Java VarArgs

To support an arbitrary number of arguments of one type for a method you have basically only one option, you have to create an array.

void multiParamArray(String[] args) {}


String[] params = new String[2];
params[0]="param1";
params[1]="param2";

multiParamArray(params);

So to use this method you have to build an array that is then used as the args parameter. Using this newer syntax, which was introduced in Java 5, hides this complete array creation.

void multiParamVarArg(String ... args);


multiParamVarArg("param1", "param2");

Those three dots indicate, that the caller could put an arbitrary number of arguments of the same type (String) to this method.

Inernally an array is here used as well. Within your method you process this parameter like a normal array.

Note: The VarArg parameter must be the last parameter of a method. There can only be one VarArg parameter per method.

Processing a VarArg parameter

Within your method you have access to normal array methods, like length and normal value access via index.

void multiParamVarArg(String ... args) {
  for (String item : args) {
      System.out.println(item);
  }
}

So lets discuss multiple different calls to this code. First of all if you don’t set this args parameter with an array, you have no possibility of getting an null array. If you provide no argument, then the length of args simply will be 0.

multiParamVarArg();       // args.length == 0
multiParamVarArg(null);   // args.length == 1; args[0]==null;
multiParamVarArg("param1", "param2")  ;   // args.length == 2;

You still have the possibility to set this parameter using an array.

multiParamVarArg(new String[]{"param1", "param2"});   // args.length == 2;

//this one is a problem
multiParamVarArg((String[]) null)  ;   // args == null;

So be very careful using this later array type of setting the VarArg. Then you have to do all those nice null checks, we all love to do :).