Compile your Java source filename.java with the -g option to create debug data for the resulting filename.class. With this option, you can perform all debug functions.
If you do not use the -g compiler option, the debugger will not have access to symbolic information. In this case, the debugger will still be able to display a Source view, if it has access to the filename.java source and to step through your Java class, but you will not be able to view the content of any variables.