Xtend: Selected Aspects

In addition to the features sketched in the "code generation with Xtend" section this section provides some selected aspects we found useful for our work.

Get more help: (Xtend Help).

Extension mechanism

Xtend has its name from the the fact that it allows to extend classes of the language without using inheritance. There are different possibilities to achieve this task. One simple example of such an extended class is java.lang.String, which was extended by Xtend to provide a method "toFirstUpper".

A simple possibility to achieve such an extension is to define a locally visible method "f(__TypeX x, ...)" (see Xtend documentation). With this, TypeX is extended by a method __f(...).

def printNTimes(String s, int n) {
    for(var i=0;i<n;i++) println(s)


Other possibilities (see Xtend documentation) allow to use static methods in a similar way ("extension imports") which can be imported by other modules: "import static extension package.Type.printNTimes".

Dispatch methods

A method (used as extension of not) can be marked with the keyword "dispatch". If such a method is defined multiple times with different specialized types as argument, the correct version of the method if called depending what object is passed (runtime polymorphism). The code behaves if an "if obj instanceof TYPE" is called to determine which version has to be called.


  • „==“ vs. „===“ (analogous „!=“ vs. „!==“):
    • „==“ uses the method "equals" to determine the result.
    • „===“ checks if the identical object is referenced (like a pointer comparison).
  • It is possible to check if an optional model element is present or not by comparing it to null.
  • It is possible to check if a model element is not yet loaded (eIsProxy==true).

EMF Parent Relationship (model navigation)

Sometimes (while generating code or validating the model) it is useful to navigate to the parent of a model object. This can be done with the attribute "eContainer" of every model element. It may be necessary to cast this parent to an appropriate type (this can also happen via a dispatch method).

List element access

Lists can be accessed, e.g., via "head" (the first element) or "get(index)" (the i.th element).

Filter and map functions, lambdas and passing lambdas

Filter functions can be used to filter a list (like the unix command grep). Map functions can be used to transform a list of elements (like the unix command sed):

    .join(', '))


  • "filter(TYPE)" filters the list to yield all entries of type "TYPE".
  • "filter(LAMBDA)" filters the list using the "LAMBDA" as selector (if the LAMBDA returns true, the element is returned).
  • "map(LAMBDA)" transforms each element using the "LAMBDA".
  • Functions expecting a lambda can be called omitting the brackets "(...)".
  • Lambda have the following syntax
    • "[a | a.name]" is the same as "[name]" or "[it.name]" ("it" is the default parameter name).
    • "return" can be omitted: the last command yields the return value.