I'm working in a project where we need to encrypt the .jar file so no one can access to the .class files which inside the jar file.... is there any java coding which can help me to encrypt the .jar file ?
Even if you encrypt the jar file, it must be decrypted before the JVM is able to run it, so you'll need another jar file containing classes that decrypt and loads in the JVM.
Since this second jar file cannot be itself encrypted, a malicious user wanting to see you class files, can simply look at classes in this second jar file, and then decrypt your super-secret jar file and have access to it.
Maybe you can increase security of your code using an obfuscator, but it will eventually protect (make it harder but not impossible) your class files from decompilation, not from being used.
If obfuscation is not enough, you could consider compiling your jar file to a DLL for windows or a SO for unix/linux, that will make it much harder to decompile, but it's not always possible to do that correctly and it's generally a PITA. GCJ is able to do this somehow, and there are other commercial products that will actually compile .class/.jar directly to machine code.
However please consider that it does not matter how much security you put in it, since the client computer MUST be able to execute it, it must be able to read it, so no matter what your code will be exposed, you can only make it harder.
If you really have an algorithm so secret you don't want to disclose no matter what, consider converting it to a web service, hosting it on your server, so that you don't have to send the actual code to the client machines and can also better prevent unauthorized copies of your application by checking access to that vital part of it.