Do you know Java: Underscore as identifier

We could use underscore as a variable name until Java 7, but it cannot be used as an identifier from Java 9 and upwards.

Java has strict rules about how to name a variable, which characters are accepted in it or which one can be the first one. Underscore is a legal character among alphabetic characters and numbers. What if I did not use any other chars just underscore? Is that still valid variable name? Let us see what the compiler says.

Single Underscore

The source looks as follows:

class Main {
    public static void main(String[] args) {
        int _ = 1;
        System.out.println(_);
    }
}

Java 7

We get the following result of the compilation of the above code with Java 7.

Yep! It is empty without any warnings!

Java 9

If we compile the code with Java 9, the result looks like:

error: as of release 9, '_' is a keyword, and may not be used as an identifier

It seems as _ was introduced as a keyword in Java 9. We cannot use it as a variable name.

Multiple Underscores

What if we put multiple underscores next to each other?

class Main {
    public static void main(String[] args) {
        int __ = 1;
        System.out.println(__);
    }
}

Java 7

Compiling the above code with Java 7, we get very similar to the previous one. No warnings again!

Java 9

The result of the compilation here in case of Java 9 is more interesting. Though we get error if we use single underscore, but it compiles swimmingly if identifier consits of multiple underscores.

Though we cannot use single underscore from Java 9 as variable name, we still have the opportunity to use multiple underscores concatenated next one another as an identifier.

Code can be found: https://github.com/torokmark/do-you-know-java