Return Optional instead of null

Return Optional instead of null everywhere, eg. in getValue() calls.

Enable this

String msg = textInput.getValue().map(value -> "Hello " + value + "!").orElseGet("Don't be shy");
greeting.setText(msg);

instead of forcing our users to do this:

String value = textInput.getValue();
if (value == null || value.isEmpty()) {
    greeting.setText("Don't be shy");
} else {
    greeting.setText("Hello " + value + "!");
}

Null checks are a code smell in Java 8, and forcing if-else checking opens up the door for bugs.

[mvysny]

For example Component.getId() returns Optional while Component.setId() takes String. This duality breaks the Java Bean spec and also borks Kotlin’s property mechanism and hence Kotlin thinks there is just a read-only property id of type Optional<String> - it ignores the setter since it is of different type.

I believe Optional is a misuse in this case since according to https://blog.jetbrains.com/idea/2017/08/code-smells-null/ this is

1. Value was never initialised (whether accidentally or on purpose)

In this case the use of Optional is discouraged.

[pleku]

I’m going to merge this issue to #3801 in case we ever get to improve the APIs or add features for better reactive/functional programming support.