How can I code for calculating a CompilationUnit lines of code in

I would like to calcualate开发者_StackOverflow a compilation unit lines of code intuitively. How can I use the method of CompilationUnit: CompilationUnit.getLineNumber([THELASTLINE].getPosition()) to get the lines of code of this compilationUnit. How can I get the ast node which contains "the last line" then?

Many thanks,