10-Builder
The goal of builder is to separate the construction from the“representation,” to allow multiple different representations. The constructionprocess stays the same, but the resulting object has different possiblerepresentations.GoF points out that the main difference with Abstract Factoryis that a Builder creates the object step-by-step, so the fact that thecreation process is spread out in time seems to be important. In addition, itseems that the “director” gets a stream of pieces that it passes to theBuilder, and each piece is used to perform one of the steps in the buildprocess.
......
//: builder:BuildMedia.java
// Example of the Builder pattern
package builder;
import java.util.*;
import junit.framework.*;
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// Different "representations"of media:
class Media extends ArrayList {}
class Book extends Media {}
class Magazine extends Media {}
class WebSite extends Media {}
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// ... contain different kinds of mediaitems:
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); }
}
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// ... but use the same basicconstruction steps:
class MediaBuilder {
? public void buildBase() {}
? public void addMediaItem(MediaItemitem) {}
? public Media getFinishedMedia() {return null; }
}
?
class BookBuilder extends MediaBuilder {
? private Book b;
? public void buildBase() {
??? System.out.println("Buildingbook framework");
??? b = new Book();
? }
? public void addMediaItem(MediaItemchapter) {
??? System.out.println("Addingchapter " + chapter);
??? b.add(chapter);
? }
? public Media getFinishedMedia() {return b; }
}?
?
class MagazineBuilder extendsMediaBuilder {
? private Magazine m;
? public void buildBase() {
??? System.out.println("Buildingmagazine framework");
??? m = new Magazine();
? }
? public void addMediaItem(MediaItemarticle) {
??? System.out.println("Addingarticle " + article);
??? m.add(article);
? }
? public Media getFinishedMedia() {return m; }
}?
?
class WebSiteBuilder extends MediaBuilder{
? private WebSite w;
? public void buildBase() {
??? System.out.println("Building website framework");
??? w = new WebSite();
? }
? public void addMediaItem(MediaItemwebItem) {
??? System.out.println("Adding webitem " + 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(newMediaItem[] {
??? new MediaItem("item1"), newMediaItem("item2"),
??? new MediaItem("item3"), newMediaItem("item4"),
? });
? public void testBook() {
??? MediaDirector buildBook =
????? new MediaDirector(newBookBuilder());
??? 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(newMagazineBuilder());
??? 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(newWebSiteBuilder());
??? 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);
? }
} ///:~
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Note that in some ways this could be seen as a morecomplicated State pattern, since the behavior of the director depends on whattype of builder you use. Instead of simply forwarding the requests through tothe underlying State object, however, the director has a sequence of operationsto perform, and it uses the State object as a Policy to fulfill its job. Thus,Builder could be described as using a Policy to create objects.
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//示例代码是比较简单的，能够很好地读懂，但是没有具体的应用实例，不是很好理解。看来具体的项目时间还是比较重要啊！整体的感觉是：Builder模式是按照自己的喜好组装好自己的目标对象，Abstract Factory模式则是一步到位得到自己的目标对象。找了一个很有意思的讲解：builder与abstract factory之间的区别
实际的样例，等以后有了实际的项目经验就好了，到时再整理咯！
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