How to map java.util.Optional<Something> to Something? in Kotlin

Question

I have a method that returns java.util.Optional<Something>. I want to use that method from Kotlin, and I want my result to be Something?, not Optional<Something>

How to do that in Kotlin, in idiomatic way?

calling .orElse(null) on Optional gives me Something? indeed, but it does not look good. Kotlin does not complain if I write val msg: Something = optional.orElse(null). (msg is declared as Something, not Something?- I loose compile-type check).

I use Kotlin 1.0.3

Answer 1

Extend the java API with a method to unwrap Optional:

fun <T> Optional<T>.unwrap(): T? = orElse(null)

Then use it like you wanted:

val msg: Something? = optional.unwrap()  // the type is enforced

See https://kotlinlang.org/docs/reference/extensions.html for details.

Answer 2

If you use com.google.common.base.Optional (Guava library) then orNull() is better.

For example,

// Java
public class JavaClass
  public Optional<String> getOptionalString() {
    return Optional.absent();
  }
}

// Kotlin
val optionalString = JavaClass().getOptionalString().orNull()

The definition of orNull()

/**
 * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
 * known to be present, use {@link #get()} instead.
 *
 * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
 * {@code Optional.orElse(null)}.
 */
@Nullable
public abstract T orNull();

Answer 3

A nullable type in Kotlin can have value or null so you can convert and Optional to nullable Type as follows:

val str: SomeType? = Optional.of(SomeType_Value).map { it }.orElse(null)

For example for SomeType of String we have

val str: String? = Optional.of("someValue").map { it }.orElse(null)