Java – iterator of wildcard type variables with upper bound

Hello, I'm trying to extend HashMap < string, string > to enforce the "all lowercase" rule

public class HttpQueryMap extends HashMap<String,String>
{    
    ...
    @Override
    public void putAll(Map<? extends String,? extends String> m)
    {       
        ...
        Iterator<Map.Entry<String,String>> iterator = m.entrySet().iterator();
        ...      
    }
    ... 
}

I received a compile time error

incompatible types
required: Iterator<Entry<String,String>>
found:    Iterator<Entry<CAP#1,CAP#2>>
where CAP#1,CAP#2 are fresh type-variables:
CAP#1 extends String from capture of ? extends String
CAP#2 extends String from capture of ? extends String

The next solution can do this, but it's actually ugly:

public class HttpQueryMap extends HashMap<String,? extends String> m)
    {       
        ...
        Map<String,String> m_str=new HashMap<String,String>();
        m_str.putAll(m);
        Iterator<Map.Entry<String,String>> iterator = m_str.entrySet().iterator();
        ...      
    }
    ... 
 }

As far as I know, the problem is the type variable string < map used in iterator Entry < string, string > > do not extend map meters

Solution

No iterators

The easiest way is to use a for - each loop Even in this case, you need to parameterize the entry with the same wildcards as in the given mapping The reason is entry is not a subtype of entry < string, string > The fact that string is the final class is irrelevant here because the compiler does not know this

for (Entry<? extends String,? extends String> entry : m.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
}

With iterator

If you really need an iterator, the compilation syntax is a little confusing:

Iterator<? extends Entry<? extends String,? extends String>> iterator =
    m.entrySet().iterator();

while (iterator.hasNext()) {
    Entry<? extends String,? extends String> entry = iterator.next();
    String key = entry.getKey();
    String value = entry.getValue();
}

I initially expected the iterator to be just iterator type < entry >, which initially looks like set < entry > this in turn seems to be in map .

But it's a little more complicated than that Here I found a possible answer:

http://mail-archives.apache.org/mod_mbox/harmony-dev/200605.mbox/%3Cbb4674270605110156r4727e563of9ce24cdcb41a0c8 @mail. gmail. com%3E

What's interesting is this: