Generics problem

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Le 09/07/2010 15:37, Ulf Zibis a écrit :
> See following code:
>
>
> /**
>  * @author Ulf Zibis, Cologne CoSoCo.de
>  */
> public class GenericsProblem {
>
>     private static abstract class Entry<V,E extends Entry<V,E>> 
> implements Comparable<E> {
>         V value;
>         int count = 0;
>
>         Entry(V value) {
>             this.value = value;
>         }
>
>         public int compareTo(E other) {
>             return count != other.count ? count > other.count ? -1 : 1 
> : 0;
>         }
>     }
>
>     private static class Word extends Entry<String,Word> {
>
>         private Word(String str) {
>             super(str);
>         }
>     }
>
>     private static abstract class Phrase<P extends Phrase<P>> extends 
> Entry<int[],P> {
>         int prefixLength;
>
>         Phrase(int[] codes, int prefixLength) {
>             super(codes);
>             this.prefixLength = prefixLength;
>         }
>
> // <P extends Phrase<P>> int matchDepth(P other) {
>         int matchDepth(Phrase<? extends Phrase> other) {
>             for (int i=0, l=Math.min(value.length, 
> other.value.length); ; )
>                 if (value[i] != other.value[i] || ++i == l)
>                     return i;
>         }
>     }
>
>     private static class UName extends Phrase<UName> {
>         private final int cp;
>
>         UName(int cp, String name) {
>             super(wordCodes(name), 1);
>             count = 1;
>             this.cp = cp;
>         }
>
>         private static int[] wordCodes(String name) {
>             String[] words = name.split("[ \\\\-]");
>             int [] codes = new int[words.length];
>             for (int i=0; i<words.length; i++)
>                 codes[i] = words[i].length();
>             return codes;
>         }
>
>         public int compareTo(UName other) {
>             int m = matchDepth(other), l1, l2, c1, c2;
>             return m == Math.min(l1 = value.length, l2 = 
> other.value.length) && l1 != l2 ? l1 < l2 ? -1 : 1 : // (3b) OK!
>                 (c1=value[m]) != (c2=other.value[m]) ? c1 < c2 ? -1 : 
> 1 : 0;
>         }
>     }
>
>     static <V,E extends Entry<V,E>> int sizeOf1(E e) { // number of 
> effective elements
>         if (e instanceof Word) // compile error ???
>             return ((Word)e).value.length();
>         else
>             return ((Phrase<? extends Phrase>)e).value.length - 
> ((Phrase<? extends Phrase>)e).prefixLength + 1;
> //        return e instanceof Word ? ((Word)e).value.length() :
> //                ((Phrase<? extends Phrase>)e).value.length - 
> ((Phrase<? extends Phrase>)e).prefixLength + 1;
>     }
> }
>
>
> I get following error:
>
> C:\Projects\Java_Bugs\JDK-7\...\GenericsProblem.java:69: inconvertible 
> types
>         if (e instanceof Word) // compile error ???
>   required: Word
>   found:    E
>   where E,V are type-variables:
>     E extends Entry<V,E> declared in method <V,E>sizeOf1(E)
>     V extends Object declared in method <V,E>sizeOf1(E)
> C:\Projects\Java_Bugs\JDK-7\...\GenericsProblem.java:70: inconvertible 
> types
> C:\Projects\Java_Bugs\JDK-7\...\GenericsProblem.java:70: inconvertible 
> types
>             return ((Word)e).value.length();
>   required: Word
>   found:    E
>   where E,V are type-variables:
>     E extends Entry<V,E> declared in method <V,E>sizeOf1(E)
>     V extends Object declared in method <V,E>sizeOf1(E)
> 2 errors
>
>
> I don't understand this because:
>     E extends Entry<V,E>
> and
>     Word extends Entry<String,Word>
>
> Any Ideas?

<V,E extends Entry<V,E>> int sizeOf1(E e) {
means for all V and E ...
clearly e instanceof Word is not valid for all E,
by example, this is invalid if E is a Phrase.

I you declare siseOf1 like this
static int sizeOf1(Entry<?,?> e)
it should be ok.


>
> -Ulf

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