Incremental compile

[Charles M�gnin]

Would someone kindly point me to the procedure
necessary to compile incrementally ? The following
snipet compiles all the sources every time but
I've seen other systems where the same code
recompiles modified source files only.
Thanks much
Charles

<target name="build" depends="init">
    <mkdir dir="${java.build}"/>
    <javac srcdir="${java.source}" destdir="${java.build}" />
</target>