What's best way to developing multi platforms app, working on web, Mobile (IOS, ANDROID, WIN, BB), Desktops (WIN, MAC,LINUX) & sync offline data?

Java is the mostly common development tool and can be used for mobile applications as well as iOS and windows based systems. The development process will be the same for different operating systems only the different will be the SDK. SDK is a package to be installed to allow the targeted platform to run Java applications. It is like a translator. Hope my answer replies your question. Thanks