How do you make a program for different operating systems , such as windows and mac os x?

Java is the object oriented, platform independent programming language which can run on any OS with JVM. Nowadays, everybody installs java on their computers. You can even check by typing "java -version" in your command prompt for windows or terminal for mac, linux. You can also write android application with it. All programming languages are started with one purpose in mind and we don't need to care about it. For example, we use vb.net to develop the application that run on windows desktop and objective-C for Mac. Still, we like to use Java or Python to develop application for all systems. It's all depends on personal preferences and area of interest. So, just learn one and expand the knowledge.