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POProblem with unbounded Java Generic interface
primarykey
Id
7118444
data
AcceptedAnswerId
7118648
AnswerCount
1
ClosedDate
CommentCount
6
CommunityOwnedDate
CreationDate
2011-08-19T07:46:54.637
FavoriteCount
2
LastActivityDate
2011-08-22T04:24:05.970
LastEditDate
2011-08-22T04:24:05.970
LastEditorUserId
364569
OwnerUserId
364569
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0
PostTypeId
1
Score
3
ViewCount
304
LastEditorDisplayName
text
Body
Consider the following simple code <pre><code>import java.util.*; public class MainTest<T extends Object1<?,?>> { List<T> list; public MainTest(List<T> l) { this.list=l; } public int testCompare() { // fails to compile here return list.get(0).compareTo(list.get(1)); } public static void main(String[]args) { List<Object1Impl> list = new ArrayList<Object1Impl>(); list.add(new Object1Impl()); list.add(new Object1Impl()); MainTest<Object1Impl> test = new MainTest<Object1Impl>(list); System.out.println(test.testCompare()); } } interface Object1<E, V> extends Comparable<Object1<E,V>> { } class Object1Impl implements Object1<Integer, Integer>{ public int compareTo(Object1<Integer, Integer> o) { return 0; } } </code></pre> I am aware that in this case the program will not compile (fails at testCompare() because T is extending unbounded <code>Object1<?,?></code>). Is there any alternative to fix this besides making <code>MainTest<T extends Object1<E,V>,E,V></code>? EDIT: the error message is <pre><code>The method compareTo(Object1<capture#1-of ?,capture#2-of ?>) in the type Comparable<Object1<capture#1-of ?,capture#2-of ?>> is not applicable for the arguments (T) </code></pre> I have read Effective Java book but still can't really think of a solution.. Also, why is it that if I change interface Object1 into an abstract class the program will compile without any problem? This really puzzles me... EDIT: when I mean changing into abstract class is as follows <pre><code>abstract class Object1<E, V> implements Comparable<Object1<E,V>>{ public int compareTo(Object1<E,V> o) { return 0; } } class Object1Impl extends Object1<Integer, Integer>{ } </code></pre> this will work (only if using Eclipse, compiling it manually using javac does not work) but I have no idea why
Tags
<java><generics><interface><abstract>
Title
Problem with unbounded Java Generic interface
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