What do the 3 dots in Java generics mean? [duplicate]

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?

Answer 1

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

Answer 2

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.

Answer 3

This concept is called varargs and explained here