Question about array of parameterized class
(Yes, another one, sorry.)
I want to construct an instance for this array:
Map<Foo,Bar> mapArray[];
This works:
mapArray = (HashMap<Foo,Bar>[])new Map[size];
but these don't work (Eclipse tells me "the cast is checking against the erased type HashMap[]"):
mapArray = (Map<Foo,Bar>[])new Map[size];
mapArray = (HashMap<Foo,Bar>[])new HashMap[size];
I'd just like to know: Why can I cast an array of an abstract type (Map) to a concrete Type (HashMap), but not cast Map to Map, or HashMap to HashMap?
This is totally counter-intuitive. It seems like it should be exactly the other way round.
# 1
I created an example file and compiled it from command line:import java.util.Map;
import java.util.HashMap;
class Test{
void test(){
int size = 10;
Map<Foo,Bar> mapArray[];
mapArray = (HashMap<Foo,Bar>[])new Map[size];
}
class Foo{}
class Bar{}
}
javac Test.java
The Test class was compiled without any errors.
I use java version "1.6.0-rc"
Which version of jdk do you use?
# 2
I am sorry. This one also compiled without any errors:import java.util.Map;
import java.util.HashMap;
class Test{
void test(){
int size = 10;
Map<Foo,Bar> mapArray[];
mapArray = (HashMap<Foo,Bar>[])new Map[size];
mapArray = (Map<Foo,Bar>[])new Map[size];
mapArray = (HashMap<Foo,Bar>[])new HashMap[size];
}
class Foo{}
class Bar{}
}
# 3
I'm using JDK 6, but I think Eclipse uses its own compiler.
Anyway, actually the code I posted did NOT work. It compiled, but at runtime Java couldn't cast Map to HashMap - just the way it should be.
Actually, to make it work I had to use the
(HashMap<Foo,Bar>[])new HashMap[size]
form, but Eclipse made be annotate the method "unchecked", in order to not get that stupid warning (that serves no purpose as far as I can see).
By the way: why do I have to annotate the whole method in Java? Now it might happen that by adding some code to the method I introduce something that's worth warning about, but because of the annotation I don't get the warning. Bad Java. No dessert. ;-)
# 4
First, you can annotate a local variable
public static void main(String[] args) {
@SuppressWarnings("unchecked") Map<String,String>[] array=
(Map<String,String>[])new HashMap<?,?>[3];
}
else i've recently discussed with Alex Buckley and Peter ah?br>on a way to modify the JLS in order to allow array of parametrized type creation.
i hope it will be integrated in jdk7.
R閙i
