Compile fails (generics/collections)

[Rachid Aourich]

Hi,
I was expecting line 1 and line 2 to compile ...

 
ArrayList<?> myList=new ArrayList<String>();
 myList.add(null);//Line 1
 myList.add(new String("")); //Line 2
 myList.add(new Object());//Line 3

However, only Line 1 is Compiled .
can you please explain why the two last lines (2&3) doesn't compile ?

[Devendra Mahra]

Since element type of myList is unknown, object cannot be added to it. Add method will take argument of type same as element type of collection. ? is unknown type and parameter passed to add needs to be subtype of ? i.e. unknown type and hence nothing can be passed to add method except null in this case.

[Rachid Aourich]

thanks ,
in that case i dont see the utility of that Instantiation [ArrayList<?> myList=new ArrayList<String>(); ] :

//Case 1  compile OK

		List<? super Integer> listOne=new ArrayList<Integer>();
		listOne.add(new Integer(0));
		
//Case 2  compile KO	
	
		List<? extends Integer> listTwo=new ArrayList<Integer>();
		listTwo.add(new Integer(0));

can you please explain why the first one compiles and the second one no ? Im very confused about that .

[Tomasz Sochanski]

		List<? super Integer> listOne=new ArrayList<Integer>();
		listOne.add(new Integer(0));     // compiles ok

Reference declared as List<? super Integer> can point List containing objects of unknown type, which is a supertype of Integer. It could be List<Object>, List<Comparable>, List<Serializable>, List<Number> or List<Integer> (any of them).
When you call List.add() in this case you can safely do it only using Integer instance, which IS-A contemporary Object, Number, Serializable, Comparable and Integer. Regardless of which supertype of Integer is used in particular listOne initialization (you have chosen ArrayList<Integer>, but it could be ie. ArrayList<Serializable>), add() method can always be called with Integer instance. In contrary you could not call listOne.add(new Object()) when declared listOne reference as List<? extends Integer>, because this declaration could mean List<Comparable>, and Object cannot be cast to Comparable when using add() method (like Integer could be).

	
		List<? extends Integer> listTwo=new ArrayList<Integer>();
		listTwo.add(new Integer(0));  // compile error

Here you declared listTwo as List of unknown types extending Integer. Let's suppose Integer is not final, and have subclass called SubInteger. List<? extends Integer> could be List<Integer> or List<SubInteger>. If it is List<SubInteger> you could not call add(Integer), because Integer is not SubInteger (SubInteger IS-A Integer, but not vice versa). It may look strange because we can see that listTwo is initialized as ArrayList<Integer> and not as ArrayList<SubInteger> (so we know the type is Integer not SubInteger), but please notice, that add() method is called on reference declared as List <? extends Integer> (uknown type).

Rachid Aourich wrote:
in that case i dont see the utility of that Instantiation [ArrayList<?> myList=new ArrayList<String>(); ] :

take a look here: https://coderanch.com/t/559717/java-programmer-SCJP/certification/List

regards
t.

[Rachid Aourich]

thanks Tomasz ,
your response was very helpful

[Tomasz Sochanski]

you are welcome ;]