Why We Use Java?

When anyone ask me why we use java?we can use .net for programming it is easy and why we use java?Then my answer is that java is open source progamming it is also multiopearting system .when we compare java ns .net progamming all topic come from java.We can say that .net is copy of java.All .net topic come java. we can run Java on many multioperating System but devloped on many operating system but .net application run on window system and developed on window system.So i say java is better than .net.why i waste money on same thing when i m getting one thing free and for same thing i will pay.