Confused by mixed generic type

[Faber Siagian]

Dear all,

I have a small program like shown below :

public class Basket<E> 
{
private E element;

public void setElement(E x) 
{
   element = x;
}

public E getElement() 
{
return element;        
}

public <A extends Apple> void insertRipe(A a, Basket<? extends A> basket) //Line 15
{
basket.setElement(a); //line 17
}
}
 
class Fruit 
{
 
}
 
class Apple extends Fruit 
{
 
}

Compilation failed at line 15 :

setElement(capture of ? extends A) in Basket<capture of ? extends A> cannot be applied to (A)

But if i change it to be

public <A extends Apple> void insertRipe(A a, Basket<? super A> basket)

the compilation succeeded.
Someone explain it to me, please.

[Keith Nagle]

At compile time, you cannot add to a collection that has
<? extends ...> for the simple reason that you could be adding something that could upset your collection.
Just remember that if you want to add something to a collection, the following are legitimate:

  public void addIt(List<Animal> l) {
   //can add anything that is an animal
   l.add(new Cat());
   l.add(new Dog());
}  
 
public void addAnother(List<? super Dog> l) {
   //here we can add anything that is a dog or
   //above in the inheritance hierarchy.
   l.add(new Dog());
   l.add(new Animal());
   l.add(new Object());
}
 
public void notGoingToAdd(List<? extends Dog> l) {
   //in a nutshell, you cannot add to the list l here.
   //In essence you are telling the compiler, Im not
   //going to add anything to this list.
   for(Dog d : l) 
      d.bark();
}

Best regards.
[ July 11, 2008: Message edited by: Keith Nagle ]

[Ralph Jaus]

At compile time, you cannot add to a collection that has <? extends ...>

Note: There is one exception: You can always add "null". For example the following will compile fine and print 1:

List<? extends Number> l = new ArrayList<Number>(); 
l.add(null); 
System.out.println(l.size());

[ July 11, 2008: Message edited by: Ralph Jaus ]

[Madhukar Ojha]

Hi Faber

It seems that you have a lot of doubts in generics.
Please go through k&B Head First Java and SCJP 5 exam guide.

When you use
void show(SomeType<? extends SomeClassOrInterface> a)
you can not put any data in the object referenced by a .

But

When you use
void show(SomeType<? super SomeClassOrInterface> a)
you can put any object which is of type SomeClassOrInterface or its
sub ype in the object referenced by a.

I hope this will help you.

[Karl Prenton]

Originally posted by Keith Nagle:
At compile time, you cannot add to a collection that has
<? extends ...> for the simple reason that you could be adding something that could upset your collection.
Just remember that if you want to add something to a collection, the following are legitimate:

  public void addIt(List<Animal> l) {
   //can add anything that is an animal
   l.add(new Cat());
   l.add(new Dog());
}  
 
public void addAnother(List<? super Dog> l) {
   //here we can add anything that is a dog or
   //above in the inheritance hierarchy.
   l.add(new Dog());
   l.add(new Animal());
   l.add(new Object());
}
 
public void notGoingToAdd(List<? extends Dog> l) {
   //in a nutshell, you cannot add to the list l here.
   //In essence you are telling the compiler, Im not
   //going to add anything to this list.
   for(Dog d : l) 
      d.bark();
}

Best regards.

[ July 11, 2008: Message edited by: Keith Nagle ]

I quickly tried this out and compiler was complaining:
$ javac GenericsPractice.java
GenericsPractice.java:25: cannot find symbol
symbol : method add(Animal)
location: interface java.util.List<capture of ? super Dog>
l.add(new Animal());
^
GenericsPractice.java:26: cannot find symbol
symbol : method add(java.lang.Object)
location: interface java.util.List<capture of ? super Dog>
l.add(new Object());
^
2 errors


I don't understand why? The code made perfect sense to me!
Any else have this bizarre problem??
It's as if the compiler cannot see "? super" ... this has got me worried now!
I'm using jdk 1.5.0_10
[ July 11, 2008: Message edited by: Frank Zito ]

[Raphael Rabadan]

Originally posted by Frank Zito:


I quickly tried this out and compiler was complaining:
$ javac GenericsPractice.java
GenericsPractice.java:25: cannot find symbol
symbol : method add(Animal)
location: interface java.util.List<capture of ? super Dog>
l.add(new Animal());
^
GenericsPractice.java:26: cannot find symbol
symbol : method add(java.lang.Object)
location: interface java.util.List<capture of ? super Dog>
l.add(new Object());
^
2 errors


I don't understand why? The code made perfect sense to me!
Any else have this bizarre problem??
It's as if the compiler cannot see "? super" ... this has got me worried now!
I'm using jdk 1.5.0_10

[ July 11, 2008: Message edited by: Frank Zito ]

I'm with you. Made sense to me and I'm not understanding it now.

