■ 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.