Heyyy 

As a beginner there isn't any prerequisites candidate should know before learning java. Only thing that a candidate should have is passion along understanding of technical terms and notations in the books. JAVA is based on this concept where every thing is consider as an object so what you really need is an introduction to Object Oriented thinking, because java's power comes from this basic nature. Though it will be helpful if you would know a few concepts like what are variables, constants ,various loops ,various operators etc