Java集合框架官方教程(6)：算法

Lesson: Algorithms

The polymorphic algorithms described here are pieces of reusable functionality provided by the Java platform. All of them come from the

Collections

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 on

List

instances, but a few of them operate on arbitrary

Collection

instances. This section briefly describes the following algorithms:

Sorting

Shuffling

Routine Data Manipulation

Searching

Composition

Finding Extreme Values

Sorting

The

sort

algorithm reorders a

List

so that its elements are in ascending order according to an ordering relationship. Two forms of the operation are provided. The simple form takes a

List

and sorts it according to its elements' natural ordering. If you're unfamiliar with the concept of natural ordering, read theObject Ordering section.

The

sort

operation uses a slightly optimized merge sort algorithm that is fast and stable:

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 guarantee

n 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.

The following

trivial program

prints out its arguments in lexicographic (alphabetical) order.

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);
}
}

Let's run the program.

% java Sort i walk the line

The following output is produced.

[i, line, the, walk]

The program was included only to show you that algorithms really are as easy to use as they appear to be.

The second form of

sort

takes a

Comparator

in addition to a

List

and sorts the elements with the

Comparator

. 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 the

sort

method.

Recall that the anagram groups are stored as values in a

Map

, in the form of

List

instances. The revised printing code iterates through the

Map

's values view, putting every

List

that passes the minimum-size test into a

List

of

List

s. Then the code sorts this

List

, using a

Comparator

that expects

List

instances, and implements reverse size-ordering. Finally, the code iterates through the sorted

List

, printing its elements (the anagram groups). The following code replaces the printing code at the end of the

main

method in the

Anagrams

example.

// Make aList 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, newComparator<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);

Running

the program

on the

same 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]

Shuffling

The

shuffle

algorithm does the opposite of what

sort

does, destroying any trace of order that may have been present in a

List

. That is, this algorithm reorders the

List

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 a

List

of

Card

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 aRandom object to use as a source of randomness. The code for this algorithm is used as an example in the

List

section.

Routine Data Manipulation

The

Collections

class provides five algorithms for doing routine data manipulation on

List

objects, all of which are pretty straightforward:

reverse

— reverses the order of the elements in a

List

.

fill

— overwrites every element in a

List

with the specified value. This operation is useful for reinitializing a

List

.

copy

— takes two arguments, a destination

List

and a source

List

, and copies the elements of the source into the destination, overwriting its contents. The destination

List

must be at least as long as the source. If it is longer, the remaining elements in the destination

List

are unaffected.

swap

— swaps the elements at the specified positions in a

List

.

addAll

— adds all the specified elements to a

Collection

. The elements to be added may be specified individually or as an array.

Searching

The

binarySearch

algorithm searches for a specified element in a sorted

List

. This algorithm has two forms. The first takes a

List

and an element to search for (the "search key"). This form assumes that the

List

is sorted in ascending order according to the natural ordering of its elements. The second form takes a

Comparator

in addition to the

List

and the search key, and assumes that the

List

is sorted into ascending order according to the specified

Comparator

. The

sort

algorithm can be used to sort the

List

prior to calling

binarySearch

.

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 the

List

, or the index of the first element greater than the value or

list.size()

if all elements in the

List

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 single

int

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, key);

Composition

The frequency and disjoint algorithms test some aspect of the composition of one or more

Collections

:

frequency

— counts the number of times the specified element occurs in the specified collection

disjoint

— determines whether two

Collections

are disjoint; that is, whether they contain no elements in common

Finding Extreme Values

The

min

and the

max

algorithms return, respectively, the minimum and maximum element contained in a specified

Collection

. Both of these operations come in two forms. The simple form takes only a

Collection

and returns the minimum (or maximum) element according to the elements' natural ordering. The second form takes a

Comparator

in addition to the

Collection

and returns the minimum (or maximum) element according to the specified

Comparator

.

Original:
http://docs.oracle.com/javase/tutorial/collections/algorithms/index.html