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 parametersprivate final int servingSize;private final int servings;// Optional parameters - initialized to default valuesprivate 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; // Defaultpublic 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();