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A Java block comment (also known as a traditional comment) is a lexical construct used to embed non-executable text within source code. During the lexical analysis phase of compilation, the Java compiler partitions the character stream into InputElements. Comments are discarded from the resulting token stream and are completely ignored during syntax parsing. Syntax A block comment begins with the forward-slash asterisk sequence /* and terminates with the asterisk forward-slash sequence */. All characters enclosed between these delimiters are discarded. 
/*
   This is a standard block comment.
   It spans multiple lines.
*/
Because block comments are evaluated at the lexical level, they can also be placed inline within a statement, provided they do not split a single token. 
int total = baseValue + /* inline block comment */ tax;
Lexical Rules and Constraints
  • No Nesting: Java does not support nested block comments. The compiler terminates the comment at the very first instance of */. Attempting to nest them results in a syntax error because the compiler attempts to parse the trailing text as executable code.
/* Outer comment
   /* Inner comment */
   This text is no longer commented and will cause a compilation error.
*/
  • String and Character Literals: The /* and */ sequences hold no special lexical meaning when placed inside string (" ") or character (' ') literals. They are treated strictly as standard literal characters.
String text = "This is not a /* comment */";
  • Unicode Escapes: The Java compiler processes Unicode escapes (e.g., \u002A for * and \u002F for /) before evaluating comments. Consequently, Unicode representations of the delimiters will successfully initiate or terminate a block comment.
  • Differentiation from Javadoc: While structurally similar, a block comment that begins with the exact sequence /** (a forward-slash followed by exactly two asterisks) is classified as a Javadoc comment. The compiler treats it as a standard block comment, but the javadoc tool parses it specifically for API documentation generation. The javadoc tool explicitly ignores comments that begin with three or more asterisks (e.g., /*** or /*******/). This intentional behavior allows developers to use large asterisk banners as visual separators in the code without accidentally generating API documentation.
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