[R] help on "plotfit" function in package "nlstools"

I encountered this problem when I was learning nonlinear regression with R : there is this function "plotfit" in package "nlstools",which "displays a superimposed plot of the dependent variable versus one the independent variables together with the fitted model.",but when I execute the example give

[R] how to make JAVA available for R use?

i've just installed the package"xlsx",which is used for read,write,format Excel 2007 files, as i loaded the package ,i was told to intall the package rJava, the problem was that i didn't have a JAVA platform on my computer, so i have to install JAVA JDK first, my problem is ,after i 've installed J