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Java Forum / General / November 2006new to generics
Hello All- In the past I have used java pre generics for some programs related to my hobbies. I have come back to it after a couple of years and am biting the bullet and moving to 1.5(5.0)... In the past I have made some type specific packages to generate some basic combinatoric structures (permutations, combinations, partitions) For my own use and education, I'm trying to figure out how to make this more usfully with generics. Specifically: class foo {...} class otherClass{ HashSet<foo> fooSet; // now comes the tricky (to me) part Iterator<???> perm = new permutation(fooSet); /* I'd like to have iterator return one permuation of the elements in fooSet at a time */ } So... Does this type of thing already exist somewhere? If so where? If not how do I go about using generics to do such a thing? Thanks in advance Collins > HashSet<foo> fooSet; > [quoted text clipped - 6 lines] > */ > } An iteration of sets of Foo? Iterator<Set<Foo>> perm = new Permutation<Foo>(); Or preferably Iterable<Set<Foo>> perm = new Permutation<Foo>(); where class Permutation<T> implements Iterable<Set<T>> { Permutation(Set<? extends T> domain) { ... Using ? extends just means that you can use Set typed with a subtype of T. So you could write class Bar extends Foo {} ... new Permutation<Foo>(new HashSet<Bar>()). Tom Hawtin By the way since HashSet makes no guarantees as to the iteration order of the set, it will be hard to do permutations on the set don't you think? You could use instead LinkedHashSet for instance. > By the way since HashSet makes no guarantees as to the iteration order > of the set, it will be hard to do permutations on the set don't you > think? > > You could use instead LinkedHashSet for instance. The obvious implementation would be to copy the set into a list. Tom Hawtin > > You could use instead LinkedHashSet for instance. > > The obvious implementation would be to copy the set into a list. I'd think an array would be better. The performance might not be much better but the code would be a lot clearer. (I speak from too much sad experience -- coding even moderately complicated array-oriented algorithms without the benefit of array syntax is painful.) -- chris >>> You could use instead LinkedHashSet for instance. >> The obvious implementation would be to copy the set into a list. [quoted text clipped - 4 lines] > (I speak from too much sad experience -- coding even moderately complicated > array-oriented algorithms without the benefit of array syntax is painful.) Certainly lack of operator overloading can make code look really nasty. The other week I wrote some code using BigIntegers. Yuck. However, I don't think it should really be much of a problem here. The real problem is ye olde arrays don't mix well with generics. It needs either an Object array and (T)set.get(i), or (T[])set.toArray(). Stick with Dr Bloch's collections, and everything will be fine. Tom Hawtin [me:] > > I'd think an array would be better. The performance might not be much > > better but the code would be a lot clearer. [quoted text clipped - 6 lines] > The other week I wrote some code using BigIntegers. Yuck. However, I > don't think it should really be much of a problem here. It depends on the algorithms you are trying to express. My own code in this sort of area (permutations, combinations, and whatnot) is rather array heavy. I know -- 'cos it's written in a language which lacks C-style array access syntax (albeit not as bad as Java Collections) -- that the lack of array syntax hurts badly. > The real problem is ye olde arrays don't mix well with generics. It > needs either an Object array and (T)set.get(i), or (T[])set.toArray(). > Stick with Dr Bloch's collections, and everything will be fine. If it comes down to a choice between writing code of adequate clarity, and using generics, then I'm afraid that generics will loose every time -- no contest at all. If Dr Bloch's Collections are so tangled up with generics that they have to be chucked out too, then so be it. There are plenty of cases where ArrayLists are to be preferred to raw arrays, I agree, but this is most definitely not one of them. -- chris Collins schreef: > Hello All- > [quoted text clipped - 23 lines] > */ > } Before you start implementing this, have a look here: http://mindprod.com/jgloss/permutation.html. You might want to change the BigIntegers to ints if you care for efficiency rather than infinite correctness. H. - -- Hendrik 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 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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