设计模式:建造模式
来源：程序员人生 发布时间：2015-01-14 09:11:50 阅读次数：3856次
原文地址:http://leihuang.org/2014/12/03/builder/
Creational 模式
物件的产生需要消耗系统资源，所以如何有效力的产生、管理 与操作物件，1直都是值得讨论的课题， Creational 模式即与物件的建立相干，在这个分类下的模式给出了1些指点原则及设计的方向。下面罗列到的全属于Creational 模式
- Simple Factory 模式
- Abstract Factory 模式
- Builder 模式
- Factory Method 模式
- Prototype 模式
- Singleton 模式
- Registry of Singleton 模式
出现的问题
The builder pattern is a good choice when designing classes whose constructors or static factories would have more than a handful of parameters.
当构造函数的参数非常多时，并且有多个构造函数时，情况以下：
Pizza(int size) { ... }
Pizza(int size, boolean cheese) { ... }
Pizza(int size, boolean cheese, boolean pepperoni) {...}
Pizza(int size, boolean cheese, boolean pepperoni ,boolean bacon) { ... }
This is called the Telescoping Constructor Pattern. 下面还有1种解决方法叫javabean模式:
Pizza pizza = new Pizza(12);
pizza.setCheese(true);
pizza.setPepperoni(true);
pizza.setBacon(true);
Javabeans make a class mutable even if it is not strictly necessary. So javabeans require an extra effort in handling thread-safety(an immutable class is always thread safety!).详见：JavaBean
Pattern
建造模式解决该问题
<span style="font-weight: normal;">public class Pizza {
// 必须的
private final int size;
// 可有可无的
private boolean cheese;
private boolean pepperoni;
private boolean bacon;
public static class Builder {
// required
private final int size;
// optional
private boolean cheese = false;
private boolean pepperoni = false;
private boolean bacon = false;
//由于size时必须的,所以设置为构造函数的参数
public Builder(int size) {
this.size = size;
}
public Builder cheese(boolean value) {
cheese = value;
return this;
}
public Builder pepperoni(boolean value) {
pepperoni = value;
return this;
}
public Builder bacon(boolean value) {
bacon = value;
return this;
}
public Pizza build() {
return new Pizza(this);
}
}
private Pizza(Builder builder) {
size = builder.size;
cheese = builder.cheese;
pepperoni = builder.pepperoni;
bacon = builder.bacon;
}
}</span>
注意到现在Pizza是1个immutable class，所以它是线程安全的。又由于每个Builder's setter方法都返回1个Builder对象，所以可以串连起来调用。以下
Pizza pizza = new Pizza.Builder(12)
.cheese(true)
.pepperoni(true)
.bacon(true)
.build();
由上面我们可以知道,建造模式,给了用户更大的自由,可以任选参数.
2014⑴1-09 18:21:17
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