Re: Generics on map.

From:
Eric Sosman <Eric.Sosman@sun.com>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 14 Mar 2008 13:35:48 -0400
Message-ID:
<1205516144.351972@news1nwk>
Mark Space wrote:

Eric Sosman wrote:

Ravi wrote:

Hi,

The following code compiles in eclipse 3.3.1. Shouldn't the compiler
throw an exception as the key doesn't pass instanceof check?

class Test {
public static void main(String args[]) {
        Map<String,String> testmap = new HashMap<String, String>();
        testmap.get(new StringBuffer());
    }
}


    No. The get() method of the Map interface takes any
Object reference as its argument, not a parameterized type.
Since testmap will only accept entries that have Strings as
keys, querying for a key of any other type will return null.
It's legal to ask, even when the answer will be "No."


Good one. Reading the OP's example, I assumed that get() took a
parameterized type as well, and I couldn't figure out why an exception
wouldn't be throw.

"When all else fails, read the documentation."


     Or as in my case, "Learn from your blunders." I'd tried
to write a Map with case-insensitive Strings as keys:

    Map<String,Thing> map = new HashMap<String,Thing>() {
        public Thing put(String key, Thing value) {
            return super.put(key.toLowerCase(), value);
        }
        public Thing get(String key) {
            return super.get(key.toLowerCase());
        }
    }

.... and then I got splinters in my fingers from scratching
my head over why it didn't work. After the penny dropped,
I changed the second method to

        public Thing get(Object key) {
            if (key instanceof String)
                key = ((String)key).toLowerCase();
            return super.get(key);
        }

.... and things worked a whole lot better. (The `if' isn't
necessary, but given my state of confusion at the time it's
perhaps forgivable.) Having been through this, I am now
"Once burned, twice shy."

--
Eric.Sosman@sun.com

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