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% cat Test.java import java.util.Collection; import java.util.List; import java.util.ArrayList; public class Test { public static <T> List<T> createEmptyL() { return new ArrayList<T>(); } public static <T> Collection<T> createEmptyC() { return new ArrayList<T>(); } public static <T> Iterable<T> createEmptyI() { return new ArrayList<T>(); } public static void main(String[] args) { final List<String> lL = createEmptyL(); final Collection<String> cL = (Collection<String>)createEmptyL(); final Iterable<String> iL = (Iterable<String>)createEmptyL(); final Collection<String> cC = createEmptyC(); final Iterable<String> iC = (Iterable<String>)createEmptyC(); final Iterable<String> iI = createEmptyI(); final Collection<String> cL2 = createEmptyL(); final Iterable<String> iC2 = createEmptyC(); } } Eclipse gives spurious errors for lines 24 & 25. Severity Description Resource In Folder Location Creation Time 2 Type mismatch: cannot convert from List<T> to Collection<String> Test.java Fooey/src line 24 November 9, 2004 4:38:49 PM 2 Type mismatch: cannot convert from Collection<T> to Iterable<String> Test.java Fooey/src line 25 November 9, 2004 4:38:49 PM 1 Type safety: The cast from List<T> to Collection<String> will not check conformance of type arguments at runtime Test.java Fooey/src line 18 November 9, 2004 4:38:49 PM 1 Type safety: The cast from List<T> to Iterable<String> will not check conformance of type arguments at runtime Test.java Fooey/src line 19 November 9, 2004 4:38:49 PM 1 Type safety: The cast from Collection<T> to Iterable<String> will not check conformance of type arguments at runtime Test.java Fooey/src line 21 November 9, 2004 4:38:49 PM Javac also gets confused and gives spurious errors for lines 18,19,21. c:/Eclipse/Fooey/src/Test.java:18: inconvertible types found : java.util.List<java.lang.Object> required: java.util.Collection<java.lang.String> final Collection<String> cL = (Collection<String>)createEmptyL(); ^ c:/Eclipse/Fooey/src/Test.java:19: inconvertible types found : java.util.List<java.lang.Object> required: java.lang.Iterable<java.lang.String> final Iterable<String> iL = (Iterable<String>)createEmptyL(); ^ c:/Eclipse/Fooey/src/Test.java:21: inconvertible types found : java.util.Collection<java.lang.Object> required: java.lang.Iterable<java.lang.String> final Iterable<String> iC = (Iterable<String>)createEmptyC(); ^ 3 errors
Inference support did not consider downcast scenarii when collecting possible substitutes. Once fixed, only following warnings are issued: ---------- 1. WARNING in d:\src\Test.java (at line 18) final Collection<String> cL = (Collection<String>)createEmptyL(); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Unnecessary cast from List<String> to Collection<String> ---------- ---------- 2. WARNING in d:\src\Test.java (at line 19) final Iterable<String> iL = (Iterable<String>)createEmptyL(); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Unnecessary cast from List<String> to Iterable<String> ---------- ---------- 3. WARNING in d:\src\Test.java (at line 21) final Iterable<String> iC = (Iterable<String>)createEmptyC(); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Unnecessary cast from Collection<String> to Iterable<String> ----------
Added regression test: GenericTypeTest#test396. Fixed
Verified in 200412140800
Actually, new implementation of the spec 15.12.2.7 behave differently, and find now errors on 3 of the cases: cL, iL, iC. Note that javac also detects the same errors: X.java:22: inconvertible types found : java.util.List<java.lang.Object> required: java.util.Collection<java.lang.String> final Collection<String> cL = (Collection<String>)emptyList(); // 2 ^ X.java:25: inconvertible types found : java.util.List<java.lang.Object> required: java.lang.Iterable<java.lang.String> final Iterable<String> iL = (Iterable<String>)emptyList(); // 3 ^ X.java:31: inconvertible types found : java.util.Collection<java.lang.Object> required: java.lang.Iterable<java.lang.String> final Iterable<String> iC = (Iterable<String>)emptyCollection(); // 5 ^ 3 errors
Ignore previous post. We compile this scenario with no error, even with new inference code in place.
Looks like we should reject cases (2), (3) & (5) based on the fact that return type inference is not meant to perform through casting conversion. Only assignment conversion is to be considered. Also see bug 97800
