1. Identifiers can use numbers, letters, underscores and dollar signs ($).

2. Identifiers cannot start with numbers.

3. Identifiers cannot contain spaces.

4. Identifiers cannot use keywords and reserved words, but they can contain keywords and reserved words.

5.java is strictly case-sensitive, but its length is infinite.

Identifier introduction:

In a programming language, an identifier is a string that is used as the name of a program element or the name of an object in a source program. This element can be a statement label, a procedure or function, a data element (such as scalar variables or arrays) or the program itself. Most commonly, the word identifier is almost synonymous with variable names.

In a system, the position of program data remains unchanged during the whole program execution. The identifier of scalar variable is associated with a memory address, which in turn points to a physical unit in the machine memory, and the physical unit contains a representation of a value. The intermediate relationship between an identifier and a value is usually transparent to programmers, so in practice, the variable name (that is, its identifier) and its value (that is, the current content of the storage unit allocated to the identifier) are confused.

In most programming languages, identifiers can be composed of any alphanumeric string, and the length is usually limited (usually 6 to 8 characters), as long as the leftmost character is a letter. Some languages also allow the use of special characters.