Effective Java~2.Builder代替多参数Constructor

Builder模式

// Builder Pattern
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 static class Builder {
		// Required parameters
		private final int servingSize;
		private final int servings;
		// Optional parameters - initialized to default values
		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 sodium(int val) {
			sodium = val;
			return this;
		}
		
		public Builder carbohydrate(int val) {
			carbohydrate = val;
			return this;
		}
		
		public NutritionFacts build() {
			return new NutritionFacts(this);
		}
	}
	
	private NutritionFacts(Builder builder) {
		servingSize = builder.servingSize;
		servings = builder.servings;
		calories = builder.calories;
		fat = builder.fat;
		sodium = builder.sodium;
		carbohydrate = builder.carbohydrate;
	}
}

调用示例

NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
			.calories(100)
			.sodium(35)
			.carbohydrate(27)
			.build();

Builder 模式非常适合类层次结构

// Builder pattern for class hierarchies
import java.util.EnumSet;
import java.util.Objects;
import java.util.Set;

public abstract class Pizza {
	public enum Topping {HAM, MUSHROOM, ONION, PEPPER, SAUSAGE}
	
	final Set<Topping> toppings;
	
	abstract static class Builder<T extends Builder<T>> {
		EnumSet<Topping> toppings = EnumSet.noneOf(Topping.class);
		
		public T addTopping(Topping topping) {
			toppings.add(Objects.requireNonNull(topping));
			return self();
		}
		
		abstract Pizza build();
		
		// Subclasses must override this method to return "this"
		protected abstract T self();
	}
	
	Pizza(Builder<?> builder) {
		toppings = builder.toppings.clone(); // See Item 50
	}
}

这里有两个具体的 Pizza 的子类，其中一个代表标准的纽约风格的披萨，另一个是半圆形烤乳酪馅饼

import java.util.Objects;

public class NyPizza extends Pizza {
	public enum Size { SMALL, MEDIUM, LARGE }
	private final Size size;
	
	public static class Builder extends Pizza.Builder<Builder> {
		private final Size size;
		
		public Builder(Size size) {
			this.size = Objects.requireNonNull(size);
		}
		
		@Override 
		public NyPizza build() {
			return new NyPizza(this);
		}
		
		@Override 
		protected Builder self() {
			return this;
		}
	}
	
	private NyPizza(Builder builder) {
		super(builder);
		size = builder.size;
	}
}

public class Calzone extends Pizza {
	private final boolean sauceInside;
	
	public static class Builder extends Pizza.Builder<Builder> {
		private boolean sauceInside = false; // Default
		
		public Builder sauceInside() {
			sauceInside = true;
			return this;
		}
		
		@Override 
		public Calzone build() {
			return new Calzone(this);
		}
		@Override 
		protected Builder self() {
			return this;
		}
	}
	
	private Calzone(Builder builder) {
		super(builder);
		sauceInside = builder.sauceInside;
	}
}

调用示例

NyPizza pizza = new NyPizza.Builder(SMALL)
	.addTopping(SAUSAGE)
	.addTopping(ONION)
	.build();

Calzone calzone = new Calzone.Builder()
	.addTopping(HAM)
	.sauceInside()
	.build();