It depends upon how you wish to call these methods. If you wish to call these methods from other Java source code, then it is considered invalid for reasons illustrated in Edwin's answer. This is a limitation of the Java Language.
However, not all classes need to be generated from Java source code (consider all the languages that use the JVM as their runtime: JRuby, Jython, etc...). At the bytecode level, the JVM can disambiguate the two methods because the bytecode instructions specify the return type they are expecting. For example, here is a class written in Jasmin that can call either of these methods:
.class public CallAmbiguousMethod
.super java/lang/Object
.method public static main([Ljava/lang/String;)V
.limit stack 3
.limit locals 1
; Call the method that returns String
aconst_null
invokestatic TestWillThatCompile/f(Ljava/util/List;)Ljava/lang/String;
; Call the method that returns Integer
aconst_null
invokestatic TestWillThatCompile/f(Ljava/util/List;)Ljava/lang/Integer;
return
.end method
I compile it to a class file using the following command:
java -jar jasmin.jar CallAmbiguousMethod.j
And call it using:
java CallAmbiguousMethod
Behold, the output is:
> java CallAmbiguousMethod
strings
numbers
Update
Simon posted an example program that calls these methods:
import java.util.Arrays;
import java.util.List;
class RealyCompilesAndRunsFine {
public static String f(List<String> list) {
return list.get(0);
}
public static Integer f(List<Integer> list) {
return list.get(0);
}
public static void main(String[] args) {
final String string = f(Arrays.asList("asdf"));
final Integer integer = f(Arrays.asList(123));
System.out.println(string);
System.out.println(integer);
}
}
Here is the Java bytecode generated:
>javap -c RealyCompilesAndRunsFine
Compiled from "RealyCompilesAndRunsFine.java"
class RealyCompilesAndRunsFine extends java.lang.Object{
RealyCompilesAndRunsFine();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static java.lang.String f(java.util.List);
Code:
0: aload_0
1: iconst_0
2: invokeinterface #2, 2; //InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
7: checkcast #3; //class java/lang/String
10: areturn
public static java.lang.Integer f(java.util.List);
Code:
0: aload_0
1: iconst_0
2: invokeinterface #2, 2; //InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
7: checkcast #4; //class java/lang/Integer
10: areturn
public static void main(java.lang.String[]);
Code:
0: iconst_1
1: anewarray #3; //class java/lang/String
4: dup
5: iconst_0
6: ldc #5; //String asdf
8: aastore
9: invokestatic #6; //Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
12: invokestatic #7; //Method f:(Ljava/util/List;)Ljava/lang/String;
15: astore_1
16: iconst_1
17: anewarray #4; //class java/lang/Integer
20: dup
21: iconst_0
22: bipush 123
24: invokestatic #8; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
27: aastore
28: invokestatic #6; //Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
31: invokestatic #9; //Method f:(Ljava/util/List;)Ljava/lang/Integer;
34: astore_2
35: getstatic #10; //Field java/lang/System.out:Ljava/io/PrintStream;
38: aload_1
39: invokevirtual #11; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
42: getstatic #10; //Field java/lang/System.out:Ljava/io/PrintStream;
45: aload_2
46: invokevirtual #12; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V
49: return
It turns out the Sun compiler is generating the bytecode necessary to disambiguate the methods (see instructions 12 and 31 in the last method).
Update #2
The Java Language Specification suggests that this may, in fact, be valid Java source code. On page 449 (§15.12 Method Invocation Expressions) we see this:
It is possible that no method is the most specific, because there are two or
more methods that are maximally specific. In this case:
- If all the maximally specific methods have override-equivalent (§8.4.2) signatures,
then:
- If exactly one of the maximally specific methods is not declared abstract,
it is the most specific method.
- Otherwise, if all the maximally specific methods are declared abstract,
and the signatures of all of the maximally specific methods have the same
erasure (§4.6), then the most specific method is chosen arbitrarily among
the subset of the maximally specific methods that have the most specific
return type. However, the most specific method is considered to throw a
checked exception if and only if that exception or its erasure is declared in
the throws clauses of each of the maximally specific methods.
- Otherwise, we say that the method invocation is ambiguous, and a compiletime
error occurs.
Unless I am mistaken, this behavior should only apply to methods declared as abstract, though...
Update #3
Thanks to ILMTitan's comment:
@Adam Paynter: Your bolded text does
not matter, because it is only a case
when two methods are
override-equivalent, which Dan showed
was not the case. Thus, the
determining factor must be if the JLS
takes generic types into account when
determining most specific method. –
ILMTitan