Answer 1

suppose that you are writing an accounting program. however, there are lots of platfotms, such as windows, linux, android or any other one. so you have two choices. first one is to write a seperate program for each of these or "use JAVA". in java you write only one code. JAVA generates "class files" from your code which can be executed with "JAVA virtual machine (JVM)". Each platform has ita own JVM, but the class files which you write are the same. so it really simplifies things.