1. 概述
建造者(Builder)設計模式用於組裝具有複雜結構的實例。在構建一個複雜實例的時候,我們很難一氣呵成,我們首先把實例的各個部分建造出來,然後分階段把它們組裝起來。
2. 示例程序
一個使用Builder設計模式來編寫“文檔的程序”的例子,文檔的結構有:一個標題、幾個字符串、條目項目。
示例程序中有如下四個類:Builder類、Director類、TextBuilder類、HTMLBuilder類。
Builder類,建造者,定義了編寫文檔的方法。Builder類是抽象類,僅僅是定義了方法,具體處理交給子類。
Director類使用該方法編寫一個具體的文檔,監工,負責使用建造者的類來生成實例。
Builder類的兩個子類TextBuilder類和HTMLBuilder類,兩個具體的建造者,TextBuilder編寫純文本文檔,HTMLBuilder類編寫HTML文檔。
Builder.java 定義了編寫文檔的方法,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,監工,負責使用建造者的類來生成實例:
package builder;
public class Director {
public Builder builder;
public Director(Builder builder) {
this.builder = builder;
}
// 編寫文檔
public void construct() {
builder.makeTitle("Hello Builder");
builder.makeString("從早上到下午");
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",
"再見"
});
builder.close();
}
}
TextBuilder.java,具體建造者類,getResult()返回建造的結果。
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,具體建造者類,getResult()返回建造的結果。
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;
}
}
使用者類,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();
}
}
運行截圖如下所示:
參考文獻:
- 圖解設計模式 - 結成浩著、楊文軒譯 - 人民郵電出版社