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

@@ -1,21 +1,21 @@
 <?xml version="1.0"?>
-<project name="LearningJava" default="compile">
+<project name="LearningJava" default="build">
 
-    <target name="clean" description="remove intermediate files">
-        <delete>
-            <fileset dir="obj" includes="**/*.class" />
-        </delete>
-    </target>
+  <target name="clean" description="remove intermediate files">
+    <delete>
+      <fileset dir="obj" includes="**/*.class"/>
+    </delete>
+  </target>
 
-    <target name="build" description="compile Java sources">
-        <javac srcdir="src" destdir="obj" />
-    </target>
+  <target name="build" description="compile Java sources">
+    <javac srcdir="src" destdir="obj" includeantruntime="yes"/>
+  </target>
 
-    <target name="jar" depends="build"
+  <target name="jar" depends="build"
         description="create Jar from compiled sources">
-        <jar destfile="dist/learning-java.jar">
-            <fileset dir="obj" includes="**/*.class" />
-        </jar>
-    </target>
+    <jar destfile="dist/learning-java.jar">
+      <fileset dir="obj" includes="**/*.class"/>
+    </jar>
+  </target>
 
 </project>