17_objects.adoc

Objects

The class de.fhg.fokus.xtensions.objects.Objects provides a bunch of static extension methods that can generally help with handling objects especially in null-safe navigation chains.

Recovering from null

To check if an object is null and provide a fallback object if the tested object is null, the Objects class provides the extension functions recoverNull(T,T) and recoverNull(T,⇒T).

The Java 9 JDK provides functions the same functionality via the methods java.util.Objects#requireNonNullElse​(T,T) and java.util.Objects#requireNonNullElseGet​(T, Supplier<? extends T>). This library provides API compatible aliases for the recoverNull functions, so that a migration to Java 9 and the built-in methods is smooth.

Example:

import static extension de.fhg.fokus.xtensions.objects.Objects.*
// ...
val person = new Person(null, null)
val first = person.getFirstName.recoverNull("John")
val last = person.getLastName.recoverNull[if(first == "John") "Doe" else "Unknown"]
println('''«first» «last»''')

In this example we initialize a person with first and last name set to null in both cases the recoverNull functions have to provide a fallback object. Since first name defaults to "John" the last name will be evaluated to "Doe".

Acting on non-null

The extension function ifNotNull will check a context object on null and if it is not will pass the object to a consumer procedure. This can be handy at the end of null-safe navigation chains.

Example:

import static extension de.fhg.fokus.xtensions.objects.Objects.*
// ...
val person = // ...
preson?.getFirstName?.toFirstUpper.ifNotNull [
	println(it)
]

Cast or null

The Xtend operator as tests if an object instance of a given class and casts the objec to this type if so. If not, the result of the operator will be null. Unfortunately this cannot be simply used in a (null-safe) navigation chain. The asType extension method wraps the same semantics in a function that can be called as part of a call chain.

Example:

val Vehicle v = // ...
v.asType(Car)?.getMake.ifNotNull [
	println("Make: " + it)
]

In the example the vehicle is casted to a Car and afterwards is either of type Car or null. Therefore subsequent calls are performed via null-safe navigation. Since the object can now be used as a car, methods of that type, such as getMake can be accessed.

Tip	Related JavaDocs: Objects