建造者模式（Builder）

<pre name="code" class="java">//: builder:BuildMedia.java// Example of the Builder patternpackage test;import java.util.*;import junit.framework.*;// Different "representations" of media:class Media extends ArrayList {}class Book extends Media {}class Magazine extends Media {}class WebSite extends Media {}// ... contain different kinds of media items:class MediaItem {private String s;public MediaItem(String s) {this.s = s;}public String toString() {return s;}}class Chapter extends MediaItem {public Chapter(String s) {super(s);}}class Article extends MediaItem {public Article(String s) {super(s);}}class WebItem extends MediaItem {public WebItem(String s) {super(s);}}// ... but use the same basic construction steps:class MediaBuilder {public void buildBase() {}public void addMediaItem(MediaItem item) {}public Media getFinishedMedia() {return null;}}class BookBuilder extends MediaBuilder {private Book b;public void buildBase() {System.out.println("Building book framework");b = new Book();}public void addMediaItem(MediaItem chapter) {System.out.println("Adding chapter " + chapter);b.add(chapter);}public Media getFinishedMedia() {return b;}}class MagazineBuilder extends MediaBuilder {private Magazine m;public void buildBase() {System.out.println("Building magazine framework");m = new Magazine();}public void addMediaItem(MediaItem article) {System.out.println("Adding article " + article);m.add(article);}public Media getFinishedMedia() {return m;}}class WebSiteBuilder extends MediaBuilder {private WebSite w;public void buildBase() {System.out.println("Building web site framework");w = new WebSite();}public void addMediaItem(MediaItem webItem) {System.out.println("Adding web item " + webItem);w.add(webItem);}public Media getFinishedMedia() {return w;}}class MediaDirector { // a.k.a. "Context"private MediaBuilder mb;public MediaDirector(MediaBuilder mb) {this.mb = mb; // Strategy-ish}public Media produceMedia(List input) {mb.buildBase();for (Iterator it = input.iterator(); it.hasNext();)mb.addMediaItem((MediaItem) it.next());return mb.getFinishedMedia();}};public class BuildMedia extends TestCase {private List input = Arrays.asList(new MediaItem[] {new MediaItem("item1"), new MediaItem("item2"),new MediaItem("item3"), new MediaItem("item4"), });public void testBook() {MediaDirector buildBook = new MediaDirector(new BookBuilder());Media book = buildBook.produceMedia(input);String result = "book: " + book;System.out.println(result);assertEquals(result, "book: [item1, item2, item3, item4]");}public void testMagazine() {MediaDirector buildMagazine = new MediaDirector(new MagazineBuilder());Media magazine = buildMagazine.produceMedia(input);String result = "magazine: " + magazine;System.out.println(result);assertEquals(result, "magazine: [item1, item2, item3, item4]");}public void testWebSite() {MediaDirector buildWebSite = new MediaDirector(new WebSiteBuilder());Media webSite = buildWebSite.produceMedia(input);String result = "web site: " + webSite;System.out.println(result);assertEquals(result, "web site: [item1, item2, item3, item4]");}public static void main(String[] args) {junit.textui.TestRunner.run(BuildMedia.class);}} // /:~