Way to sort / enforce order for Map.entrySet?
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From: Lew on 24 Apr 2010 09:18 Mike Schilling wrote: > Lew wrote: >> Mike Schilling wrote: >>> Lew wrote: >>>> Feed a >>>> man a meal and you feed him for an hour; send him to the diner and >>>> you feed him for a lifetime. >>> Build a man a fire, and he's warm for a night, set him on fire, and >>> he's warm his whole life. >>> >>>>> I do use a TreeMap -- >>>>> "final Map<String, String> map = new >>>>> TreeMap<String, String>();", >>>>> but the order is not getting preserved when I call "map.entrySet". >>>> And you didn't even provide an SSCCE! What is wrong with you? >>> Since he wanted to sort by value, not key, he's correct -- the entry >>> set won't be sorted the way he wanted. >> Unless, of course, he uses a custom Comparator that does sort the way >> he wants. > > Not Map.entrySet(); that always sorts by the map's keys. (You can *copy* > the result of entrySet() and sort that copy however you like, of course.) And that would be exactly the advice that several people gave him. -- Lew
From: Eric Sosman on 24 Apr 2010 10:35 On 4/24/2010 2:36 AM, Mike Schilling wrote: > [...] > Not Map.entrySet(); that always sorts by the map's keys. (You can *copy* > the result of entrySet() and sort that copy however you like, of course.) Two points to add: First, the Set returned by entrySet() only "sorts" the entries if the underlying Map implementation makes a guarantee of some kind. Not all do, and LinkedHashMap (at least) promises[*] an order that is not based on the keys. Second, a Map.Entry object is only valid as long as the Map from which it was derived remains unchanged. Modify the Map, and any existing Map.Entry instances become so much junk. [*] Actually, the Javadoc for LinkedHashMap says only that its iterator() method produces Iterators that visit entries in non-key order. The order of an Iterator on its entrySet() is not explicitly described, so I'm really only guessing that it might be the same. -- Eric Sosman esosman(a)ieee-dot-org.invalid
From: Mike Schilling on 24 Apr 2010 12:57 Eric Sosman wrote: > On 4/24/2010 2:36 AM, Mike Schilling wrote: >> [...] >> Not Map.entrySet(); that always sorts by the map's keys. (You can >> *copy* the result of entrySet() and sort that copy however you like, >> of course.) > > Two points to add: First, the Set returned by entrySet() only > "sorts" the entries if the underlying Map implementation makes a > guarantee of some kind. Right. In the part you snipped, it was stated explicitly that the underlying map was a TreeMap.
From: Mike Schilling on 24 Apr 2010 12:58 Lew wrote: > Mike Schilling wrote: >> Lew wrote: >>> Mike Schilling wrote: >>>> Lew wrote: >>>>> Feed a >>>>> man a meal and you feed him for an hour; send him to the diner and >>>>> you feed him for a lifetime. >>>> Build a man a fire, and he's warm for a night, set him on fire, and >>>> he's warm his whole life. >>>> >>>>>> I do use a TreeMap -- >>>>>> "final Map<String, String> map = new >>>>>> TreeMap<String, String>();", >>>>>> but the order is not getting preserved when I call >>>>>> "map.entrySet". >>>>> And you didn't even provide an SSCCE! What is wrong with you? >>>> Since he wanted to sort by value, not key, he's correct -- the >>>> entry set won't be sorted the way he wanted. >>> Unless, of course, he uses a custom Comparator that does sort the >>> way he wants. >> >> Not Map.entrySet(); that always sorts by the map's keys. (You can >> *copy* the result of entrySet() and sort that copy however you like, >> of course.) > > And that would be exactly the advice that several people gave him. My point is that it doesn't require an SSCCE to demonstrate the the entrySet() of a TreeMap isn't sorted by the values.
From: Mike Schilling on 24 Apr 2010 13:28
Tom Anderson wrote: > > I have a terrible vice of optimism. I think that for someone who > doesn't know about the collections API, and hasn't built up a general > understanding of the 'Tao of Java', if you will, that will let them > grok it from reading the docs, advice such as you gave, correct > though it is, might as well be in Greek. In that case, a picture is > worth a thousand words, and in programming a picture is a snippet. Good Lodr, Tom, are you really suggesting that it isn't intuitively obvious to the rawest beginner that the new set should be created via SortedSet<Map.Entry<String, String>> results = new TreeSet<Map.Entry<String, String>>( new Comparator <Map.Entry<String, String>>() {... }; |