GENERICS

[Abhi vijay]

Source: Inquisition

  // Line 1:
List<? super String> list = new ArrayList <String> (); 
 

// Line 2:
list.add(new Object()); 
 
Drag and Drop the following options over Line 1 and Line 2. Drag the relevant fragment over the line to indicate whether or not the Line will not compile, or if will compile, whether it will throw Exceptions.  

Here according to me Line2 should compile and run.
But the answer says. Compile time error.

But <? super String> means anything that is a super class of String can be added into the collection, Object is a superclass of String?

[Punit Singh]

You can add only String or subtypes of String.

list.add(new String());

[Ankit Garg]

Abhi vijay wrote:But <? super String> means anything that is a super class of String can be added into the collection, Object is a superclass of String?

No. ? super String means that the List can be of any super type of String. It might be CharSequence, Serializable, Comparable etc or even String itself. So you are not sure about which type it is. But one thing is sure. The type is a super type of String. So you can store objects of String and it's sub-classes into the list. Since String is a final class, so you can store only String class's objects in the list...

[Punit Singh]

Abhi vijay wrote:

But <? super String> means anything that is a super class of String can be added into the collection, Object is a superclass of String?

This means List reference can take any list reference that is String or SuperClass of String.

List<? super String> list = new ArrayList<Object>();
	List<? super String> list1 = new ArrayList<CharSequence>();
	List<? super String> list2 = new ArrayList<Comparable>();
	List<? super String> list3 = new ArrayList<Serializable>();

[Chandra Bhatt]

List<? super String> list = new ArrayList <String> ();   

list parameterized reference variable can refer to object parameterized with either <String> or Super class of String.
super just gives you flexibility of adding objects to it unlike extends where there is NO NO to add.

[Djonatah Stiegler]

Abhi vijay wrote:Source: Inquisition

  // Line 1:
List<? super String> list = new ArrayList <String> (); 
 

// Line 2:
list.add(new Object()); 
 
Drag and Drop the following options over Line 1 and Line 2. Drag the relevant fragment over the line to indicate whether or not the Line will not compile, or if will compile, whether it will throw Exceptions.  

Here according to me Line2 should compile and run.
But the answer says. Compile time error.

But <? super String> means anything that is a super class of String can be added into the collection, Object is a superclass of String?

You are wrong.

List<? super String> l = new ArrayList<String>();

This means the List can accept only collections of string super types, or strings. It is not about the elements that will be inserted into the collection.

<><

[Abhi vijay]

So how do I replace

  list.add(new Object());     

so that it compiles fine?

[Punit Singh]

 

  List<Object> list = new ArrayList<Object>();
	                               list.add(new Object());

 

[Djonatah Stiegler]

list.add(new Object());   

It 's going to compile fine when you have a collection that stores Objects
like that:

    st<Object> l = new ArrayList<Object>(); 

or that

    st<? super Object> l = new ArrayList<Object>(); 

But since Object class doens't have a super type it is useless...

<><

[Abhi vijay]

No, I mean

 
 List<? super String> list = new ArrayList <String> ();   
  
  

 list.add(new Object());  //Line2   

I mean Line 2 can be replaced with list.add(new String()); what else can it be replaced with?

[Punit Singh]

By subchilds of String, but String is a final class so it cannot be replaced with anything else.

Ya but you can replace with this:

   list.add(null); 

[Abhi vijay]

Thanks, a lot guys!!

Now its clear, actually I was trying to mix up LINE1 AND LINE2.

In line1,

List<? super String> list = new ArrayList<Object>(); // Line1.
Here on the Right hand Side , we can declare either String or any supertype of String, namely String,Serializable, CharSequence, Comparable<String>.

Whereas in Line2,

we can add only String or subtypes of String.

[Punit Singh]

Ya now you got 100%.