generic method write to a list

Question

I am trying to create a generic method which accepts a list of numbers in the format below. I know that I should be able to add any class to the list "o" which is at least a number (floats, integers, doubles) BUT also I should be able to add any object because all classes extend from object. In other words, object is a super class of any classs. So I wonder, why do I get an error on the line o.add(p); ?

public int checkType(List<? super Number> o) 
{
   Object p = new Object();
   //error
   o.add(p);
   return - 1;
 }

I followed the explanation of generics from Difference between <? super T> and <? extends T> in Java which is the accepted answer.

I thought it should be `List extends Number> o` as the parameter. So its applied to any object that extends Number, such as those native wrapper objects. — Ferdinand Neman, Sep 03 '15 at 08:50

score 2 · Accepted Answer · edited Dec 08 '15 at 09:26

2

The type-parameter List<? super Number> o specifies that any List which can hold a Number can be passed to it.

Which could be List<Object>, but that could also be List<Number>.

So compiler will allow you to add only Number (or subtype) objects to it.

To test, call your methods with the following arguments.

checkType(new ArrayList<Object>()); // Works fine (and it can hold Object type).
checkType(new ArrayList<Number>()); // Works fine (But it can NOT hold Object type).

So, as you see a List<? super Number> means that you are trying to add a Number to the list (Rule of PECS).

edited Dec 08 '15 at 09:26

Buhake Sindi

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answered Sep 03 '15 at 08:52

Codebender

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checkType(new ArrayList – george Sep 03 '15 at 08:59
@george, that's the whole point of PECS... The parameter is not restricted to just `List – Codebender Sep 03 '15 at 09:02

score 0 · Answer 2 · answered Sep 03 '15 at 08:52

0

One consideration?

List<Object> myObjectList = Arrays.asList(1, 2, new SuperObject(), 4, 5);

Java comes standard with a utility class Arrays.

answered Sep 03 '15 at 08:52

Buhake Sindi

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generic method write to a list

2 Answers2