Why isn't isEmpty in Kotlin Collections a property?

Question

I'm starting to learn Kotlin, and just noticed that Collections.isEmpty is a function, rather than a property. However, Collections.size is a property and not a function.

In most (all?) collections, I expect these two to be related semantically, and the implementation of isEmpty can simply be size === 0.

So... why isn't isEmpty a property? Is it simply a design bug?

Answer 1

When deciding which methods of java.util.Collection will be properties in kotlin.collections.Collection the following considerations were taken into account:

• both size and isEmpty can be qualified as properties according to Kotlin coding conventions, however size is inherent to the collection and isEmpty is just a derivative of the size property.
• when implementing a collection one can override size val property with a var property (likely with a private setter), however the same is hardly ever needed for isEmpty.
• exposing a method as a property requires additional methods generated in derived classes, better to keep their number as small as possible.