After some testing, the problem just happens when I don't have the return value
in the list of possible types, and boolean wasn't.
Anyway I think it should add them to the list automaitcally.

SVN commit 509191 by okellogg:

defaultDatatypes(): New. Override method from CodeGenerator.
BUG:115991


 M  +1 -0      ChangeLog  
 M  +14 -0     umbrello/codegenerators/javawriter.cpp  
 M  +5 -0      umbrello/codegenerators/javawriter.h  


--- branches/KDE/3.5/kdesdk/umbrello/ChangeLog #509190:509191
@@ -5,6 +5,7 @@
 * fixed problem with font size computation/word wrap in note widgets
   http://sourceforge.net/mailarchive/forum.php?thread_id=9558795&forum_id=472
 * Bugs/wishes from http://bugs.kde.org:
+* Umbrello doesnt allow some keywords for return types (115991)
 * Multiplicity labels positioned incorrectly when moving entities (120598)
 * Types of entity's attributes are displayed instead of their names (120742)
 * Unable to delete entity from entity list in "UML Model" frame (120750)
--- branches/KDE/3.5/kdesdk/umbrello/umbrello/codegenerators/javawriter.cpp #509190:509191
@@ -649,6 +649,20 @@
     return string;
 }
 
+QStringList JavaWriter::defaultDatatypes() {
+    QStringList l;
+    l.append("int");
+    l.append("char");
+    l.append("boolean");
+    l.append("float");
+    l.append("double");
+    l.append("byte");
+    l.append("short");
+    l.append("long");
+    l.append("String");
+    return l;
+}
+
 void JavaWriter::writeOperations(UMLClassifier *c, QTextStream &java) {
     UMLOperationList opl;
     UMLOperationList oppub,opprot,oppriv;
--- branches/KDE/3.5/kdesdk/umbrello/umbrello/codegenerators/javawriter.h #509190:509191
@@ -57,6 +57,11 @@
      */
     virtual Uml::Programming_Language getLanguage();
 
+    /**
+     * Overrides method from class CodeGenerator
+     */
+    QStringList defaultDatatypes();
+
 private:
 
     /**