Re: java generics bug?

From:

robertjparks@gmail.com

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 07 Aug 2007 17:51:06 -0000

Message-ID:

<1186509066.855721.14650@d30g2000prg.googlegroups.com>

On Aug 7, 12:02 pm, Daniel Pitts <googlegrou...@coloraura.com> wrote:

On Aug 7, 8:48 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

I think what you want to do is:

public static <T> void printOneDimension(Map<String,T> map){

Hope this helps.

Daniel.

Excellent! This worked. I guess I need to learn more about this T
notation. For anyone following the thread here is the final solution:

public static void printOneDimension(Map<String,?> map){
        for(String k1:map.keySet())System.out.println(k1);
    }
public static <T> void
printTwoDimensionsNew(Map<String,Map<String,T>> 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",m1);
        m2.put("y",m1);
        printOneDimension(m2);
        printTwoDimensionsNew(m2);

}

Here is the output:
b
a
y
x
y,b
y,a
x,b
x,a

Thanks,
Rob