Couple of Generics questions

[UAF]

public class Try
{
    public static void main(String[] prms)
    {
        ArrayList<ArrayList<A>> lla = new ArrayList<ArrayList<A>>();        
        ArrayList<ArrayList<B>> llb = new ArrayList<ArrayList<B>>();
        
        ArrayList<A> la = new ArrayList<A>();        
        ArrayList<B> lb = new ArrayList<B>();
                
        
        stam(lla);        //compiler error
        stam(llb);        //compiler error
        
        stam2(lla);        
        stam2(llb);
                
        stam3(lla);        
        stam3(llb);
        
        stam4(la);        
        stam4(lb);
        
        stam5(la);        
        stam5(lb);
    }

    static void stam(ArrayList<ArrayList<? extends A>> list){}
    
    static void stam2(ArrayList<? extends ArrayList<? extends A>> list){}
    
    static <T extends A> void stam3(ArrayList<ArrayList<T>> list){}
    
    static <T extends A> void stam4(ArrayList<T> list){}
    
    static void stam5(ArrayList<? extends A> list){}
}

class A
{
    
}

class B extends A
{
    
}

=======================================

In the code above, the calls to stam() don't work.

My main question is why the calls to Stam() don't work, while the calls to stam2() just below it do work.


Also, if someone could clarify the difference between using wildcards and using T I'll be greatful.

[toadaly]

This seems to me like a compiler bug to me.

[r035198x]

Wildcards (The Java™ Tutorials > Learning the Java Language > Generics)

[UAF]

I read it, I still don't get it.
Could you please try to exlpain in your own words.

Why is

ArrayList<ArrayList<? extends A>>

not good, and

ArrayList<? extends ArrayList<? extends A>

good?

I'm not trying to send something that extends ArrayList, I as sending one.