The polymorphic algorithms described here are pieces of reusable functionality provided by the Java platform. All of them come from theCollections
class, and all take the form of static methods whose first argument is the collection on which the operation is to be performed. The great majority of the algorithms provided by the Java platform operate onList
instances, but a few of them operate on arbitraryCollection
instances. This section briefly describes the following algorithms:
Thesort
algorithm reorders aList
so that its elements are in ascending order according to an ordering relationship. Two forms of the operation are provided. The simple form takes aList
and sorts it according to its elements' natural ordering. If you're unfamiliar with the concept of natural ordering, read the Object Ordering section.The
sort
operation uses a slightly optimized merge sort algorithm that is fast and stable:The following
- Fast: It is guaranteed to run in
n log(n)
time and runs substantially faster on nearly sorted lists. Empirical tests showed it to be as fast as a highly optimized quicksort. A quicksort is generally considered to be faster than a merge sort but isn't stable and doesn't guaranteen log(n)
performance.- Stable: It doesn't reorder equal elements. This is important if you sort the same list repeatedly on different attributes. If a user of a mail program sorts the inbox by mailing date and then sorts it by sender, the user naturally expects that the now-contiguous list of messages from a given sender will (still) be sorted by mailing date. This is guaranteed only if the second sort was stable.
trivial program
prints out its arguments in lexicographic (alphabetical) order.Let's run the program.import java.util.*; public class Sort { public static void main(String[] args) { List<String> list = Arrays.asList(args); Collections.sort(list); System.out.println(list); } }The following output is produced.% java Sort i walk the lineThe program was included only to show you that algorithms really are as easy to use as they appear to be.[i, line, the, walk]The second form of
sort
takes aComparator
in addition to aList
and sorts the elements with theComparator
. Suppose you want to print out the anagram groups from our earlier example in reverse order of size largest anagram group first. The example that follows shows you how to achieve this with the help of the second form of thesort
method.Recall that the anagram groups are stored as values in a
Map
, in the form ofList
instances. The revised printing code iterates through theMap
's values view, putting everyList
that passes the minimum-size test into aList
ofList
s. Then the code sorts thisList
, using aComparator
that expectsList
instances, and implements reverse size-ordering. Finally, the code iterates through the sortedList
, printing its elements (the anagram groups). The following code replaces the printing code at the end of themain
method in theAnagrams
example.Running// Make a List of all anagram groups above size threshold. List<List<String>> winners = new ArrayList<List<String>>(); for (List<String> l : m.values()) if (l.size() >= minGroupSize) winners.add(l); // Sort anagram groups according to size Collections.sort(winners, new Comparator<List<String>>() { public int compare(List<String> o1, List<String> o2) { return o2.size() - o1.size(); }}); // Print anagram groups. for (List<String> l : winners) System.out.println(l.size() + ": " + l);the program
on thesame dictionary
as in The Map Interface section, with the same minimum anagram group size (eight), produces the following output.12: [apers, apres, asper, pares, parse, pears, prase, presa, rapes, reaps, spare, spear] 11: [alerts, alters, artels, estral, laster, ratels, salter, slater, staler, stelar, talers] 10: [least, setal, slate, stale, steal, stela, taels, tales, teals, tesla] 9: [estrin, inerts, insert, inters, niters, nitres, sinter, triens, trines] 9: [capers, crapes, escarp, pacers, parsec, recaps, scrape, secpar, spacer] 9: [palest, palets, pastel, petals, plates, pleats, septal, staple, tepals] 9: [anestri, antsier, nastier, ratines, retains, retinas, retsina, stainer, stearin] 8: [lapse, leaps, pales, peals, pleas, salep, sepal, spale] 8: [aspers, parses, passer, prases, repass, spares, sparse, spears] 8: [enters, nester, renest, rentes, resent, tenser, ternes, treens] 8: [arles, earls, lares, laser, lears, rales, reals, seral] 8: [earings, erasing, gainers, reagins, regains, reginas, searing, seringa] 8: [peris, piers, pries, prise, ripes, speir, spier, spire] 8: [ates, east, eats, etas, sate, seat, seta, teas] 8: [carets, cartes, caster, caters, crates, reacts, recast, traces]
Theshuffle
algorithm does the opposite of whatsort
does, destroying any trace of order that may have been present in aList
. That is, this algorithm reorders theList
based on input from a source of randomness such that all possible permutations occur with equal likelihood, assuming a fair source of randomness. This algorithm is useful in implementing games of chance. For example, it could be used to shuffle aList
ofCard
objects representing a deck. Also, it's useful for generating test cases.This operation has two forms: one takes a
List
and uses a default source of randomness, and the other requires the caller to provide a Random object to use as a source of randomness. The code for this algorithm is used as an example in theList
section.
TheCollections
class provides five algorithms for doing routine data manipulation onList
objects, all of which are pretty straightforward:
reverse
reverses the order of the elements in aList
.
fill
overwrites every element in aList
with the specified value. This operation is useful for reinitializing aList
.
copy
takes two arguments, a destinationList
and a sourceList
, and copies the elements of the source into the destination, overwriting its contents. The destinationList
must be at least as long as the source. If it is longer, the remaining elements in the destinationList
are unaffected.
swap
swaps the elements at the specified positions in aList
.
addAll
adds all the specified elements to aCollection
. The elements to be added may be specified individually or as an array.
ThebinarySearch
algorithm searches for a specified element in a sortedList
. This algorithm has two forms. The first takes aList
and an element to search for (the "search key"). This form assumes that theList
is sorted in ascending order according to the natural ordering of its elements. The second form takes aComparator
in addition to theList
and the search key, and assumes that theList
is sorted into ascending order according to the specifiedComparator
. Thesort
algorithm can be used to sort theList
prior to callingbinarySearch
.The return value is the same for both forms. If the
List
contains the search key, its index is returned. If not, the return value is(-(insertion point) - 1)
, where the insertion point is the point at which the value would be inserted into theList
, or the index of the first element greater than the value orlist.size()
if all elements in theList
are less than the specified value. This admittedly ugly formula guarantees that the return value will be>= 0
if and only if the search key is found. It's basically a hack to combine a boolean(found)
and an integer(index)
into a singleint
return value.The following idiom, usable with both forms of the
binarySearch
operation, looks for the specified search key and inserts it at the appropriate position if it's not already present.int pos = Collections.binarySearch(list, key); if (pos < 0) l.add(-pos-1);
The frequency and disjoint algorithms test some aspect of the composition of one or moreCollections
:
frequency
counts the number of times the specified element occurs in the specified collectiondisjoint
determines whether twoCollections
are disjoint; that is, whether they contain no elements in common
Themin
and themax
algorithms return, respectively, the minimum and maximum element contained in a specifiedCollection
. Both of these operations come in two forms. The simple form takes only aCollection
and returns the minimum (or maximum) element according to the elements' natural ordering. The second form takes aComparator
in addition to theCollection
and returns the minimum (or maximum) element according to the specifiedComparator
.