. You want to write a Kotlin function formatMessage that takes a message and an optional prefix with default value "Info:". It should return the combined string. Which function correctly implements this and allows calling with or without the prefix?

hard

A. fun formatMessage(prefix: String = "Info:", message: String) = "$prefix $message"

B. fun formatMessage(message: String, prefix: String) = "$prefix $message"

C. fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message"

D. fun formatMessage(message: String = "Info:", prefix: String) = "$prefix $message"

Solution

1. Step 1: Define function with default parameter last

   The optional parameter prefix with default value should come after the required message parameter.

2. Step 2: Check each option

   fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message" correctly places prefix last with default. fun formatMessage(prefix: String = "Info:", message: String) = "$prefix $message" places default before required, which can cause call issues. Options C and D lack default for prefix or have wrong order.

3. Final Answer:

   fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message" -> Option C

4. Quick Check:

   Default last parameter enables optional argument [OK]

Hint: Put default parameters after required ones for easy calls [OK]

Common Mistakes:

• Placing default parameter before required parameter
• Not providing default value for optional parameter
• Confusing parameter order causing call errors