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A Java single-line comment is a lexical construct that instructs the Java compiler to ignore all text following a double forward slash (//) up to the line terminator. During the lexical analysis phase of compilation, the compiler strips these comments from the source code as discarded input elements, ensuring they produce no bytecode and have zero impact on runtime execution. The syntax requires no closing delimiter; the comment automatically terminates at the end of the line, defined by a carriage return (\r), a line feed (\n), a carriage return followed by a line feed (\r\n), or the End of File (EOF). 
// This is a standalone single-line comment
int threadCount = 4; 

double timeout = 5.0; // This is a trailing single-line comment
Because the compiler lexically analyzes source code strictly from left to right, any valid Java syntax placed after the // character sequence on the same line is treated as part of the comment and will not be compiled. 
// int activeConnections = 10; (This statement is ignored by the compiler)
If the // character sequence appears within a string literal, the compiler treats it as standard character data rather than a comment. 
String endpoint = "https://api.example.com/v1"; // The // inside the string literal is not a comment
According to the Java Language Specification (JLS 3.2), lexical translation processes Unicode escapes before parsing comments. If a single-line comment contains a Unicode escape sequence representing a line terminator (such as \u000A for a line feed or \u000D for a carriage return), the escape is translated into an actual line terminator first. This prematurely ends the comment, causing the compiler to parse any text immediately following the escape sequence as executable Java code. 
// The following escape sequence acts as a newline \u000A int x = 10;
In the example above, the compiler translates \u000A into a line feed. The comment terminates at that exact position, and int x = 10; is parsed as a standard variable declaration despite appearing on the same physical line in the source file.
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