Problem looping over iterator of Map with bounded type

[Edward Ross]

Hi
Does anyone have any idea why in Java6 I get the following error:

incompatible types
required: java.util.Iterator<java.util.Map.Entry<? extends java.lang.Number, java.lang.String>>
found: java.util.Iterator<java.util.Map.Entry<capture#19 of ? extends java.lang.Number, java.lang.String>>

when trying to compile this:

public void test1(HashMap<? extends Number, String> stuff)
{      
   for(Iterator<Entry<? extends Number, String>> it = stuff.entrySet().iterator(); it.hasNext();)
   {
      Entry<? extends Number, String> e = it.next();
      System.out.println("Number: " + e.getKey() + "  String; " + e.getValue());
   }
}

while the following compiles fine?

public void test2(HashMap<? extends Number, String> stuff)
{
   for(Entry<? extends Number, String> e : stuff.entrySet())
   {
       System.out.println("Number: " + e.getKey() + "  String; " + e.getValue());
   }
}

Thanks,
Edward

[Mykhailo Kozik]

stuff.entrySet().iterator();

returns actual passed type.

For example, if you call test1() with param new HashMap<Integer, String>() it means that stuff.entrySet().iterator() returns Iterator<Entry<Integer, String>>.
But in general Iterator<Entry<Integer, String>> not compatible with Iterator<Entry<? extends Number, String>>

[Campbell Ritchie]

... and welcome to the Ranch

[Stephan van Hulst]

Okay, this is going to be *really* difficult to explain, but here's my best shot.

stuff.entrySet().iterator() returns an Iterator<T>, where T is the concrete type (or capture) of the entry set.

Your variable expects an Iterator<X>. So X and T need to be *exactly* the same type.

Here, X is Entry<? extends Number, String>, and T is Entry<SomeNumber, String>. This is *not* exactly the same type.

So, your variable should not be an Iterator<X>, but Iterator<? extends T>.

The final solution is:

Iterator<? extends Entry<? extends Number, String>> it = stuff.entrySet().iterator();

[Stephan van Hulst]

Easier would probably be to do the capture ahead of time, so your variable is sure to match up:

public <T extends Number> void test1 (HashMap<T, String> stuff) {
  for(Iterator<Entry<T, String>> it = stuff.entrySet().iterator(); it.hasNext();) {
    Entry<T, String> e = it.next();
    System.out.println("Number: " + e.getKey() + "  String; " + e.getValue());
  }
}

[Edward Ross]

Campbell thanks for the welcome.
Stephan and Mykhailo, thank you, thank you, thank you. That really explains it.

Edward