Kotlin Functions vs. Properties: When to use what?

Learn when to use functions and when to use read-only properties in your Kotlin class.

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You should always prefer to use read-only properties (using val keyword) over using functions if your code block meets all the following criteria:

• No arguments (function parameters) are required
• Throwing an exception is not required
• Your code block performs Cheap (simple) computation
• Your code block returns the same result when executed (provided the object state hasn’t changed)

If any one of the above-mentioned criteria is not met, then blindly use a function.

Let’s understand it using an example.

Let’s try this way. Given below is a class Box that contains two code blocks. Code block 1 is a getVolume function and Code block 2 is a read-only property with a getter.

Your task is to identify if both the code blocks need to change as per the 4 criteria mentioned above or not?

class Box (val width: Int, val height: Int, val length: Int) {
    var usedSpace: Int = 0     // Initially box is empty
    // code block 1 
    fun getVolume(): Int {
        return width * height * length 
    }
    // code block 2 
    val availableSpace: Int 
        get() = volume – usedSpace
}

Solution

class Box (val width: Int, val height: Int, val length: Int) {
    var usedSpace: Int = 0     // Initially box is empty
    // Replace code block 1 by this read-only property 
    val volume: Int 
        get() = width * height * length
    // Replace code block 2 by this function 
    fun getAvailableSpace(): Int {
        return volume – usedSpace 
    }
}

Explanation for Code Block 1

Code Block 1 originally was getVolume function. This code meets all the 4 criteria mentioned above. So we replaced it with a read-only property.

• It had no function parameters.
• It does not throw any exception.
• It does a simple calculation to find the volume of box.
• width, height, and length are initialised during the box object creation. Hence each time the getVolume method is called, it always returns the same result.

Therefore, Code Block 1 should use a read-only property instead of a function.

Explanation for Code Block 2

Code Block 2 was originally a property.

• It meets the first three criteria but fails to conform to the last criteria. i.e. Each time it is called, it might return a different value depending on the value of usedSpace property.

Hence, we should use a function for this code instead of using a read-only property.

How does making such a change affect your application performance?

Well, it does not. It is just a code convention for better Kotlin programming.

And this article is based on the Coding Convention article on the official kotlinlang.org website.

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