Can I pass an array as arguments to a method with variable arguments in Java

Question

I d like to be able to create a function like   class A     private String extraVar    public String myFormat String format  Object     args       return String format format  extraVar  args           The problem here is that args is treated as Object   in the method myFormat  and thus is a single argument to String format  while I d like every single Object in args to be passed as a new argument  Since String format is also a method with variable arguments  this should be possible   If this is not possible  is there a method like String format String format  Object   args   In that case I could prepend extraVar to args using a new array and pass it to that method

User · Answer

jasonmp85 is right about passing a different array to String format  The size of an array can t be changed once constructed  so you d have to pass a new array instead of modifying the existing one   Object newArgs   new Object args length 1   System arraycopy args  0  newArgs  1  args length   newArgs 0    extraVar   String format format  extraVar  args

User · Answer

Yes  a T    is only a syntactic sugar for a T    JLS 8 4 1 Format parameters  The last formal parameter in a list is special  it may be a variable arity parameter  indicated by an elipsis following the type  If the last formal parameter is a variable arity parameter of type T  it is considered to define a formal parameter of type T    The method is then a variable arity method  Otherwise  it is a fixed arity method  Invocations of a variable arity method may contain more actual argument expressions than formal parameters  All the actual argument expressions that do not correspond to the formal parameters preceding the variable arity parameter will be evaluated and the results stored into an array that will be passed to the method invocation   Here s an example to illustrate  public static String ezFormat Object    args        String format   new String new char args length            replace  quot  0 quot    quot    s   quot        return String format format  args     public static void main String    args        System out println ezFormat  quot A quot    quot B quot    quot C quot            prints  quot   A    B    C   quot     And yes  the above main method is valid  because again  String    is just String    Also  because arrays are covariant  a String   is an Object    so you can also call ezFormat args  either way  See also  Java language guide varargs   Varargs gotchas  1  passing null How varargs are resolved is quite complicated  and sometimes it does things that may surprise you  Consider this example  static void count Object    objs        System out println objs length      count null  null  null      prints  quot 3 quot  count null  null      prints  quot 2 quot  count null      throws java lang NullPointerException     Due to how varargs are resolved  the last statement invokes with objs   null  which of course would cause NullPointerException with objs length  If you want to give one null argument to a varargs parameter  you can do either of the following  count new Object     null        prints  quot 1 quot  count  Object  null      prints  quot 1 quot   Related questions The following is a sample of some of the questions people have asked when dealing with varargs   bug with varargs and overloading  How to work with varargs and reflection Most specific method with matches of both fixed variable arity  varargs    Vararg gotchas  2  adding extra arguments As you ve found out  the following doesn t  quot work quot       String   myArgs      quot A quot    quot B quot    quot C quot         System out println ezFormat myArgs   quot Z quot            prints  quot    Ljava lang String  13c5982    Z   quot   Because of the way varargs work  ezFormat actually gets 2 arguments  the first being a String    the second being a String  If you re passing an array to varargs  and you want its elements to be recognized as individual arguments  and you also need to add an extra argument  then you have no choice but to create another array that accommodates the extra element  Here are some useful helper methods  static  lt T gt  T   append T   arr  T lastElement        final int N   arr length      arr   java util Arrays copyOf arr  N 1       arr N    lastElement      return arr    static  lt T gt  T   prepend T   arr  T firstElement        final int N   arr length      arr   java util Arrays copyOf arr  N 1       System arraycopy arr  0  arr  1  N       arr 0    firstElement      return arr     Now you can do the following      String   myArgs      quot A quot    quot B quot    quot C quot         System out println ezFormat append myArgs   quot Z quot             prints  quot   A    B    C    Z   quot       System out println ezFormat prepend myArgs   quot Z quot             prints  quot   Z    A    B    C   quot    Varargs gotchas  3  passing an array of primitives It doesn t  quot work quot       int   myNumbers     1  2  3        System out println ezFormat myNumbers           prints  quot    I 13c5982   quot   Varargs only works with reference types  Autoboxing does not apply to array of primitives  The following works      Integer   myNumbers     1  2  3        System out println ezFormat myNumbers           prints  quot   1    2    3   quot

User · Answer

The underlying type of a variadic method function Object    args  is function Object   args   Sun added varargs in this manner to preserve backwards compatibility  So you should just be able to prepend extraVar to args and call String format format  args

User · Answer

I was having same issue   String   arr  new String      A    B    C     Object obj   arr    And then passed the obj as varargs argument  It worked

User · Answer

It s ok to pass an array - in fact it amounts to the same thing  String format   s  s    hello    world       is the same as  String format   s  s   new Object      hello    world        It s just syntactic sugar - the compiler converts the first one into the second  since the underlying method is expecting an array for the vararg parameter   See   Varargs - J2SE 1 5 Documentation

[java] Can I pass an array as arguments to a method with variable arguments in Java?

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