Is Unified Modelling Language(UML) Necessary For Java Programming(Core) ?

No, but if you are going to build a program that is more than just a simple little thing with one or two classes, then you will find it helpful. The  more classes, the more likely UML will help.

Java is an object orientated language. Designing a program before you attempt to write it is quite simply the only way you can expect to succeed. Too many novice programmers just jump straight in and start coding, with little or no idea of the expected structure of the program.

The standard way to design an object orientated program is to use the unified modelling language.  This is a set of diagrams which illustrate various ways in which the objects in the program communicate with each other. There are many types of diagram, but there is no need to draw every single one to design your program.

With a good design laid out on paper, coding the program suddenly becomes quite straight forward.  Several of the diagrams include terms that are really just pseudocode, but laid out in a systematic way. Only a fool would attempt to create a complex program without some sort of a design, so  uml diagrams are a must.

Well... If you are working with a group, you need to model the program you are going to do. UML is not a programming language, and it is not mandatory. If you don't have much experience or don't feel that you will tackle the concept at once, still you need to go for a modeling language. UML is the best. If you are working with a group, you need to stick to the standards. But actually who makes the standards. I don't think a wrong, changing standards if working on a small project with a small team. If I'm doing it alone, if I feel that I can tackle without paper and pen, I directly start coding. But when there is a small trouble, then I need to draw a diagram to solve it. UML is a set of guidelines to draw such diagrams. Not only for Java, whatever programming language you use, it is healthy to use some diagrams to design before you go for coding, especially with a group of people; then it is easier to understand what he/she really means.