What do the 3 dots in Java generics mean?

Question

for example I have a code like this: (from here)

private class LongOperation extends AsyncTask<String, Void, String> {

      @Override
      protected String doInBackground(String... params) {}      

      @Override
      protected void onPostExecute(String result) {}

      @Override
      protected void onPreExecute() {}

      @Override
      protected void onProgressUpdate(Void... values) {
      }
}

and what do the 3 dots in the parameter of the method do?

Note that varargs (variable arity arguments) have nothing to do with generics. — Joachim Sauer, Jul 12 '13 at 20:12
See also [the wikipedia article on variadic functions](http://en.wikipedia.org/wiki/Variadic_function) — Richard JP Le Guen, Jul 12 '13 at 20:12
Nothing to do with generics. None of the methods with the `...` explicit use generic types (thus they can be viewed simply as only using non-generic types). Make sure to classify questions appropriately. — user2246674, Jul 12 '13 at 20:12
okay thank you. thought that because I just see them in connection with generics — maysi, Jul 12 '13 at 20:15
hey people.. just for info, can they be used as arrays, i mean all features including foreach?? — Sachin Verma, Jul 12 '13 at 20:19

William Morrison · Accepted Answer · 2017-03-07T13:07:33.360

9

The three dots are referred to as varargs and here, allow you to pass more than one string to the method like so:

doInBackground("hello","world");
//you can also do this:
doInBackground(new String[]{"hello","world"});

Documentation on that here.

Within the method doInBackground you can enumerate over the varargs variable, params like so:

for(int i=0;i<params.length;i++){
    System.out.println(params[i]);
}

So its basically an array of strings within the scope of doInBackground

edited Mar 07 '17 at 13:07

answered Jul 12 '13 at 20:10

William Morrison

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ok. and how do I access to this strings? just like an array? – maysi Jul 12 '13 at 20:13
2

@Simon Yup. In fact, at runtime it _is_ just an array, and not even a special one. The `...` stuff is purely compile-time syntactic sugar. You can even invoke the method by passing it a `String[]` (or whatever) instead of a bunch of `String`s, as this answer shows. – yshavit Jul 12 '13 at 20:16

score 4 · Answer 2 · answered Jul 12 '13 at 20:12

4

The compiler treats the three dots ... as taking in an array of that object. In this case String and Void. The amount of objects you pass in is the size of the array.

Thus:

doInBackground("Hi", "Hello", "Bye") will create an array of String of length 3.

answered Jul 12 '13 at 20:12

chancea

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so void will generate an array of the type Object? – maysi Jul 12 '13 at 20:16
1

@Simon not `void` this is `Void` it is an object itself different that the `void` your thinking of – chancea Jul 12 '13 at 20:17
Okay and parameters like `Void... objects` would be the same as `Object[] objects` ? – maysi Jul 12 '13 at 20:20
1

@Simon no the `Void` class is different than the `Object` class. Look at http://stackoverflow.com/questions/643906/uses-for-the-java-void-reference-type – chancea Jul 12 '13 at 20:21

score 2 · Answer 3 · answered Jul 12 '13 at 20:13

2

This concept is called varargs and explained here

answered Jul 12 '13 at 20:13

Michael Lang

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What do the 3 dots in Java generics mean?

3 Answers3