重叠构造器模式
// Telescoping constructor pattern
public class NutritionFacts {
private final int servingSize; // (mL) required
private final int servings; // (per container) required
private final int calories; // (per serving) optional
private final int fat; // (g/serving) optional
private final int sodium; // (mg/serving) optional
private final int carbohydrate; // (g/serving) optional
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) {
this.servingSize = servingSize;
this.servings = servings;
this.calories = calories;
this.fat = fat;
this.sodium = sodium;
this.carbohydrate = carbohydrate;
}
public static void main(String[] args) {
NutritionFacts cocaCola =
new NutritionFacts(240, 8, 100, 0, 35, 27);
}
}
重叠构造器的弊端
JavaBeans模式
// JavaBeans Pattern - allows inconsistency, mandates mutability
public class NutritionFacts {
// Parameters initialized to default values (if any)
private int servingSize = -1; // Required; no default value
private int servings = -1; // Required; no default value
private int calories = 0;
private int fat = 0;
private int sodium = 0;
private int carbohydrate = 0;
public NutritionFacts() { }
// Setters
public void setServingSize(int val) {
servingSize = val;
}
public void setServings(int val) {
servings = val;
}
public void setCalories(int val) {
calories = val;
}
public void setFat(int val) {
fat = val;
}
public void setSodium(int val) {
sodium = val;
}
public void setCarbohydrate(int val) {
carbohydrate = val;
}
public static void main(String[] args) {
NutritionFacts cocaCola = new NutritionFacts();
cocaCola.setServingSize(240);
cocaCola.setServings(8);
cocaCola.setCalories(100);
cocaCola.setSodium(35);
cocaCola.setCarbohydrate(27);
}
}
JavaBeans模式的弊端
- 在构造过程中，JavaBeans模式可能处于不一致的状态。
- JavaBeans模式使得把类做成不可变的可能性不复存在。
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;
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;
}
public static void main(String[] args) {
NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8)
.calories(100).sodium(35).carbohydrate(27).build();
}
}
Builder模式的优势
- 代码容易编写，易于阅读。
- Builder模式模拟了具名的可选参数。
- Builder模式也适用于类层次模式。
Pizza class
import java.util.*;
// Builder pattern for class hierarchies
// Note that the underlying "simulated self-type" idiom allows for arbitrary fluid hierarchies, not just builders
public abstract class Pizza {
public enum Topping {HAM, MUSHROOM, ONION, PEPPER, SAUSAGE}
final Set toppings;
abstract static class Builder> {
EnumSet 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
}
}
NyPizza class
import java.util.Objects;
// Subclass with hierarchical builder
public class NyPizza extends Pizza {
public enum Size {SMALL, MEDIUM, LARGE}
private final Size size;
public static class Builder extends Pizza.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;
}
@Override
public String toString() {
return "New York Pizza with " + toppings;
}
}
Calzone class
// Subclass with hierarchical builder
public class Calzone extends Pizza {
private final boolean sauceInside;
public static class Builder extends Pizza.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;
}
@Override
public String toString() {
return String.format("Calzone with %s and sauce on the %s",
toppings, sauceInside ? "inside" : "outside");
}
}
PizzaTest class
// Using the hierarchical builder (Page 16)
public class PizzaTest {
public static void main(String[] args) {
NyPizza pizza = new NyPizza.Builder(SMALL)
.addTopping(SAUSAGE).addTopping(ONION).build();
Calzone calzone = new Calzone.Builder()
.addTopping(HAM).sauceInside().build();
System.out.println(pizza);
System.out.println(calzone);
}
}
与构造器相比
- 它可以有多个可变参数
- 可以利用单个builder构建多个对象。
Builder模式的弊端
- 对了创建对象，必须先创建它的构建器。具有性能开销。
- 比重叠构造器模式更加冗长。
简而言之，如果累的构造器或者静态工厂中具有多个参数（4个或以上），设计这种类时，Builder模式是一种不错的选择。
