Effective Kotlin: Item 27 — Eliminate unchecked warnings

In the excellent Effective Java, item 27 covers unchecked warnings with the main take-home being you should remove as many as you can. Kotlin already helps by reducing the potential number of warnings we get, for example, with its use of smart casts and star-projections instead of raw types.

if (obj is String) {
println(obj.length) // Smart-cast to a String automatically

Where you cannot remove a warning but know it to be type-safe then in Java you would annotate the class with @SuppressWarnings(“unchecked”), the Kotlin equivalent being @Suppress(“UNCHECKED_CAST”), and of course document why it is type-safe.

One of the other tools up Kotlin’s sleeve is inline functions with reified type parameters which allow you to perform is T checks for the type parameter along with access to T::class.java. The following example is from Android KTX:

inline fun <reified T> Context.systemService() = 
// somewhere in your code
val connectivityManager = systemService<ConnectivityManager>()

Inline functions with reified types cannot be called from Java whereas standard inline functions can of course so if interoperability is a concern then it is vital to provide a Java-compatible version of your function too.

Each week I am looking at “items” from Joshua Bloch’s well-respected book, Effective Java to see how it applies to Kotlin. You can find the rest of the items I’ve covered at Effective Kotlin. Please let me know your thoughts.

Matt Dolan has been eating doughnuts and developing with Android since the dark days of v1.6.

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