What non-Unicode letter characters can be the first character of an identifier?

• A. A and B.
• B. $ and _.
• C. ! and @.
• D. The numeric character 1 and the character #.

Answer: (B)

In Java, the rules for valid identifiers specify that the first character of an identifier must be a letter (either uppercase or lowercase), a dollar sign ($), or an underscore (_). This means that while letters from the alphabet can initiate identifiers, the special characters that are permissible as the first character are limited to the dollar sign and the underscore.

Choosing the dollar sign is particularly useful in certain scenarios, like when interfacing with code generated by tools, or defining constants in some libraries. The underscore is frequently used by developers to enhance the readability of variable or method names by separating words (though Java naming conventions often recommend using camelCase for this purpose).

Since the other options include characters that are not allowed as starting characters for identifiers in Java, they do not conform to the language's naming rules. Therefore, the correct answer reflects the specific characters that are allowed, which are the dollar sign and the underscore.