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当一个类有大量的可选属性是,要考虑用构建器替代构造器,构造器模式会让类的实例化变得太过复杂。
构建器可以见名之意,并且有很大的灵活性。
构建器构造对象不直接生成想要的对象,而是让客户端利用所有必要的参数调用构造器(或者静态工厂),
得到一个builder对象。然后客户端在builder对象上调用类似于setter的方法,来设置每个相关的可选参数。
最后,客户端调用无参的build方法来生成不可变的对象。这个builder是它构建的类的静态成员类。
示例:
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; /** * 这里必须用静态方法,否则在没有外部类的情况下无法新建内部类 */ public static class Builder { //必选属性 private final int servingSize; private final int servings; //可选属性 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; } public Builder fat(int val) { fat = val; return this; } //内部类可以访问外部类的私有成员 public NutritionFacts build() { return new NutritionFacts(this); } } /** * 外部类可以访问内部类的私有属性 */ private NutritionFacts(Builder builder) { servingSize = builder.servingSize; servings = builder.servings; calories = builder.calories; fat = builder.fat; sodium = builder.sodium; carbohydrate = builder.carbohydrate; } public String toString() { return "[" + servingSize + "," + servings + "," + calories + "," + fat + "," + sodium + "," + carbohydrate + "]"; }}
public class Main { public static void main(String[] args){ /** * 初始化时,能够见名之意,并且可选属性可以连环赋值,避免了像setter方法一样需要很长的代码。 */ NutritionFacts nutritionFacts=new NutritionFacts.Builder(10,20).calories(30).sodium(35).build(); System.out.println(nutritionFacts.toString()); }}
[10,20,30,0,35,0]
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