Java identifiers can only begin with three things:

a letter (A-Z or a-z)

a dollar sign ($)

an underscore ( _ )


No digits or punctuations.


  Identifiers can contain the aforementioned three things plus digits (0-9) inside their bodies or at the end.  Anything else is illegal.  i.e.


double d_1;    and 

float f$;             will both compile.


int no.1 = 1;          and

String @address;  will not compile


■ You cannot use a reserved word  as an identifier.  i.e.


double do = 2; 

float for = 1;

byte switch = 0;    None of these three statements will compile.