What is the ellipsis for in this method signature

Question

In the App Engine docs  what is the ellipsis  JID     for in this method signature   public MessageBuilder withRecipientJids JID    recipientJids    What s the function of those three dots

User · Answer

The three dot       notation is actually borrowed from mathematics  and it means     and so on    As for its use in Java  it stands for varargs  meaning that any number of arguments can be added to the method call  The only limitations are that the varargs must be at the end of the method signature and there can only be one per method

User · Answer

Those are Java varargs  They let you pass any number of objects of a specific type  in this case they are of type JID    In your example  the following function calls would be valid   MessageBuilder msgBuilder    There should probably be a call to a constructor here    MessageBuilder msgBuilder2  msgBuilder withRecipientJids jid1  jid2   msgBuilder2 withRecipientJids jid1  jid2  jid78 a  someOtherJid     See more here  http   java sun com j2se 1 5 0 docs guide language varargs html

User · Answer

Those are varargs they are used to create a method that receive any number of arguments   For instance PrintStream printf method uses it  since you don t know how many would arguments you ll use   They can only be used as final position of the arguments   varargs was was added on Java 1 5

User · Answer

The way to use the ellipsis or varargs inside the method is as if it were an array   public void PrintWithEllipsis String   setOfStrings        for  String s   setOfStrings          System out println s       This method can be called as following   obj PrintWithEllipsis       prints nothing obj PrintWithEllipsis  first       prints  first  obj PrintWithEllipsis  first    second       prints  first nsecond    Inside PrintWithEllipsis  the type of setOfStrings is an array of String  So you could save the compiler some work and pass an array   String   argsVar     first    second    obj PrintWithEllipsis argsVar     For varargs methods  a sequence parameter is treated as being an array of the same type  So if two signatures differ only in that one declares a sequence and the other an array  as in this example   void process String   s    void process String   s      then a compile-time error occurs   Source  The Java Programming Language specification  where the technical term is variable arity parameter rather than the common term varargs

User · Answer

It means that the method accepts a variable number of arguments   varargs   of type JID  Within the method  recipientJids is presented   This is handy for cases where you ve a method that can optionally handle more than one argument in a natural way  and allows you to write calls which can pass one  two or three parameters to the same method  without having the ugliness of creating an array on the fly   It also enables idioms such as sprintf from C  see String format    for example

[java] What is the ellipsis (...) for in this method signature?

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