Java syntax <?> explanation

Question

Hi I came across a code in documentation of android in Grid view for the following code.

gridview.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
            Toast.makeText(HelloGridView.this, "" + position, Toast.LENGTH_SHORT).show();
        }
    });

In this what does the < ?> (space added as stackoverflow didn't allow without space) indicate/do?

This has nothing to do with Android. But you should add the generics tag — Miserable Variable
– Miserable Variable, Commented Jul 9, 2011 at 13:56

Oscar Gomez · Accepted Answer · 2011-07-09 05:16:32Z

3

The < ? > is a wild card for the generic type, meaning the generic type for AdapterView can be anything at all.

More specifically in this case the parameter on the method may receive an AdapterView with absolutely any generic type. As a note if you wanted to limit the generic type you could do:

AdapterView<? extends myClass)

This limits the generic type to myClass or anything that extends myClass.

Just as a note:

 AdapterView <?> and AdapterView<? extends Object>

Are identical.

You can find additional information here in the wildcards section

Java generics documentation

edited Jul 9, 2011 at 5:16

answered Jul 9, 2011 at 5:01

Oscar Gomez

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2 Comments

footy Over a year ago

So basically its used when we dont want to specify the type of the object(where required like in a Collection or Vector) and should we want it to be general case method?

Oscar Gomez Over a year ago

@Footy Yes it is used when you want to allow your parameter to be of any generic type without limiting it at all. If you wanted to limit it you can use <? extends myClass>, then it would only accept generics that extends or that are myClass.

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