Why can't Java variable names start with a number?

Question

In Java, variable names start with a letter, currency character ($) etc. but not with number, :, or .

Simple question: why is

Answer 1

Simply put, it would break facets of the language grammar.

For example, would 7f be a variable name, or a floating point literal with a value of 7?

You can conjure others too: if . was allowed then that would clash with the member selection operator: would foo.bar be an identifier in its own right, or would it be the bar field of an object instance foo?