Re: java generics bug?
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.