Effective Kotlin: Item 24 — Favor static member classes over nonstatic

Matthew Dolan

Aug 25, 2018·1 min read

With item 24, Joshua Bloch in Effective Java talks about preferring static member classes over non-static member classes which have a hidden reference to the enclosing instance that can prevent garbage collection (item 7).

On Android, non-static member classes and anonymous classes have been the cause of many memory leaks as, for example, the code below shows an AsyncTask that implicitly holds a reference to the Activity until the long-running task completes:

public class LeakyActivity extends Activity {

    …

    private class LongRunningTask extends AsyncTask<Void, Void, String> {

        @Override
        protected String doInBackground(Void... params) {
            // Long running task occurs here
            
            return "Task complete";
        }
    }
}

Kotlin’s approach to classes means that member classes are static by default with the inner modifier being explicitly added to a class for it to become a non-static member class and thus have an implicit reference to the outer class, i.e. the opposite to Java’s static modifier.

class OuterClass {
    // static member class
    class NestedClass {}
    
    // non-static member class
    inner class InnerClass {}
}

Anonymous classes and lambdas in Kotlin are inner classes, so caution still applies when using those.

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.