Class not found: javac1.8

Class not found: javac1.8

/home/NetBeansProjects/WebApplication1/nbproject/build-impl.xml:887: The
following error occurred while executing this line:
/home/NetBeansProjects/WebApplication1/nbproject/build-impl.xml:309: Class
not found: javac1.8 BUILD FAILED (total time: 0 seconds) i clean and built
the project.i have istalled netbeans 7.3.1 on ubuntu..how to solve this
error