Keywords are the reserved words in Java that have a predefined meaning. They form an important part of Java syntax and cannot be used as names for variables, classes, or methods.

What makes a keyword special

• Keywords are predefined and reserved by the language.
• Each keyword has a special meaning in Java.
• Keywords are always written in lowercase.
• Keywords cannot be used as identifiers.

Examples of Java keywords

• class
• while
• for
• interface
• abstract

Rules for identifiers

Identifiers are the names you assign to variables, constants, functions, classes, and arrays. They must follow Java naming rules and must not match reserved keywords.

• The first character must be a letter, underscore, or dollar sign.
• Identifiers cannot start with digits, but they may contain digits later.
• Whitespace cannot be included.
• Identifiers are case sensitive.

Valid and invalid examples

• Valid: PhoneNumber, PRICE, radius, a, a1
• Invalid: 1sum, Sum 1