okay, so I'm not a total moron ... it was just my ridiculously muddled CLASSPATH that was to blame (along with the Flex SDK, maybe)

.gitignore

@@ -0,0 +1 @@
+*.class

build.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<project name="LearningJava" default="compile">
+<project name="LearningJava" default="build">
 
   <target name="clean" description="remove intermediate files">
     <delete>
@@ -8,7 +8,7 @@
   </target>
 
   <target name="build" description="compile Java sources">
-        <javac srcdir="src" destdir="obj" />
+    <javac srcdir="src" destdir="obj" includeantruntime="yes"/>
   </target>
 
   <target name="jar" depends="build"