無法確定 E 的類型參數；對於上限爲 int,java.lang.Object 的類型變量 E，不存在唯一最大實例

在Eclipse中能夠編譯運行，同樣的代碼，使用ant編譯會失敗報錯：無法確定 <E>E 的類型參數；對於上限爲 int,java.lang.Object 的類型變量 E，不存在唯一最大實例

通過在網上查找資料，有提到eclipse中是用到JDT編譯而ant javac使用JDK，找到了幾種解決方案：

1.配置使用JDK編譯[0]。

2.通過加強制類型轉換，可以臨時繞過該問題，參考[1][2]。

3.[2][3]同時指出這是JDK1.6版本的問題，[2]中指出在升級到jdk1.6.0_38版本後，測試打包就沒有問題，而我這邊沒有成功，根據[3]中參考的stackoverflow鏈接中有人指出這是JDK的一個bug，在JDK1.7中被修復了[4]。我使用JDK1.7後問題沒有出現。

bug_id=6302954

JDK-6302954 : Inference fails for type variable return constraint

Details

Type:	Bug	Submit Date:	2005-07-28
Status:	Closed	Updated Date:	2011-03-08
Project Name:	JDK	Resolved Date:	2011-03-08
Component:	tools	OS:	generic,windows_xp,windows_2000
Sub-Component:	javac	CPU:	x86,generic
Priority:	P3
Resolution:	Fixed
Affected Versions:	5.0,6,6u20
Fixed Versions:	7 (b03)

Related Reports

Backport:	JDK-2204263 - Inference fails for type variable return constraint
Backport:	JDK-2171748 - Inference fails for type variable return constraint
Backport:	JDK-2204974 - Inference fails for type variable return constraint
Duplicate:	JDK-6456971 - Varargs and inference problem
Duplicate:	JDK-6476073 - Capture using super wildcard of type variables doesn't work
Relates:	JDK-6794295 - javac does not compile correct source code with generics.

Sub Tasks

Description

https://bugs.eclipse.org/bugs/show_bug.cgi?id=98379

This program doesn't compile:

public class X {
    <T extends X> T f1() throws Exception{
    	return null;
    }
    <U extends X> U f2() throws Exception {
        return f1();
    }
}

X.java:6: type parameters of <T>T cannot be determined; no unique maximal instance exists for type variable T with upper bounds U,X
        return f1();
                 ^
1 error

                                    

Comments

SUGGESTED FIX

Index: j2se/src/share/classes/com/sun/tools/javac/comp/Infer.java
--- /tmp/geta16053	2006-10-18 23:36:54.000000000 -0700
+++ /tmp/getb16053	2006-10-18 23:36:54.000000000 -0700
@@ -140,7 +140,7 @@
 		     bs.nonEmpty() && that.inst == null;
 		     bs = bs.tail) {
 		    // System.out.println("hibounds = " + that.hibounds);//DEBUG
-		    if (isSubClass(bs.head, that.hibounds, types))
+		    if (isSubClass(bs.head, that.hibounds))
 			that.inst = types.fromUnknownFun.apply(bs.head);
 		}
 		if (that.inst == null || !types.isSubtypeUnchecked(that.inst, that.hibounds, warn))
@@ -151,19 +151,25 @@
 	}
     }
     //where
-	private boolean isSubClass(Type that, List<Type> those, Types types) {
-	    Type that1 = that.baseType();
-	    boolean isSub = true;
-	    if (that1.tag == TYPEVAR) {
-		for (List<Type> l = types.getBounds(((TypeVar)that1)); isSub && l.nonEmpty(); l = l.tail) {
-		    isSub = isSubClass(l.head, those, types);
+        private boolean isSubClass(Type t, final List<Type> ts) {
+            t = t.baseType();
+            if (t.tag == TYPEVAR) {
+                List<Type> bounds = types.getBounds((TypeVar)t);
+                for (Type s : ts) {
+                    if (!types.isSameType(t, s.baseType())) {
+                        for (Type bound : bounds) {
+                            if (!isSubClass(bound, List.of(s.baseType())))
+                                return false;
+                        }
+                    }
 		}
 	    } else {
-		for (List<Type> l = those; isSub && l.nonEmpty(); l = l.tail) {
-		    isSub = that1.tsym.isSubClass(l.head.baseType().tsym, types);
+                for (Type s : ts) {
+                    if (!t.tsym.isSubClass(s.baseType().tsym, types))
+                        return false;
 		}
 	    }
-	    return isSub;
+            return true;
 	}
 
     /** Instaniate undetermined type variable to the lub of all its lower bounds.
@@ -192,7 +198,7 @@
 	    else for (List<Type> bs = that.hibounds;
 		      bs.nonEmpty() && hb == null;
 		      bs = bs.tail) {
-		if (isSubClass(bs.head, that.hibounds, types))
+		if (isSubClass(bs.head, that.hibounds))
 		    hb = types.fromUnknownFun.apply(bs.head);
 	    }
 	    if (hb == null ||
                                     

2006-10-19

SUGGESTED FIX

Webrev of changes: http://sa.sfbay/projects/langtools/bugid_summary.pl?bugid=6302954
See also attachment 6302954.tar.gz.
                                     

2006-10-19

EVALUATION

A bug.  The compiler should infer U.