Why is this Kotlin condition marked as always 'true'?

Question

I have this bit of code to check if my mongoDB connection is still active:

val isConnected: Boolean
            get() = if (instance!!.connection != null) {
                try {
                    instance!!.connection!!.getDatabase(instance!!.databaseName!!) != null
                } catch (e: Exception) {
                    Logger.handle("Error verifying mongoDB connection status", e)
                    false
                }
            } else false

I get a warning saying this condition is always true:

Condition 'instance!!.connection!!.getDatabase(instance!!.databaseName!!) != null' is always 'true'

I know that Kotlin has nullable types, but the getDatabase() method is referencing a Java method inside MongoClient. I'm using jdk-8u291 which doesn't have nullable return types for Java as far as I'm aware (excluding the @Nullable annotation). Here's the decompiled MongoClient::getDatabase() method:

public interface MongoClient extends Closeable {

    
    /**
     * Gets a {@link MongoDatabase} instance for the given database name.
     *
     * @param databaseName the name of the database to retrieve
     * @return a {@code MongoDatabase} representing the specified database
     * @throws IllegalArgumentException if databaseName is invalid
     * @see MongoNamespace#checkDatabaseNameValidity(String)
     */
    MongoDatabase getDatabase(String databaseName);

   // ...
}

Why is this condition marked as always "true" if the return type of the Java method is obviously nullable?

Answer 1

Is because MongoDatabase getDatabase(String databaseName); is not marked with @Nullable. So that type is not nulable for Kotlin.

And since you are saying everything is not nullable !! then the result is not returned null by the shorthand ? which you are not using.