Question: Why java mostly used for the web? What's its characteristics? 

Answer: Java is used in many places on the web because it happens to be platform independent.  That means you can create a web page and it doesn't matter if the viewer it using a MAC, UNIX, LINUX, or Windows machine, nor the browser they are using and they can still see all of the functionality of the site.  Unlike, for example, Flash which requires a plugin that is specific to the browser and the OS you are using.
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