Re: Map question
On 5/31/2010 9:55 AM, laredotornado wrote:
Hi,
I'm using Java 6. The Javadocs suggest that Map uses containsKey, and
hence an Object's equals method to determine if the object is already
a key in the map.
Where do you find this suggestion? The Javadoc says that the
Map interface *requires* that any class implementing it *provide*
a containsKey() method, but doesn't say anything about how the class
uses the method internally, or even whether it's used internally.
However, I'm noticing that when I define this
equals method on an object I'm inserting as a key in my Map (a
HashMap):
public boolean equals(Object obj) {
SearchResultHotels hotel = (SearchResultHotels) obj;
boolean ret = obj != null&& ((SearchResultHotels) obj).getId() ==
getId();
log.debug("\t comparing " + hotel.getName() + " to " + getName() +
":" + ret);
return ret;
}
I never see my log statement printed out when I call "map.put". What
am I missing?
One thing you're missing -- it's a side-issue to your question,
but it's important anyhow -- is that this equals() method will throw
ClassCastException if `obj' is a String or a JButton or anything other
than a SearchResultHotels. Throwing CCE from equals() is a no-no; the
method should simply return `false' meaning "The HoundOfTheBaskervilles
object you handed me is unequal to `this'."
To the question at hand, consider how a HashMap works: It computes
the hashCode() of the key, and using that value it chooses a "bucket."
If the bucket already holds a key/value pair with the same key, the
new value replaces the old. If the bucket holds no pair with the given
key, the new pair is added. And ...
... (drum roll, please) ...
.... if the bucket is *empty*, how many times must HashMap call equals()
to determine that the new key is not a duplicate?
--
Eric Sosman
esosman@ieee-dot-org.invalid