and in Java Comments

Question

What s the difference between          comment             and             comment          in Java  When should I use them

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For the Java programming language  there is no difference between the two  Java has two types of comments  traditional comments             and end-of-line comments           See the Java Language Specification  So  for the Java programming language  both           and            are instances of traditional comments  and they are both treated exactly the same by the Java compiler  i e   they are ignored  or more correctly  they are treated as white space    However  as a Java programmer  you do not only use a Java compiler  You use a an entire tool chain  which includes e g  the compiler  an IDE  a build system  etc  And some of these tools interpret things differently than the Java compiler  In particular             comments are interpreted by the Javadoc tool  which is included in the Java platform and generates documentation  The Javadoc tool will scan the Java source file and interpret the parts between            as documentation   This is similar to tags like FIXME and TODO  if you include a comment like    TODO  fix this or    FIXME  do that  most IDEs will highlight such comments so that you don t forget about them  But for Java  they are just comments

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Java supports two types of comments       multiline comment      The compiler ignores everything from    to     The comment can span over multiple lines     single line   The compiler ignores everything from    to the end of the line    Some tool such as javadoc use a special multiline comment for their purpose  For example     doc comment    is a documentation comment used by javadoc when preparing the automatically generated documentation  but for Java it s a simple multiline comment

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Single comment e g     comment Multi Line comment e g     comment    javadoc comment  e g      comment

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First one is for Javadoc you define on the top of classes  interfaces  methods etc  You can use Javadoc as the name suggest to document your code on what the class does or what method does etc and generate report on it   Second one is code block comment  Say for example you have some code block which you do not want compiler to interpret then you use code block comment   another one is    this you use on statement level to specify what the proceeding lines of codes are supposed to do   There are some other also like   TODO  this will mark that you want to do something later on that place    FIXME you can use when you have some temporary solution but you want to visit later and make it better   Hope this helps

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The first is Javadoc comments  They can be processed by the javadoc tool to generate the API documentation for your classes  The second is a normal block comment

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Reading the section 3 7 of JLS well explain all you need to know about comments in Java      There are two kinds of comments             text            A traditional comment  all the text from the ASCII characters    to the ASCII characters    is ignored  as in C and C               text         An end-of-line comment  all the text from the ASCII characters    to the end of the line is ignored  as in C         About your question   The first one               is used to declare Javadoc Technology      Javadoc is a tool that parses the declarations and documentation   comments in a set of source files and produces a set of HTML pages   describing the classes  interfaces  constructors  methods  and fields    You can use a Javadoc doclet to customize Javadoc output  A doclet is   a program written with the Doclet API that specifies the content and   format of the output to be generated by the tool  You can write a   doclet to generate any kind of text file output  such as HTML  SGML    XML  RTF  and MIF  Oracle provides a standard doclet for generating   HTML-format API documentation  Doclets can also be used to perform   special tasks not related to producing API documentation    For more information on Doclet refer to the API   The second one  as explained clearly in JLS  will ignore all the text between    and    thus is used to create multiline comments     Some other things you might want to know about comments in Java   Comments do not nest     and    have no special meaning in comments that begin with        has no special meaning in comments that begin with    or      The lexical grammar implies that comments do not occur within character literals    3 10 4  or string literals    3 10 5     Thus  the following text is a single complete comment      this comment           ends here

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I don t think the existing answers adequately addressed this part of the question      When should I use them    If you re writing an API that will be published or reused within your organization  you should write comprehensive Javadoc comments for every public class  method  and field  as well as protected methods and fields of non-final classes   Javadoc should cover everything that cannot be conveyed by the method signature  such as preconditions  postconditions  valid arguments  runtime exceptions  internal calls  etc   If you re writing an internal API  one that s used by different parts of the same program   Javadoc is arguably less important   But for the benefit of maintenance programmers  you should still write Javadoc for any method or field where the correct usage or meaning is not immediately obvious   The  killer feature  of Javadoc is that it s closely integrated with Eclipse and other IDEs   A developer only needs to hover their mouse pointer over an identifier to learn everything they need to know about it   Constantly referring to the documentation becomes second nature for experienced Java developers  which improves the quality of their own code   If your API isn t documented with Javadoc  experienced developers will not want to use it

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Comments in the following listing of Java Code are the greyed out characters           The HelloWorldApp class implements an application that    simply displays  Hello World   to the standard output      class HelloWorldApp       public static void main String   args            System out println  Hello World       Display the string            The Java language supports three kinds of comments      text      The compiler ignores everything from    to          documentation      This indicates a documentation comment  doc comment  for short   The compiler ignores this kind of comment  just like it ignores comments that use    and     The JDK javadoc tool uses doc comments when preparing automatically generated documentation      text   The compiler ignores everything from    to the end of the line   Now regarding when you should be using them   Use    text when you want to comment a single line of code   Use    text    when you want to comment multiple lines of code   Use     documentation     when you would want to add some info about the program that can be used for automatic generation of program documentation

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The first form is called Javadoc   You use this when you re writing formal APIs for your code  which are generated by the javadoc tool   For an example  the Java 7 API page uses Javadoc and was generated by that tool   Some common elements you d see in Javadoc include     param   this is used to indicate what parameters are being passed to a method  and what value they re expected to have  return   this is used to indicate what result the method is going to give back  throws   this is used to indicate that a method throws an exception or error in case of certain input  since   this is used to indicate the earliest Java version this class or function was available in   As an example  here s Javadoc for the compare method of Integer          Compares two   code int  values numerically     The value returned is identical to what would be returned by      lt pre gt        Integer valueOf x  compareTo Integer valueOf y       lt  pre gt         param  x the first   code int  to compare     param  y the second   code int  to compare     return the value   code 0  if   code x    y              a value less than   code 0  if   code x  lt  y   and            a value greater than   code 0  if   code x  gt  y      since 1 7     public static int compare int x  int y        return  x  lt  y    -1     x    y    0   1       The second form is a block  multi-line  comment   You use this if you want to have multiple lines in a comment   I will say that you d only want to use the latter form sparingly  that is  you don t want to overburden your code with block comments that don t describe what behaviors the method complex function is supposed to have   Since Javadoc is the more descriptive of the two  and you can generate actual documentation as a result of using it  using Javadoc would be more preferable to simple block comments

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