As you might know Java 8 will finally bring an important new feature to the Java programming language :

Lambda Expressions, a feature already supported by many other languages, such as Xtend

Xtend is a statically typed JVM language that translates to readable Java source code. It's very different to other JVM languages in the sense that aims to be a better Java by better supporting existing Java idioms and fully supporting the existing ecoystem. Xtend doesn't force you to rewrite your existing Java apps or drop your beloved and well-proven frameworks, but instead allows for using existing Java APIs in a much nicer way without any interoperability issues. In contrast to e.g. Scala or Clojure it is more like an extension to Java than a replacement.

Identifying and supporting existing Java idioms is not too hard given the huge existing code base but what about the future? Will Xtend also be a better match for the upcoming Java 8 libraries which were explicitly defined for the new Java 8 lambda syntax?

To find out I simply looked at Brian Goetz’s latest version of State of the Lambda : Libraries Edition and translated the contained Java code examples to Xtend.

In the following you see the code for Java as written by Brian and the translation to Xtend. Both are using the exact same Java 8 APIs and contain the same amount of static type information!

Example 1

Java 8: shapes.forEach(s -> { s.setColor(RED); }); Xtend: shapes.forEach[color = RED]

This first example already reveals most of the important differences.

First the general syntax for a lambda expression in Java is using an arrow ( -> ), while Xtend uses squared brackets (like smalltalk). Also in Xtend a lambda expression doesn't need to be passed using braces. Although you could write shapes.forEach([color = RED]) you don't need to.

In Java 8 the body of a lambda is either a block or a single expression. So whenever you need to use a statement you have to add the curly braces and semicolons. In Xtend everything is an expression and the lambda itself is already a block expression.

In Xtend you can omit the declaration of a parameter name. In that case the parameter is called ' it ' and is implicit (similarly to ' this '). If you want to give it a name you do it like this : shapes.forEach[ s| s.color = RED]

Unrelated to Lambdas but interesting in this case: Xtend allows to use assignments to call setter methods.

Example 2

Java 8: shapes.stream() .filter(s -> s.getColor() == BLUE) .forEach(s -> { s.setColor(RED); }); Xtend: shapes.stream .filter[color == BLUE] .forEach[color = RED]

Not too much new stuff in here. Xtend lets you omit empty parenthesis and you can call a getter using the property's name.

Example 3

Java 8: List<Shape> blue = shapes.stream() .filter(s -> s.getColor() == BLUE) .into(new ArrayList<>()); Xtend: val blue = shapes.stream .filter[color == BLUE] .into(new ArrayList)

Here we see type inference in action. While you can use the diamond operator in Java, you can leave it out in Xtend. Also the variable doesn't need to be explicitly typed as the type can be fully inferred from the right hand side.

Example 4

Java 8: Set<Box> hasBlueShape = shapes.stream() .filter(s -> s.getColor() == BLUE) .map(s -> s.getContainingBox()) .into(new HashSet<>()); Xtend: val hasBlueShape = shapes.stream .filter[color == BLUE] .map[containingBox] .into(new HashSet)

Note how readable the lambdas get when you can leave out all the cryptic clutter. Let's compare a last example from a later section of the document:

Example 5

Java 8: List<Album> sortedFavs = albums.stream() .filter(a -> a.tracks.anyMatch(t -> (t.rating >= 4))) .sorted(comparing(a -> a.name)) .into(new ArrayList<>()); Xtend: val sortedFavs = albums.stream .filter[tracks.anyMatch[rating >= 4]] .sorted(comparing[name]) .into(new ArrayList)

Although the libraries as well as the examples have been written for Java 8, the Xtend code is still much less cluttered with symbols and therefore significantly more readable. Yet the type information is exactly the same! Xtend is just a bit smarter with type inference and is syntactically less rigid.

Besides that and the fact the Xtend compiles to readable Java 5 code, it has many other important features to offer.