java

With Kotlin 1.3, a new experimental feature called “Inline Classes” is now available. This post is a somewhat deep dive into the nature of the implementation, how it works, where the edges are, and what limitations currently exist. Let’s take a look!

Recently I wrote an article about de-structuring in TypeScript, and hinted that Kotlin has a similar feature, but in more of a “strongly typed” language style.

Kotlin has an annotation called @JvmDefault which, like most of the “Jvm” prefixed annotations, exists to help massage the Kotlin compiler output to match Java classes in a certain way. This annotation was added in 1.2.40, and has now seen some experimental enhancements in 1.2.50, so it seems worth exploring what this is all about.

Tail recursion can be a very elegant way to solve problems that involve a significant number of iterations but are better organized using recursion, without paying the cost you normally pay for recursion. Kotlin can enable the use of tail recursion. To understand the benefits of this, a quick recap would be of value.

As most Java programmers know, Java generics are erased at compile-time. This has trade-offs, but the two main reasons for this are:

• Compatibility - Java 1.4 and earlier dealt exclusively in raw types at the VM level. Generics were able to compile without introducing significant new typing to the bytecode and classfile format spec, and having to deal with older classes generated without that typing.
• Simplicity - By erasing to raw types, the JVM doesn’t have to understand specialization; something that has its own complexities and downsides. For example, specialized types are much more challenging to optimize with a just-in-time compiler.

However, knowing the type parameters used at runtime can have real value, and it’s something Java simply doesn’t offer. Kotlin would like to help in this area, but there are many challenges in doing so.