What do 3 dots next to a parameter type mean in Java

Question

What do the 3 dots following String in the following method mean  public void myMethod String    strings          method body

User · Accepted Answer

It means that zero or more String objects (or a single array of them) may be passed as the argument(s) for that method.

See the "Arbitrary Number of Arguments" section here: http://java.sun.com/docs/books/tutorial/java/javaOO/arguments.html#varargs

In your example, you could call it as any of the following:

myMethod(); // Likely useless, but possible
myMethod("one", "two", "three");
myMethod("solo");
myMethod(new String[]{"a", "b", "c"});

Important Note: The argument(s) passed in this way is always an array - even if there's just one. Make sure you treat it that way in the method body.

Important Note 2: The argument that gets the ... must be the last in the method signature. So, myMethod(int i, String... strings) is okay, but myMethod(String... strings, int i) is not okay.

Thanks to Vash for the clarifications in his comment.

User · Answer

Arguably  it is an example of syntactic sugar  since it is implemented as an array anyways  which doesn t mean it s useless  - I prefer passing an array to keep it clear  and also declare methods with arrays of given type  Rather an opinion than an answer  though

User · Answer

Just think of it as the keyword params in C   if you are coming from that background

User · Answer

This is the Java way to pass varargs  variable number arguments    If you are familiar with C  this is similar to the     syntax used it the printf function   int printf const char   format          but in a type safe fashion  every argument has to comply with the specified type  in your sample  they should be all String    This is a simple sample of how you can use varargs   class VarargSample       public static void PrintMultipleStrings String    strings          for  String s   strings               System out println s                    public static void main String   args          PrintMultipleStrings  Hello    world             The     argument is actually an array  so you could pass a String   as the parameter

User · Answer

Also to shed some light  it is important to know that var-arg parameters are limited to one and you can t have several var-art params  For example this is illigal   public void myMethod String    strings  int     ints      method body

User · Answer

Adding to the other well-written answers  an advantage of varagrs I found useful is that  when I call a method with array as a parameter type  it takes away the pain of creating an array  add elements and then send it   Instead  I can just call the method with as many values as I want  from zero to many

User · Answer

That feature is called varargs  and it s a feature introduced in Java 5  It means that function can receive multiple String arguments  myMethod  quot foo quot    quot bar quot    myMethod  quot foo quot    quot bar quot    quot baz quot    myMethod new String    quot foo quot    quot var quot    quot baz quot        you can even pass an array  Then  you can use the String var as an array  public void myMethod String    strings       for String whatever   strings              do what ever you want               the code above is equivalent to     for  int i   0  i  lt  strings length  i                classical for  In this case you use strings i           This answer borrows heavily from kiswa s and Lorenzo s    and also from Graphain s comment

User · Answer

It means zero or multiple parameter of same data type  It must be the last parameter of the constructor or function  We can have only one of this type parameter in respective constructor or function    Example 1   public class quest1        public quest1 String    mynum            System out println  yee haa               public static void main String   args            quest1 q new quest1             quest1 q1 new quest1  hello               Example 2   public class quest1        public quest1 int    at            System out println  yee haa               public quest1 String    at            System out println  yee haa               public static void main String   args            quest1 q new quest1  value             quest1 q1 new quest1 1               public void name String     s                  output       yee haa      yee haa

