使用带有Type参数的通配符需要额外的转换

Question

I have created two classes , Parent and child . My Parent class is implementing Comparable interface as "class Parent implements Comparable"

I have also written a method which returns the maximum element of a list , here i have ? super T wild card to accept any Child class of Parent .

The following code works fine on a list of Child Objects .

public static <T extends Comparable<? super T>> T max(List <T> list){
    Iterator< T> itr = list.iterator();   // line xx
    T result = itr.next();
    T temp ;
    while(itr.hasNext()){
        temp = itr.next();
        if( temp.compareTo(result) > 0)
            result = temp ;
    }
    return result ;
}

But if i change the proto type of max method to accept any List of SubType of T by using < ? extends T > then the above code does not compile .

Compliation Error is : cannot convert from Iterator to Iterator

on changing line xx to " Iterator< T> itr = (Iterator) list.iterator();" the code works with < ? extends T > argument .

public static <T extends Comparable<? super T>> T max(List <? extends T> list){
    Iterator< T> itr = (Iterator<T>) list.iterator(); // works fine
    Iterator< T> itr1 =  list.iterator();            // generates compile error .
    T result = itr.next();
    T temp ;
    while(itr.hasNext()){
        temp = itr.next();
        if( temp.compareTo(result) > 0)
            result = temp ;
    }
    return result ;
}

I am not getting it that why i compile time error is thrown .

Complete Code follows as :

package Generics;

import java.util.*;

public class WildCard {
public static void main(String[] args) {
    List<Child> cList = Arrays.asList(new Child("c1"),new Child("c3"),new Child("c2"));
    System.out.println(max(cList));
}

public static <T extends Comparable<? super T>> T max(List <? extends T> list){
    Iterator< T> itr = (Iterator<T>) list.iterator();
    T result = itr.next();
    T temp ;
    while(itr.hasNext()){
        temp = itr.next();
        if( temp.compareTo(result) > 0)
            result = temp ;
    }
    return result ;
}
}
class Parent implements Comparable<Parent>{
    private String val ;
    public Parent(String val){
        this.val = val ;
    }
    @Override
    public int compareTo(Parent o) {
        // TODO Auto-generated method stub
        return this.val.compareTo(o.val);
    }
    public String toString(){
        return val ;
    }
}
class Child extends Parent{

    public Child(String val) {
        super(val);
    }

}

My doubt is even when i was not using ? extends T in function argument , i was able to accept the subtypes and i did not need any casting . But why on changing that syntax i need type casting ?

Answer 1

Since you have a List<? extends T> List<? extends T> , you should use an Iterator<? extends T> Iterator<? extends T> :

Iterator<? extends T> itr = list.iterator();

And Iterator is a producer of T anyway, so according to the PECS rule (Producer extends , Consumer super ), so it's natural that Iterator should with extends anyway.