Valid Characters in a Java Class Name

Valid characters in a Java class name

You can have almost any character, including most Unicode characters! The exact definition is in the Java Language Specification under section 3.8: Identifiers.

An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter. ...

Letters and digits may be drawn from the entire Unicode character set, ... This allows programmers to use identifiers in their programs that are written in their native languages.

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), or a compile-time error occurs.

However, see this question for whether or not you should do that.

Can we have class name with alphanumeric character?

Generally, classes can be named only with alphanumeric characters, meaning a-z, A-Z and 0-9. You should also make the first letter of a class Capital.

What are the valid characters for a Java method name?

Yes, method names and variable names are what's called "identifiers". Identifiers all share the same rules regarding accepted characters. Take a look at §3.8 from the Java Language Specification to find out exactly what an identifier may contain, and §6.2 for an explanation about how identifiers are used.

Java - class name starts with numbers

The first character needs to be a "Java letter", which includes letters, underscore and dollar sign.

See https://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.8

An identifier is an unlimited-length sequence of Java letters and Java
digits, the first of which must be a Java letter.
[...]

The "Java letters" include uppercase and lowercase ASCII Latin letters
A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical
reasons, the ASCII underscore (_, or \u005f) and dollar sign ($, or
\u0024). The $ character should be used only in mechanically generated
source code or, rarely, to access pre-existing names on legacy
systems.
[...]

Letters and digits may be drawn from the entire Unicode character set,
which supports most writing scripts in use in the world today,
including the large sets for Chinese, Japanese, and Korean. This
allows programmers to use identifiers in their programs that are
written in their native languages.

Illegal characters in class names

All alphanumeric unicode characters and underscore are valid, but it cannot begin with a number, so anything not belonging in there is invalid.

• Similar question here
• More info on C# language spec here

Which characters are valid in CSS class names/selectors?

You can check directly at the CSS grammar.

Basically¹, a name must begin with an underscore (_), a hyphen (-), or a letter(a–z), followed by any number of hyphens, underscores, letters, or numbers. There is a catch: if the first character is a hyphen, the second character must² be a letter or underscore, and the name must be at least 2 characters long.

-?[_a-zA-Z]+[_a-zA-Z0-9-]*

In short, the previous rule translates to the following, extracted from the W3C spec.:

In CSS, identifiers (including element names, classes, and IDs in
selectors) can contain only the characters [a-z0-9] and ISO 10646
characters U+00A0 and higher, plus the hyphen (-) and the underscore
(_); they cannot start with a digit, or a hyphen followed by a digit.
Identifiers can also contain escaped characters and any ISO 10646
character as a numeric code (see next item). For instance, the
identifier "B&W?" may be written as "B&W?" or "B\26 W\3F".

Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity.

¹ It's all made a bit more complicated by the inclusion of escaped unicode characters (that no one really uses).

² Note that, according to the grammar I linked, a rule starting with TWO hyphens, e.g. --indent1, is invalid. However, I'm pretty sure I've seen this in practice.

Is 'Fruit...' a valid class name in Java Naming convention?

class Fruit {
   // Class implementation
}

public void extractJuice(Fruit... args) {
   // This method can take variable arguments of type Fruit
}

So what you are looking at is varargs short for Variable Arguments.

Futher references:
- When do you use varargs in Java?

---

Related Topics