java-创建和销毁对象（二）

第2条：遇到多个构造器参数时要考虑用构造器

静态工厂和构造器有个共同的局限性：它们都不能很好地扩展到大量的可选参数。

那么有大量可选参数时该如何构造对象呢？

 

第一种方法：重叠构造器（telescope constructer）当你想要创建实例的时候，就利用参数列表最短的构造器，其它的值会赋默认值。

 

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 servingSize, int servings, int calories) {
	    this(servingSize, servings,calories,0);
	}
	public NutritionFacts(int servingSize, int servings, int calories, int fat) {
		this(servingSize,servings,calories,fat,0);
	}
	public NutritionFacts(int servingSize, int servings, int calories, int fat,
			int sodium) {
		this(servingSize,servings,calories,fat,sodium,0);

	}
	public NutritionFacts(int servingSize, int servings, int calories, int fat,
			int sodium, int carbohydrate) {
		super();
		this.servingSize = servingSize;
		this.servings = servings;
		this.calories = calories;
		this.fat = fat;
		this.sodium = sodium;
		this.carbohydrate = carbohydrate;
	}
	
}

 

缺点：如果参数多了的话，就失去了控制。

 

第2中：JavaBean模式。

 

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 int getServingSize() {
		return servingSize;
	}
	public void setServingSize(int servingSize) {
		this.servingSize = servingSize;
	}
	public int getServings() {
		return servings;
	}
	public void setServings(int servings) {
		this.servings = servings;
	}
	public int getCalories() {
		return calories;
	}
	public void setCalories(int calories) {
		this.calories = calories;
	}
	public int getFat() {
		return fat;
	}
	public void setFat(int fat) {
		this.fat = fat;
	}
	public int getSodium() {
		return sodium;
	}
	public void setSodium(int sodium) {
		this.sodium = sodium;
	}
	public int getCarbohydrate() {
		return carbohydrate;
	}
	public void setCarbohydrate(int carbohydrate) {
		this.carbohydrate = carbohydrate;
	}
}
/***客服端代码**/
public static void main(String[] args) {
		NutritionFacts cocaCola=new NutritionFacts();
		cocaCola.setServingSize(240);
		cocaCola.setServings(8);
		cocaCola.setCalories(100);
		
}

 

缺点：构造过程被分到了几个调用中，在构造过程中JavaBean可能处于不一致的状态。

 

第3种  Builder模式。

 

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;
	
	/**
	 * 静态内部类
	 * @author weix
	 *
	 */
	public static class Builder{
		private final int servingSize;
		private final int servings;
		private  int calories=0;
		private  int fat=0;
		private  int sodium=0;
		private  int carbohydrate=0;
		
		public Builder(int servingSize, int servings) {
			this.servingSize = servingSize;
			this.servings = servings;
		}
		
		public Builder calories(int val){
			calories=val;
			return this;
		}
		public Builder fat(int val){
			fat=val;
			return this;
		}
		public Builder carbohydrate(int val){
			carbohydrate=val;
			return this;
		}
		public Builder sodium(int val){
			sodium=val;
			return this;
		}
		
		public NutritionFacts build(){
			return new NutritionFacts(this);
		}

		
	}
	
	public NutritionFacts(Builder builder) {
		this.servingSize=builder.servingSize;
		this.servings=builder.servings;
		this.calories=builder.calories;
		this.fat=builder.fat;
		this.sodium=builder.sodium;
		this.carbohydrate=builder.carbohydrate;
	}
/****客户端****/
	public static void main(String[] args) {
		NutritionFacts cocaCola=new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carbohydrate(27).build();
	}

}

 

缺点：为了创建对象，必须先创建他的构造器，在十分注重性能的情况下，可能成问题。比重叠构造器模式更加冗长，因此他只在有很多参数的时候才使用4个或者更多。