Are List<Object> and List<? extends Object> the same thing?

[Robert O'Leary]

They both can accept a reference type of any Object or subclass right?

[Himalay Majumdar]

Yes..you are right.

But only List<Object> allows you to add elements.

[Robert O'Leary]

Himalay Majumdar wrote:Yes..you are right.

But only List<Object>allows you to add elements.

Cool...and if you were using them as a method parameters, only List<? extends Object> can accept an argument other than List<Object>e.g as in Line 5.

class test {  
	public static void main(String args[]){
		List<Animal> a = new ArrayList<Animal>();
		method(a); //doesnt compile
		method2(a); //compiles
	}
	
	public static void method(List<Object> l)
	{ 
		l.add(new Animal("Toby"));	//compiles
	}
	
	public static void method2(List<? extends Object> l)
	{ 
		l.add(new Animal("Magilla")); //doesnt compile
	}
} 
     

[Himalay Majumdar]

:deleted:

[Himalay Majumdar]

Robert O'Leary wrote:
Cool...and if you were using them as a method parameters, only List<? extends Object> can accept an argument other than List<Object>e.g as in Line 5

Not true.. List<? extends Object> can accept ANY argument..including..List<Object>

public class Ranch {

	public static void main(String[] args) {

		List<Object> a = new ArrayList<Object>(); 
		methoda(a);
	}

	public static void methoda(List<? extends Object> l){
			
	}
}

Check out this link for more practice

[Ruben Soto]

Himalay,

I think you and Robert are saying the same thing. Maybe there is a little misunderstanding going on.

As for whether List<? extends Object> is the same as List<Object>, they are similar but quite different. You basically can't add anything to List<? extends Object>.

I however think that List<? super Object> and List<Object> are exactly the same. If I'm mistaken, someone will correct me.

[Himalay Majumdar]

Hi Ruben..how are you doin buddy.

Robert mentioned..List<? extends Object> can accept an argument other than List<Object>.

but List<? extends Object> can accept List<Object>

I dont see any difference between List<? super Object> and List<Object>,

but List<? super Dog> and List<Dog> are not same.

[Rex Tan]

Himalay Majumdar wrote:Hi Ruben..how are you doin buddy.

Robert mentioned..List&lt;? extends Object&gt; can accept an argument other than List&lt;Object&gt;.

but List&lt;? extends Object&gt; can accept List&lt;Object&gt;

I dont see any difference between List&lt;? super Object&gt; and List&lt;Object&gt;,

but List&lt;? super Dog&gt; and List&lt;Dog&gt; are not same.

List<? super Object> is very similar to List<Object>

but given Foo extends Object

List<? super Foo> list = new ArrayList<Object>(); is legal
List<Foo> list = new ArrayList<Object>(); is not legal

also for method calls

public void doStuff(List<Foo> l)
{
l.add (new Foo()); //Legal
}

public void doStuff(List<? extends Foo> l)
{
l.add (new Foo()); //Not legal
}

public void doStuff(List<? super Foo> l)
{
l.add(new Foo()); //Legal
}

It's really confusing but.. good luck

[Himalay Majumdar]

Adding to Rex's code

public void doStuff(List<? super Foo> l) 
{ 
l.add(new Object()); //illegal 
} 

Thats the reason I said

List<? super Object> is SAME as List<Object>,

but List<? super Dog> and List<Dog> are NOT SAME.

[Ruben Soto]

Himalay Majumdar wrote:Hi Ruben..how are you doin buddy.

Robert mentioned..List&lt;? extends Object&gt; can accept an argument other than List&lt;Object&gt;.

but List&lt;? extends Object&gt; can accept List&lt;Object&gt;

I dont see any difference between List&lt;? super Object&gt; and List&lt;Object&gt;,

but List&lt;? super Dog&gt; and List&lt;Dog&gt; are not same.

Himalay,

Doing good man, how's the studying going?

I think Robert didn't mean that List<? extends Object> can't accept Object references, I think he meant that the only difference between List<? extends Object> and List<Object> is that List<? extends Object> can accept a reference of a type other than Object. But maybe it's me who misunderstood.

[Ruben Soto]

Rex Tan wrote:
List<? super Object> is very similar to List<Object>

Hi Rex,

Actually I think they are exactly the same, because the only thing the wildcard can be is Object, since Object doesn't have a superclass. But for other classes, then List<? super A> would be different from List<A>. I think Himalay said the same thing also.

[Himalay Majumdar]

Yeah Ruben..I dint interpret his statement as whole... By "other than List<Object>" he means everything including List<Object>.
Now his code makes sense to me.

By the way..April 4th is my Judgement day..how about you.

[Ruben Soto]

Himalay Majumdar wrote:Yeah Ruben..I dint interpret his statement as whole... By "other than List&lt;Object&gt;" he means everything including List&lt;Object&gt;.
Now his code makes sense to me.

By the way..April 4th is my Judgement day..how about you.

I am sure you will do great, you have come a long way since the beginning of the journey.
I haven't set up a date yet myself, but it will be soon.

[Himalay Majumdar]

Thank you so much Ruben. Will keep you posted.

[Robert O'Leary]

Himalay Majumdar wrote:

Robert O'Leary wrote:
Cool...and if you were using them as a method parameters, only List&lt;? extends Object&gt; can accept an argument other than List&lt;Object&gt;e.g as in Line 5

Not true.. List&lt;? extends Object&gt; can accept ANY argument..including..List&lt;Object&gt;

public class Ranch {

	public static void main(String[] args) {

		List<Object> a = new ArrayList<Object>(); 
		methoda(a);
	}

	public static void methoda(List<? extends Object> l){
			
	}
}

Check out this link for more practice

yep that was i saying, i could have been clearer though! thanks for the discussion gents