When do you use varargs in Java

Question

I m afraid of varargs  I don t know what to use them for    Plus  it feels dangerous to let people pass as many arguments as they want    What s an example of a context that would be a good place to use them

User · Answer

A good rule of thumb would be    Use varargs for any method  or constructor  that needs an array of T  whatever type T may be  as input    That will make calls to these methods easier  no need to do new T           You could extend this rule to include methods with a List lt T gt  argument  provided that this argument is for input only  ie  the list is not modified by the method    Additionally  I would refrain from using f Object    args  because its slips towards a programming way with unclear APIs   In terms of examples  I have used it in DesignGridLayout  where I can add several JComponents in one call   layout row   grid new JLabel  Label    add field1  field2  field3     In the code above the add   method is defined as add JComponent    components    Finally  the implementation of such methods must take care of the fact that it may be called with an empty vararg  If you want to impose at least one argument  then you have to use an ugly trick such as   void f T arg1  T    args          I consider this trick ugly because the implementation of the method will be less straightforward than having just T    args in its arguments list   Hopes this helps clarifying the point about varargs

User · Answer

Varargs is the feature added in java version 1 5   Why to use this    What if  you don t know the number of arguments to pass for a method  What if  you want to pass unlimited number of arguments to a method    How this works   It creates an array with the given arguments  amp  passes the array to the method   Example    public class Solution          public static void main String   args            add 5 7           add 5 7 9              public static void add int    s           System out println s length           int sum 0          for int num s              sum sum num          System out println  sum is   sum               Output    2   sum is 12   3  sum is 21

User · Answer

Varargs can be used when we are unsure about the number of arguments to be passed in a method  It creates an array of parameters of unspecified length in the background and such a parameter can be treated as an array in runtime   If we have a method which is overloaded to accept different number of parameters  then instead of overloading the method different times  we can simply use varargs concept    Also when the parameters  type is going to vary then using  Object   test  will simplify the code a lot    For example    public int calculate int   list        int sum   0      for  int item   list            sum    item            return sum      Here indirectly an array of int type  list  is passed as parameter and is treated as an array in the code   For a better understanding follow this link it helped me a lot in understanding this concept clearly    http   www javadb com using-varargs-in-java  P S  Even I was afraid of using varargs when I didn t knw abt it  But now I am used to it  As it is said   We cling to the known  afraid of the unknown   so just use it as much as you can and you too will start liking it

User · Answer

Varargs are useful for any method that needs to deal with an indeterminate number of objects   One good example is String format   The format string can accept any number of parameters  so you need a mechanism to pass in any number of objects   String format  This is an integer   d   myInt   String format  This is an integer   d and a string   s   myInt  myString

User · Answer

I use varargs frequently for constructors that can take some sort of filter object  For example  a large part of our system based on Hadoop is based on a Mapper that handles serialization and deserialization of items to JSON  and applies a number of processors that each take an item of content and either modify and return it  or return null to reject

User · Answer

In Java doc of Var-Args it is quite clear the usage of var args   http   docs oracle com javase 1 5 0 docs guide language varargs html  about usage it says    So when should you use varargs   As a client  you should take advantage of them whenever the API offers them  Important uses in core APIs include reflection  message formatting  and the new printf facility   As an API designer  you should use them sparingly  only when the benefit is truly compelling   Generally speaking  you should not overload a varargs method  or it will be difficult for programmers to figure out which overloading gets called

User · Answer

I use varargs frequently for outputting to the logs for purposes of debugging   Pretty much every class in my app has a method debugPrint     private void debugPrint Object    msg        for  Object item   msg  System out print item       System out println        Then  within methods of the class  I have calls like the following   debugPrint  for assignment    hwId     student    studentId     question        serialNo     the grade is    grade     When I m satisfied that my code is working  I comment out the code in the debugPrint   method so that the logs will not contain too much extraneous and unwanted information  but I can leave the individual calls to debugPrint   uncommented   Later  if I find a bug  I just uncomment the debugPrint   code  and all my calls to debugPrint   are reactivated   Of course  I could just as easily eschew varargs and do the following instead   private void debugPrint String msg        System out println msg      debugPrint  for assignment     hwId      student     studentId      question         serialNo      the grade is     grade     However  in this case  when I comment out the debugPrint   code  the server still has to go through the trouble of concatenating all the variables in every call to debugPrint    even though nothing is done with the resulting string   If I use varargs  however  the server only has to put them in an array before it realizes that it doesn t need them   Lots of time is saved

User · Answer

I have a varargs-related fear  too   If the caller passes in an explicit array to the method  as opposed to multiple parameters   you will receive a shared reference to that array    If you need to store this array internally  you might want to clone it first to avoid the caller being able to change it later    Object   args   new Object     1  2  3      varArgMethod args       not varArgMethod 1 2 3     args 2     something else       this could have unexpected side-effects   While this is not really different from passing in any kind of object whose state might change later  since the array is usually  in case of a call with multiple arguments instead of an array  a fresh one created by the compiler internally that you can safely use  this is certainly unexpected behaviour

[java] When do you use varargs in Java?

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