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1. 概述
建造者(Builder)设计模式用于组装具有复杂结构的实例。在构建一个复杂实例的时候,我们很难一气呵成,我们首先把实例的各个部分建造出来,然后分阶段把它们组装起来。
2. 示例程序
一个使用Builder设计模式来编写“文档的程序”的例子,文档的结构有:一个标题、几个字符串、条目项目。
示例程序中有如下四个类:Builder类、Director类、TextBuilder类、HTMLBuilder类。
Builder类,建造者,定义了编写文档的方法。Builder类是抽象类,仅仅是定义了方法,具体处理交给子类。
Director类使用该方法编写一个具体的文档,监工,负责使用建造者的类来生成实例。
Builder类的两个子类TextBuilder类和HTMLBuilder类,两个具体的建造者,TextBuilder编写纯文本文档,HTMLBuilder类编写HTML文档。
Builder.java 定义了编写文档的方法,如下所示:
package builder;
public abstract class Builder {
public abstract void makeTitle(String title);
public abstract void makeString(String str);
public abstract void makeItems(String[] items);
public abstract void close();
}
Director.java,监工,负责使用建造者的类来生成实例:
package builder;
public class Director {
public Builder builder;
public Director(Builder builder) {
this.builder = builder;
}
// 编写文档
public void construct() {
builder.makeTitle("Hello Builder");
builder.makeString("从早上到下午");
builder.makeItems(new String[] {
"早上好",
"中午好",
"下午好"
});
builder.makeString("晚上");
builder.makeItems(new String[] {
"晚上好",
"晚安",
"再见"
});
builder.close();
}
}
TextBuilder.java,具体建造者类,getResult()返回建造的结果。
package builder;
public class TextBuilder extends Builder{
private StringBuffer buffer = new StringBuffer();
@Override
public void makeTitle(String title) {
buffer.append("==================================\n");
buffer.append("[" + title + "]\n");
buffer.append("\n");
}
@Override
public void makeString(String str) {
buffer.append('*' + str + "\n");
buffer.append("\n");
}
@Override
public void makeItems(String[] items) {
for (int i=0; i<items.length; i++) {
buffer.append(" ." + items[i] + "\n");
}
}
@Override
public void close() {
buffer.append("==================================\n");
}
public String getResult() {
return buffer.toString();
}
}
HTMLBuilder.java,具体建造者类,getResult()返回建造的结果。
package builder;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
public class HTMLBuilder extends Builder{
private String filename;
private PrintWriter writer;
@Override
public void makeTitle(String title) {
filename = title + ".html";
try {
writer = new PrintWriter(new FileWriter(filename));
} catch (IOException e) {
e.printStackTrace();
}
writer.println("<html><head><title>" + title + "</title></head></html>");
writer.println(title);
}
@Override
public void makeString(String str) {
writer.println("<p>" + str + "</p>");
}
@Override
public void makeItems(String[] items) {
writer.println("<ul>");
for (int i=0; i<items.length; i++) {
writer.println("<li>" + items[i] + "</li>");
}
writer.println("</ul>");
}
@Override
public void close() {
writer.println("</body></html>");
writer.close();
}
public String getResult() {
return filename;
}
}
使用者类,User.java。
package builder;
public class User {
public static void main(String args[]) {
TextBuilder textBuilder = new TextBuilder();
Director director = new Director(textBuilder);
director.construct();
String result = textBuilder.getResult();
System.out.println(result);
HTMLBuilder htmlBuilder = new HTMLBuilder();
Director director01 = new Director(htmlBuilder);
director01.construct();
}
}
运行截图如下所示:
参考文献:
- 图解设计模式 - 结成浩著、杨文轩译 - 人民邮电出版社