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Java Forum / First Aid / August 2007How can i implements a class like an HashMap but with 2 keys ?
Hi all, i am a newbie, i want to create a class that works like an HashMap but with 2 keys, get(key1,key2) -> value I was thinking to create a class "Pair" that override equals and hashcode and then store here the two keys and then a class that holds an HashMap with Pair as key and the value class, can be this a solution? or what can i use? Bye thanks for the help > i want to create a class that works like an HashMap but > with 2 keys, get(key1,key2) -> value > > I was thinking to create a class "Pair" that override equals and hashcode > and then store here the two keys and then a class that holds an HashMap > with Pair as key and the value class, can be this a solution? Sounds good to me, actually you could just use a plain hashmap with that pair class as key, no need to wrap it inside another class. What you could also do is generating a string from the two keys which is then used as combined key, but the pair thing looks cleaner to me... Best regards, Jan Thomä  Signature_________________________________________________________________________ insOMnia - We never sleep... http://www.insOMnia-hq.de Rob wrote: >> i [sic] want to create a class that works like an HashMap but >> with 2 keys, get(key1,key2) -> value >> I was thinking to create a class "Pair" >> that override equals and hashcode [sic] That's "hashCode". >> and then store here the two keys and then a class that holds an HashMap >> with Pair as key and the value class, can be this a solution? This would be the canonical solution. Make Pair immutable and generic if your keys are of general type, non-generic if you know what the key types are. SignatureLew Jan Thomä schreef: >> i want to create a class that works like an HashMap but >> with 2 keys, get(key1,key2) -> value [quoted text clipped - 7 lines] > also do is generating a string from the two keys which is then used as > combined key, but the pair thing looks cleaner to me... Have a look at Jakarta Commons Collections, it has MultiKey and MultiKeyMap, which works exactly as you describe. If you want a generic version, let me know. H. SignatureHendrik Maryns http://tcl.sfs.uni-tuebingen.de/~hendrik/ ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html > Have a look at Jakarta Commons Collections, it has MultiKey and > MultiKeyMap, which works exactly as you describe. > > If you want a generic version, let me know. > > H. Yes, i read in the api and seems perfect for what i have to do. A generic version is better, do you know where can i get it? Thanks Rob schreef: >> Have a look at Jakarta Commons Collections, it has MultiKey and >> MultiKeyMap, which works exactly as you describe. [quoted text clipped - 6 lines] > > A generic version is better, do you know where can i get it? The Jakarta team has been very reluctant in starting a generic version. Finally, there has been some advancement there. However, I have made my own generic version, if you send me an e-mail, I can send it to you. Mention which classes you would like. Alternatively, there is ‘larvalabs’. H. SignatureHendrik Maryns http://tcl.sfs.uni-tuebingen.de/~hendrik/ ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html >i am a newbie, i want to create a class that works like an HashMap but >with 2 keys, get(key1,key2) -> value Here are three solutions: 1. use two HashMaps on the same set of objects. Then you can look up by either key. 2. create a Pair class that contains key1 and key2 with an ^ (xor) combined hashCode. Then Pair is a single key for both building and lookup. You need both keys to lookup, like a safety deposit box. 3. concatenate your two String keys into one. See http://mindprod.com/jgloss/hashcode.html http://mindprod.com/jgloss/hashmap.html http://mindprod.com/jgloss/hashtable.html SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >> i am a newbie, i want to create a class that works like an HashMap but >> with 2 keys, get(key1,key2) -> value ... > 3. concatenate your two String keys into one. I'd avoid this one unless there is at least one char bit pattern that cannot appear in the first input, to use as separator. For example, using "_" for the separator, the key "A_B_C" would be used for both ("A_B","C") and ("A","B_C"), making it impossible to distinguish those pairs. This can be worked around by an encoding step before the concatenation, but that makes it all more complicated, and simplicity is the main merit of this method. Patricia >> 3. concatenate your two String keys into one. > > I'd avoid this one unless there is at least one char bit pattern > that cannot appear in the first input, to use as separator. Why is it necessary to separate them? The cost of not doing so is only an increased risk of collisions. In fact that cost might be zero or so negligible that adding the separator at each operation costs more than the potential collisions it attempts to avoid. I'm assuming that the concatenation is only used internally in the HashMap, and that the separate components are used elsewhere in their original, separate state. /gordon -- >>> 3. concatenate your two String keys into one. >> I'd avoid this one unless there is at least one char bit pattern [quoted text clipped - 10 lines] > HashMap, and that the separate components are used elsewhere in their > original, separate state. If it were only for purposes of calculating hashCode(), I would completely agree. However, my understanding is that this is to be used as the key in a single key HashMap substituting for a map keyed by the pair. In the portion I snipped, the first two items in the list were: >> 1. use two HashMaps on the same set of objects. Then you can look up >> by either key. >> >> 2. create a Pair class that contains key1 and key2 with an ^ (xor) >> combined hashCode. Then Pair is a single key for both building and >> lookup. You need both keys to lookup, like a safety deposit box. Both of those solutions would treat <"A_B",C> and <"A","B_C"> as different keys. Both could be present as keys, with different values, at the same time. Patricia > If it were only for purposes of calculating hashCode(), I would > completely agree. However, my understanding is that this is to be used > as the key in a single key HashMap substituting for a map keyed by the > pair. Thanks. My thoughts were stuck on hashCode() when I posted earlier, I forgot about (in)equality of the keys themselves. /gordon -- > Why is it necessary to separate them? The cost of not doing so is only > an increased risk of collisions. Please ignore that. After posting I realized that the concatenated keys themselves need to be unique, not the hashcodes... /gordon -- >Why is it necessary to separate them? The cost of not doing so is only >an increased risk of collisions. You could get false hits: applespear would match apple-spear and apples-pear SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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