EDIT: I was making some tests and trying to figure out what's was happening and then a discovered.

When we use a List<? super Xxxx> as a parameter of a method the only type allowed to be add to the List is the Xxxxx. In the example where was used Dog we can ONLY ADD DOGS.
[ July 11, 2008: Message edited by: Raphael Rabadan ]

[Denis Bogdanov]

Hallo Raphael,

I think it is an inaccuracy in explanation. Suppose we have a method
<blockquote>code:

---

<pre name="code" class="java"> public void addItem(List<? super Dog> list) {
list.add(new Dog());
}</pre>

---

</blockquote> and Dog extends Animal & Animal extends Object

This means that you can pass to the method lists of type Object, Animal and Dog and the new object of the class Dog can be added to passed lists.

You can not invoke list.add(new Animal()) because, for example, for the list of Dog the cast is needed.

[Raphael Rabadan]

Hello, Dennis.

You said what i've said :-)
He can only add Dog's in the example, you just give the explanation for it :-)

I didn't said I could pass only a List of Dogs :-)

[Karl Prenton]

I guess the code was a little misleading; probably the type of code to find in an exam!

obviously, code compiles for the more general case:
public void addAnother(List<? super Object> l)

which is equivalent as non-generic :
public void addAnother(List l)

[Raphael Rabadan]

Originally posted by Frank Zito:
I guess the code was a little misleading; probably the type of code to find in an exam!

obviously, code compiles for the more general case:
public void addAnother(List<? super Object> l)

which is equivalent as non-generic :
public void addAnother(List l)

The equivalent to:

<blockquote>code:

---

<pre name="code" class="java">public void addAnother(List l)</pre>

---

</blockquote>

Wouldn't be:

<blockquote>code:

---

<pre name="code" class="java">public void addAnother(List<? extends Object> l)</pre>

---

</blockquote>

??

[ July 11, 2008: Message edited by: Raphael Rabadan ]
[ July 11, 2008: Message edited by: Raphael Rabadan ]

[Denis Bogdanov]

Originally posted by Raphael Rabadan:

You said what i've said :-)
He can only add Dog's in the example, you just give the explanation for it :-)

Hallo Raphael,
You are right. This was an aim of my post. You are discovered WHATand i have tried to explain WHY

[Karl Prenton]

couple of useful links that may help explain my earlier statement:

http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeArguments.html#Which%20types%20are%20permitted%20as%20wildcard%20bounds?

https://coderanch.com/t/385198/java/java/upper-bound-wildcard-readonly-but

[Faber Siagian]

Oooops, it is more confusing now.
Yes i know that we can't add any element into a collection, but in the code, nothing is added into basket. I just set basket's element.

Or there's something else that i missed about collection?

[Raphael Rabadan]

Originally posted by Faber Siagian:
Oooops, it is more confusing now.
Yes i know that we can't add any element into a collection, but in the code, nothing is added into basket. I just set basket's element.

Or there's something else that i missed about collection?

You can't add, or change it. You could remove.

Let's think about it. Having a List<? super Dog> as a parameter of a method considering this hierarchy:

class Animal{ }
class Dog extends Animal{ }

you could pass a List<Dog>, a List<Animal> and a List<Object>.
What would happen if you pass a List<Dog> considering this method:

static void addObject(List<? super Dog) list){
  list.add(new Object());
}

Would it be possible? An Object for a List of Dogs?? Neither could be possible An Animal, because it could be a Cat, or anything else.

Thats why you can't use list.set(0,new Object()) too. Because you would be setting an Object to a list that could be of Dogs

I hope you understand now.

Raphael Rabadan
[ July 12, 2008: Message edited by: Raphael Rabadan ]