what is main work "java"?

Answer 1

Well, hard question. Some time ago, Java main work was introduced with the slogan "Write once, run everywhere"..So the same program coded in Java could run on many operations system. But this way was not effective, these programs, mainly with a huge computes, or windows were slowly and did not look good in different systems.
So, now the Java programs work in many systems, in dektops, older mobiles, new smartphons and tablets, or backround in web servers, or many others automatic systems.. But for every such that environment you have to write its difference code in Java with another functions another libraries, another techniques of programming...

Answer 2

how many type java?