Why can an identifier not start with a number?

Question

Why in java (I dont know any other programming languages) can an identifier not start with a number and why are the following declarations also not allowed?

Answer 1

That's because section 3.8 of the Java Language Specification says so.

An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter. An identifier cannot have the same spelling (Unicode character sequence) as a keyword (§3.9), boolean literal (§3.10.3), or the null literal (§3.10.7).

As for why this decision was made: probably because this simplifies parsing, avoids ambiguous grammar, allows introduction of special syntax in a later version of the language and/or for historical reasons (i.e. because most other languages have the same restrictionsimilar restrictions). Note that your examples example with -d is especially clear:

int -d = 7;
System.out.println("Some number: " + (8 + -d));

Is the minus the first part of an identifier, or the unary minus?

Furthermore, if you had both -d and d as variables, it would be completely ambiguous:

int -d = 7;
int d = 2;
System.out.println("Some number: " + (8 + -d));

Is the result 15 or 6?