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静态工厂和构造器都有个共同的局限性，就是它们都不能够很好的扩展到大量的可选参数。
如：
public class NutritionFacts{ private final int servingSize; private final int servings; private final int calories; private final int fat; private final int sodium; private final int carbohydrate; public NutritionFacts(int servingSize, int servings){ this(servingSize, servings, 0); } public NutritionFacts(int srvingSize, int servings, int calories){ this(servingSize, servings, calories, fat, 0); } public NutritionFacts(int servingSize, int servings, int calories, int fat ){ this(servingsSize, servings, calories, fat, 0); } public NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium){ this(servingsSize, servings, calories, fat, sodium,0); } public NutritionFacts(int servingSize, int servings, int calories, int fat, int sodium, int carbohydrate){ this.servingSize = servingSize; this.servings = servings; this.calories = caloriers; this.fat = fat; this.sodium = sodium; this.carbohydrate = carbohydrate; } }
我们可以看到利用构造器方式，有多个参数时，很难控制其灵活性。因而我们选择尝试给每个值设置默认值：
public class NutritionFacts{ private int servingSize = -1; private int servings = -1; private int calories = 0; private int fat = 0; private int sodium = 0; private int carbohydrate = 0; public NutritionFacts(){ // setters public void setServingSize(int val){ servingSize = val; } public void setServings(int val){ servings = val; } //.... } }
调用初始化
NutritionFacts cocaCola = new NutritionFacts(); cocaCola.setServingSize(240); cocaCola.setServings(8); cocaCola.setCalories(100); cocaCola.setSodium(35); cocaCola.setCarbohydrate(27);
这是常见的javaBean模式，通过setter方法来构建对象，但是这也阻止了类做成不可变的可能，会有多线程安全问题。
那么我们来看看另外一种方式：
public class NutritionFacts{ private final int servingSize; private final int srvings; private final int calories; private final int fat; private final int sodium; private final int carbohydrate;
// 静态内部类 public static class Builder{
// 必须元素 private final int servingSize; private final int servings; // 默认元素 private int calories = 0; private int fat = 0; private int carbohydrate = 0; private int sodium = 0; public Builder(int servingSize, int servings){ this.servingSize = servingSize; this.servings = servings; } public Builder calories(int val){ calories = val; return this; } public Builder carbohydrate(int val){ carbohydrate = val; return this; } public Builder sodium(int val){ sodium = val; return this; }
// 在内部类中返回外部类对象的接口 public NutritionFacts build(){ return new NutritionsFacts(this); } } private NutritionFacts(Builder builder){ servingSize = builder.servingSize; servings = builder.servings; calories = builder.calories; fat = builder.fat; sodium = builder.sodium; carbodhydrate = builder.carbodhydrate; } }
调用方式：
NutritionFacts fact = new NutritionFacts.builder(240,8).calories(100).sodium(46).carbohydrate(27).build();
这样通过builder模式，实现了可选参数构建对象实例。
第二条 遇到多个构造器参数时，要考虑用构建器,码迷,mamicode.com
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原文地址：http://www.cnblogs.com/goupb/p/3695525.html