User · Answer

It s Varargs     The varargs short for variable-length arguments is a feature that allows the method to accept variable number of arguments  zero or more   With varargs it has become simple to create methods that need to take a variable number of arguments  The feature of variable argument has been added in Java 5   Syntax of varargs  A vararg is secified by three ellipsis  three dots  after the data type  its general form is  return type method name data type     variableName         Need for varargs  Prior to Java 5  in case there was a need of variable number of arguments  there were two ways to handle it  If the max number of arguments  a method can take was small and known  then overloaded versions of the method could be created  If the maximum number of arguments a method could take was large or and unknown then the approach was to put those arguments in an array and pass them to a method which takes array as a parameter  These 2 approaches were error-prone - constructing an array of parameters every time and difficult to maintain - as the addition of new argument may result in writing a new overloaded method   Advantages of varargs  Offers a much simpler option  Less code as no need to write overloaded methods   Example of varargs  public class VarargsExample    public void displayData String     values     System out println  Number of arguments passed     values length     for String s   values      System out println s                  public static void main String   args      VarargsExample vObj   new VarargsExample         four args   vObj displayData  var    args    are    passed        three args   vObj displayData  Three    args    passed         no-arg   vObj displayData         Output  Number of arguments passed 4 var  args  are  passed  Number of arguments passed 3 Three  args  passed  Number of arguments passed 0   It can be seen from the program that length is used here to find the number of arguments passed to the method  It is possible because varargs are implicitly passed as an array  Whatever arguments are passed as varargs are stored in an array which is referred by the name given to varargs  In this program array name is values  Also note that method is called with different number of argument  first call with four arguments  then three arguments and then with zero arguments  All these calls are handled by the same method which takes varargs   Restriction with varargs  It is possible to have other parameters with varargs parameter in a method  however in that case  varargs parameter must be the last parameter declared by the method   void displayValues int a  int b  int     values     OK    void displayValues int a  int b  int     values  int c     compiler error   Another restriction with varargs is that there must be only one varargs parameter   void displayValues int a  int b  int     values  int     moreValues     Compiler error   Overloading varargs Methods  It is possible to overload a method that takes varargs parameter   Varargs method can be overloaded by -  Types of its vararg parameter can be different  By adding other parameters  Example of overloading varargs method  public class OverloadingVarargsExp       Method which has string vararg parameter  public void displayData String     values     System out println  Number of arguments passed     values length     for String s   values      System out println s                     Method which has int vararg parameter  public void displayData int     values     System out println  Number of arguments passed     values length     for int i   values      System out println i                     Method with int vararg and one more string parameter  public void displayData String a  int     values     System out println   a     a     System out println  Number of arguments passed     values length     for int i   values      System out println i                  public static void main String   args      OverloadingVarargsExp vObj   new OverloadingVarargsExp         four string args   vObj displayData  var    args    are    passed          two int args   vObj displayData 10  20         One String param and two int args   vObj displayData  Test   20  30        Output  Number of arguments passed 4 var  args  are  passed   Number of arguments passed 2 10  20   a Test Number of arguments passed 2 20  30    Varargs and overloading ambiguity  In some cases call may be ambiguous while we have overloaded varargs method  Let s see an example  public class OverloadingVarargsExp       Method which has string vararg parameter  public void displayData String     values     System out println  Number of arguments passed     values length     for String s   values      System out println s                     Method which has int vararg parameter  public void displayData int     values     System out println  Number of arguments passed     values length     for int i   values      System out println i                  public static void main String   args      OverloadingVarargsExp vObj   new OverloadingVarargsExp         four string args   vObj displayData  var    args    are    passed          two int args   vObj displayData 10  20         This call is ambiguous   vObj displayData           In this program when we make a call to displayData   method without any parameter it throws error  because compiler is not sure whether this method call is for displayData String     values  or displayData int     values   Same way if we have overloaded methods where one has the vararg method of one type and another method has one parameter and vararg parameter of the same type  then also we have the ambiguity -  As Exp -  displayData int     values  and displayData int a  int     values   These two overloaded methods will always have ambiguity

User · Answer

String    is the same as String    import java lang     public class MyClassTest          public static void main String    args          public static void main String   args            for String str  args                System out println str

User · Answer

A really common way to see a clear example of the use of the three dots it is present in one of the most famous methods in android AsyncTask   that today is not used too much because of RXJAVA  not to mention the Google Architecture components   you can find thousands of examples searching for this term  and the best way to understand and never forget anymore the meaning of the three dots is that they express a    doubt    just like in the common language  Namely it is not clear the number of parameters that have to be passed  could be 0  could be 1 could be more  an array

User · Answer

Syntax   Triple dot       --  Means we can add zero or more objects pass in an arguments or pass an array of type object   public static void main String   args    public static void main String    args      Definition   1  The Object     argument is just a reference to an array of Objects   2    String     or String       It can able to handle any number of string objects  Internally it uses an array of reference type object   i e  Suppose we pass an Object array to the     argument - will the resultant argument value be a two-dimensional array - because an Object   is itself an Object    3  If you want to call the method with a single argument and it happens to be an array  you have to explicitly wrap it in   another  method new Object   array     OR  method  Object array   which will auto-wrap    Application   It majorly used when the number of arguments is dynamic number of arguments know at runtime  and in overriding  General rule - In the method we can pass any types and any number of arguments  We can not add object      arguments before any specific arguments  i e   void m1 String      String s  this is a wrong approach give syntax error  void m1 String s  String       This is a right approach  Must always give last order prefernces

[java] What do 3 dots next to a parameter type mean in Java?

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