How to get Executor for main thread on API level < 28

You can use new HandlerExecutor(Looper.getMainLooper()); from com.google.android.gms.common.util.concurrent.HandlerExecutor...in the end it's the same answer as atarasenko.

I added an extension in Kotlin for that:

fun Context.mainExecutor(): Executor {
    return if (VERSION.SDK_INT >= VERSION_CODES.P) {
        mainExecutor
    } else {
        HandlerExecutor(mainLooper)
    }
}

You can use (in activity for example):

ContextCompat.getMainExecutor(this);

https://developer.android.com/reference/androidx/core/content/ContextCompat.html#getMainExecutor(android.content.Context)

You can use code snippet from retrofit https://github.com/square/retrofit/blob/master/retrofit/src/main/java/retrofit2/Platform.java

public class MainThreadExecutor implements Executor {
    private final Handler handler = new Handler(Looper.getMainLooper());

    @Override 
    public void execute(Runnable r) {
        handler.post(r);
    }
}