java-对象的创建

一、考虑用静态工厂方法来替代构造器

• 静态工厂与设计模式中的工厂方法模式不同
• 与构造器比第一大优势：他们有具体的名称，而不是类名
• 与构造器比第二大优势：不必在每次调用它的时候都创建一个新对象
• 与构造器比第三大优势：它可以返回原返回类型的任何子类型的对象
• 与构造器比第四大优势：在创建参数化实例时，可以使代码变得更加简洁

举例如下：

public static Boolean valueof(boolean a){
		return a?Boolean.TRUE:Boolean.FALSE;
	}

二、服务提供者框架

• 服务接口：提供者实例
• 提供者注册API：系统用来注册实现，让客服端访问它们
• 服务访问API：客服端用来获取服务的实例，是一个灵活的静态工厂

举例如下：

public interface Service{
		//service-specific methods go here
	}
	
	public interface Provider{
		Service newService();
	}
	
	public  class Services{
		private Services(){}
		
		private static final Map<String, Provider> providers=new ConcurrentHashMap<String, Provider>();
		public static final String DEFAULT_PROVIDER_NAME="<def>";
		
		//provider registration API
		public static void  registerDefaultProvider(Provider p){
			registerProvider(DEFAULT_PROVIDER_NAME, p);
		}
		public static void registerProvider(String name,Provider p){
			providers.put(name, p);
		}
		
		//service access API
		public static Service newInstance(){
			return newInsance(DEFAULT_PROVIDER_NAME);
		}
		public static Service newInsance(String name){
			Provider provider= providers.get(name);
			if(provider==null){
				throw new IllegalArgumentException("No provider registered with name:"+ name);
			}
			return provider.newService();
		}
	}

三、Builder 模式

• 当遇到多个构造器参数时考虑使用构建器
• 与重叠构造器模式、JavaBean模式相比，既能保证像重叠构造器模式那样安全，又能保证JavaBeans模式那么好的可读性，而且也更易于扩展

重叠构造器模式举例：

public class NutritionFacts
{
	private  int servingSize; //required;
	private  int servings;    //required;
	private int calorier;    //optioned;
	private  int fat;         //optioned;
	private int sodium;      //optioned;
	private  int carbohydrate;//optioned;
	
	public NutritionFacts(int servingSize,int servings){
		this(servingSize, servingSize, 0);
	}
	
	public NutritionFacts(int servingSize,int servings,int calorier){
		this(servingSize, servings, calorier,0);
	}
	
	public NutritionFacts(int servingSize,int servings,int calorier,int fat){
		this(servingSize, servings, calorier,fat,0,0);
	}
	
	public NutritionFacts(int servingSize,int servings,int calorier,int fat,int sodium){
		this(servingSize, servings, calorier,fat, sodium,0);
	}
	
	public NutritionFacts(int servingSize,int servings,int calorier,int fat,int sodium,int carbohydrate){
		this.servingSize=servingSize;
		this.servings=servings;
		this.calorier=calorier;
		this.fat=fat;
		this.sodium=sodium;
		this.carbohydrate=carbohydrate;
	}
}

javaBeans模式举例：

public class NutritionFacts
{
	private  int servingSize; //required;
	private  int servings;    //required;
	private int calorier;    //optioned;
	private  int fat;         //optioned;
	private int sodium;      //optioned;
	private  int carbohydrate;//optioned;
	
	public NutritionFacts(){}

	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 getCalorier()
	{
		return calorier;
	}

	public void setCalorier(int calorier)
	{
		this.calorier = calorier;
	}

	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;
	}
	
	
}

Builder Pattern举例：

public class NutritionFacts
{
	private  int servingSize; //required;
	private  int servings;    //required;
	private  int calorier;    //optioned;
	private   int fat;         //optioned;
	private int sodium;      //optioned;
	private   int carbohydrate;//optioned;
	
	public static class Builder{
		private int servingSize;
		private int servings;
		
		private int calorier=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){
			calorier=val;
			return this;
		}
		public Builder fat(int val){
			fat=val;
			return this;
		}
		public Builder carhohydrate(int val){
			carbohydrate=val;
			return this;
		}
		public Builder sodium(int val){
			sodium=val;
			return this;
		}
		
		public NutritionFacts build(){
			return new NutritionFacts(this);
		}
	}
	
	private NutritionFacts(Builder builder){
		servingSize=builder.servingSize;
		servings=builder.servings;
		calorier=builder.calorier;
		fat=builder.fat;
		sodium=builder.sodium;
		carbohydrate=builder.carbohydrate;
	}
	
	public static void main(String[] args)
	{
		NutritionFacts coca=new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carhohydrate(27).build();
		
		System.out.println(coca.servingSize+";"+coca.servings+";"+coca.calorier+";"+coca.fat+";"+coca.sodium+";"+coca.carbohydrate);
	}
}