Set String keySet = props.keySet() not compiled, can someone explain why?

Question

I just ran into a problem with javac, this code compiles fine in jdeveloper 10.1.3 but doesn't compile with the javac task in Ant 1.6.5
Compiling these 2 lines
Properties props = new Properties();
Set<String> keySet2 = props.keySet();
results in this error.
incompatible types
[javac] found : java.util.Set<java.lang.Object>
[javac] required: java.util.Set<java.lang.String>
[javac] Set<String> keySet2 = props.keySet();
Given the assignment context, why can't the compiler infer that the Set returns by keySet is a Set<String>?
However, casting p to a Map before calling keySet() compiles fine.
Properties props = new Properties();
Set<String> keySet2 = ((Map)props).keySet();
This is really weird. Could it be just a bug in the javac implementation that comes with Ant? I'm wondering if there's some fundamental about generics that I'm missing. Can anyone shed some light on this?
Edited by: 982572 on Jan 17, 2013 12:25 PM

Answer 1

Ah, thanks. I think that answers some of what I didn't get. The part about discarding the generic information makes sense.
There's something else I'm not understanding.
Consider this line of code
Set<String> keySet2 = (Set<String>)props.keySet();
JDeveloper 10.1.3.3. for some reason is able to compile this code (it's using jdk 1.5.0_06).
I tried the same code in Eclipse with jdk 1.6.0_26 and it doesn't compile.
I believe the latter is correct. Set<String> and Set<Object> aren't related so the compiler shouldn't have allowed a cast from one to the other ... but somehow the code is compiling in JDeveloper 10.1.3.3. This is really throwing me off ...
What's also really weird is that JDeveloper can't compile this line
Set<String> keySet2 = props.keySet();
but to me this is pretty much identical to
Set<String> keySet2 = (Set<String>)props.keySet(); // although it's an explicit cast
and JDeveloper can compile this fine ...
So anyway, if props.keySet() returns Set<Object> and at compile time the compiler doesn't allow casting to Set<String>, how would I go about putting everything in the Set<Object> returned by keySet() into a Set<String>? I know I can iterate through the Set<Object> returned and then just add each String to Set<String>.
This would work,
Set<? super String> keySet = props.keySet();
but then it's not really accurate ... even though Object is the only ancestor of the String class, the <? super String> bound suggests that one can put something other than String into keySet and operate on the elements as though they are String, which is not the correct message to convey.
I wonder why Properties doesn't extend Hashtable<String,String> ... but there's probably a reason and given that it extends Hashtable<Object, Object> instead, is there no way to assign the return value of Properties.keySet() to a Set<String> by using some Java generic notation/syntax?
I know this would work as well, but that's just discarding the generic information in the first place
Set<String> keySet = (Set<String>)(Set)props.keySet();
I suppose going back to the iterator is the only way to basically copy props.keySet() into a Set<String>?
Edited by: 982572 on Jan 17, 2013 1:56 PM
Edited by: 982572 on Jan 17, 2013 2:19 PM
Edited by: 982572 on Jan 17, 2013 2:37 PM
Edited by: 982572 on Jan 17, 2013 2:38 PM
Edited by: 982572 on Jan 17, 2013 2:42 PM