Xtend Syntax in Java

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Xtend Syntax
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The syntax for Xtend is simplistic because most of an extension file s content is made up of the underlying expression language syntax
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Import
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The import keyword is used at the top of a file and indicates the name space of different types used within the extension file As usual, double colon (::) characters delimit elements in the namespace, and a semicolon ends the statement For example, the following import statement imports our mindmap model namespace
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import mindmap;
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No support exists for wildcard or static imports in Xtend, so the following examples are incorrect:
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import org::eclipse::*; import mindmap::Map; // incorrect // also incorrect
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Extension Import
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It is possible to import another extension file using the EXTENSION keyword at the top of the file Again, fully qualified namespace declaration is required Following is an example
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extension org::eclipse::mindmap::Util; // full path with no *ext
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141 Xpand Language
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If you want to export an extension used in a file with your own extensions, add the reexport keyword to the end of the extension statement
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extension org::eclipse::mindmap::Util reexport;
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Comments
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Comments in Xtend come in two flavors: single line and multiline Single-line comments are like those in Java: two forward slashes (//) demark the start of a comment that goes until the end of the line
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// An example single line comment
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A multiline comment is also like comments in Java, with /* marking the start and */ marking the end of comments that can span multiple lines
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/* * A multi-line comment */
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Extensions
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The basic syntax for an extension expression indicates an optional return type, extension name, parameter list, and expression body following a single colon and terminated with a semicolon Following is the general syntax
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ReturnType expressionName(ParamType1 param1, ) : expression-body;
Consider this simple example that returns a standard setter method name for a passed element:
String setterName(NamedElement element) : 'set' + elementnametoFirstUpper();
Extension Invocation
You can invoke an extension in two ways The first is by passing the element instance as a parameter, as shown here:
setterName(myFeature);
CHAPTER 14 Xpand Template Language
The implicit first parameter represents the element instance, so we can use the member syntax to seemingly invoke an extension on the instance This gives us more of the feeling that we re truly extending the underlying metamodel This is an equivalent example to the first:
myFeaturesetterName();
Remember that even though we re seemingly extending the metamodel, these extensions are not available using reflection and cannot be used for specialization of metamodel operations During evaluation, an operation matched in the metamodel takes precedence in execution
Type Inference
It s not strictly required that a return type be declared because the type can be inferred from the expression and depends on its context of use For example, consider this expression:
asList(Object o) : {o};
When invoking this extension and passing an Integer, as shown next, it has the static type List[Integer] So the use of upTo() is statically type safe
asList(55)get(0)upTo(60);
Recursion
One exception to the rule regarding type declaration is with recursive expressions Because the type cannot be inferred, it must be stated explicitly, as shown in this example:
String fullyQualifiedName(JavaPackage p) : peSuperPackage == null pname : fullyQualifiedName(peSuperPackage) + '' + pname ;
Consider another example from GMF s xpt::editor::palette::Utils extensions:
141 Xpand Language
private List[gmfgen::ToolGroup] collectSubGroups(gmfgen::ToolGroup group) : let rv = groupentriestypeSelect(gmfgen::ToolGroup) : rvaddAll(rvcollect(g| gcollectSubGroups())flatten()) ;
Cached Extensions
If you expect to have an extension called multiple times for a given set of parameters, you might want to have the result cached for performance reasons This is accomplished by adding the cached keyword to the expression, as in this example from GMF s xpt::navigator::Utils extensions:
cached List[gmfgen::GenCommonBase] getNavigatorContainerNodes(gmfgen::GenNavigator navigator) : getNavigatorNodes(navigator)select( n | getChildReferencesFrom(navigator, n)size() > 0) ;
Private Extensions
If you do not want to expose an extension outside its file, you can add the private keyword, as shown in this example from GMF s xpt::navigator:: Utils extensions:
private List[gmfgen::GenCommonBase] getNavigatorNodes(gmfgen::GenNavigator navigator) : let diagram = navigatoreditorGendiagram : { diagram } addAll(diagramtopLevelNodes) addAll(diagramchildNodes) addAll(diagramlinks) addAll(diagramcompartments) typeSelect(gmfgen::GenCommonBase) ;