Comments are descriptive text which are not compiled or executed, but are provided as an aid to the reader of the code to understand the code better. It is better to write meanigful comments for the code you write.

There are three types of comments used in Java. These are:

// single line comment
The line starting from slashes (two slashes) to the end of line is considered as a comment. Example:
// This comment extends to the end of the line.

 /* Block comment */
To add a comment of more than one line, we can precede our comment using /* and end with delimiter */.  Everything in between these two delimiters is discarded by the Java compiler. For instance

/* line one
* line 2
*/
This type of comment may also be placed between any Java tokens:
int i = /* maximum integer */ Integer.MAX_VALUE;

/** documentation comment */
 This is a special type of comment that indicates documentation comment. This type of comment is readable to both, computer and human. To start the comment, use /** instead of /* and end with */. This type of comment is a documentation which is interpreted as an official document on how the class and its public method work. For instance

/**
* These are used to extract documentation from the Java source.

*/

Be careful, a block comment might not always reliably comment out a section of code

A block comment does not reliably comment out a section of code. We should use a sequence of single-line comments instead. Most IDEs automate this process.

Consider an example:

        /*
            /* Operators not supported yet*/
            if ("+-*/&|!=".indexOf(ch) >= 0)
            return "OPERATOR ";
        */

The resulting program is syntactically invalid. String literals are not treated specially within comments. Block comments do not nest. In our program, the comment that start with /* ends with */ at the end of /* Operators not supported yet */.
Even if we remove this line, the program will fail to compile. The text inside of comments is not treated specially in any way. The comment will now end seeing */ inside if condition.