why java is useful for us?

Clarifications

... it allows examiners to set questions.

There are numerous programming languages being used, obviously not all are useful, but most have their pros and cons.

I'm sure all the information you need will be in your course notes.

Failing that, search google for your question, or even look in wikipedia.

Damn silly question

What are you using it for?

If it does it well, that's why it's useful.

than a good cup of java in the morning to get the day started

Because in Russia, programs install you!