Any name injava program is called as identifiers. It may be a variable name, a method name, a class etc.

Example:

Class Test
{
	public Static void main(String[] args)
	{
		int X=10;
	}
}

NOTE: ALL THE ABOVE ARE IDENTIFIERS.
RULES & REGULATIONS FOR DECLARING AN IDENTIFIER:

Example:

int String=10;
System.out.println(String) ;  // 10
Here String is a valid identifier but not recomnded.
int Runnable=20;
System.out.println(Runnable);  //20

Here Runnable(an inbuilt interface) is a valid identifier, but not recommended.
5. There is no length limit of identifier.
Exa: int xxxxxxxxxxxyyyyyxz=10;

Reserved Words: