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

Such things aren't allowed in just about any language (I can't think of one right now), mostly to prevent confusion.

Your example -d is an excellent example. How does the compiler know if you meant "the variable named -d" or "the negative of the number in the variable d"? Since it can't tell (or worse yet, it could so you couldn't be sure what would happened when you typed that without reading the rest of the file), it's not allowed.

The example 7g is the same thing. You can specify numbers as certain bases or types by adding letters to the end. The number 8357 is an int in Java, where as 8357L is a long (since there is an 'L' on the end). If variables could start with numbers, there would be cases where you couldn't tell if it was supposed to be a variable name or just a literal.

I would assume the others you listed have similar reasons behind them, some of which may be historical (i.e. C couldn't do it for reason X, and Java is designed to look like C so they kept the rule).

In practice, they are almost never a problem. It's very rare you find a situation where such things are annoying. The one you'll run into the most is variables starting with numbers, but you can always just spell them out (i.e. oneThing, twoThing, threeThing, etc.).