Some help with unchecked types

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Le Tubs - 23 Jan 2006 14:41 GMT

Hi

I'm relatively new to Java (from a C/C++ background, so please don't
flame me). I having trouble with some warnings like (the code compiles
_but_ I treat warnings as errors).

[javac] Compiling 10 source files to D:\class
[javac] xxxx\Oracle.java:294: warning: [unchecked] unchecked
[javac] found : java.util.Iterator
[javac] required: java.util.Iterator<java.lang.String>
[javac] Iterator<String> it =
this.hashmap.keySet().iterator();

The Code that generating it is below

Now I've gone out onto the www and a quite extensive search and sort of
getting quite confused about the whole issue. Can somebody shed some
light on what I am missing.
Thanking you in advance for your time and consideration.

Thanks
David

TreeMap hashmap;
public ArrayList getSortedKeys()
{
ArrayList<String> keys = new ArrayList<String>();
Iterator<String> it = this.hashmap.keySet().iterator();
while (it.hasNext())
{
keys.add( (String)it.next() );
}
return keys;
}

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Joe Attardi - 23 Jan 2006 14:48 GMT

> [javac] found : java.util.Iterator
> [javac] required: java.util.Iterator<java.lang.String>
[quoted text clipped - 4 lines]
> light on what I am missing.
> Thanking you in advance for your time and consideration.

Your problem is here:

> TreeMap hashmap;

If you want an Iterator<String> with no warnings, you'll need to do
this:
TreeMap<String, Object>

where the second parameter, Object, is whatever type the String keys in
the TreeMap will map to.

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Timbo - 23 Jan 2006 14:49 GMT

> Hi
>
[quoted text clipped - 15 lines]
> light on what I am missing.
> Thanking you in advance for your time and consideration.

You problem is that you are declaring the iterator, 'it', as
having its generic type instantiated to 'String', but 'hashMap'
does not have its generic type constrained at all, so you could
add any type of object in there. So you need to declare it like so:

TreeMap<String> hashMap;

Also, you can then remove the downcast like so:

keys.add(it.next());

Hope that clarifies things!
Tim

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Jeffrey Schwab - 23 Jan 2006 15:03 GMT

> Hi
>
[quoted text clipped - 8 lines]
> [javac] Iterator<String> it =
> this.hashmap.keySet().iterator();

...

> TreeMap hashmap;
> public ArrayList getSortedKeys()
[quoted text clipped - 7 lines]
> return keys;
> }

The compiler can't guarantee that only strings will come from the keySet
iterator, unless of course it can first guarantee that only Strings
will be used as keys. When you create the Map, specify the types of
keys and values you want to allow.

As further niceties:

- You should question the wisdom of calling a TreeMap "hashmap."

- For the benefit of the method's caller, you might also want to
change the compile-time return type of getSortedKeys to ArrayList<String>.

Here's some code:

import java.util.*;
public class Main {
    TreeMap<String, Object> map;
    public ArrayList<String> getSortedKeys()
    {
        ArrayList<String> keys = new ArrayList<String>();
        Iterator<String> it = this.map.keySet().iterator();
        while (it.hasNext())
        {
            keys.add(it.next());
        }
        return keys;
    }
}

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Le Tubs - 23 Jan 2006 15:11 GMT

Thanks for all your replies, you have cleared up the problems I was
having I'm actually cleaning up code that a DBA wrote ... (I think its
the last time that I "volunteer" for this type of exercise & I won't
bore everyone with a tirade of DBA jokes )
Thanks again for your help it is much appreciated.

David

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Le Tubs - 23 Jan 2006 15:12 GMT

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Le Tubs - 23 Jan 2006 15:28 GMT

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Hendrik Maryns - 24 Jan 2006 12:09 GMT

Jeffrey Schwab schreef:

>> Hi
>>
[quoted text clipped - 51 lines]
> }
> }

How about

return new ArrayList(this.map.keySet())?

Does all the iterations for you...

H.

- --
Hendrik Maryns

==================
www.lieverleven.be
http://aouw.org

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Jeffrey Schwab - 24 Jan 2006 12:26 GMT

>>import java.util.*;
>>public class Main {
[quoted text clipped - 10 lines]
>> }
>>}

> How about
>
> return new ArrayList(this.map.keySet())?
>
> Does all the iterations for you...

Oooo... You're good. :)

I think most of us were just concentrating on helping the OP understand
generics. Now I'm kicking myself for not being more clever.

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Some help with unchecked types

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