Identifiers（標識符） in Java are symbolic(符號化) names used for identification.
They can be a class name, variable name, method name, package name, constant（常量） name, and more.
However, In Java, There are some reserved words（保留字） that can not be used as an identifier.
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There are some rules and conventions（慣例） for declaring the identifiers in Java.
If the identifiers are not properly declared, we may get a compile-time error.
Following are some rules and conventions for declaring identifiers:

A valid identifier must have characters [A-Z] or [a-z] or numbers [0-9], and underscore(_) or a dollar sign ($). for example, @javatpoint is not a valid identifier because it contains a special character which is @.

There should not be any space in an identifier. For example, java tpoint is an invalid identifier.
An identifier should not contain a number at the starting. For example, 123javatpoint is an invalid identifier.

An identifier should be of length 4-15 letters only. However, there is no limit on its length. But, it is good to follow the standard conventions.

We can’t use the Java reserved keywords as an identifier such as int, float, double, char, etc. For example, int double is an invalid identifier in Java.

An identifier should not be any query language（查詢語言） keywords such as SELECT, FROM, COUNT, DELETE, etc.

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