除了使用构造函数,一个类可以通过提供静态工厂方法,返回自身实例,如:
public static Boolean valueOf(boolean b) {
return b ? Boolean.TRUE : Boolean.FALSE;
}
使用静态工厂方法而非构造函数创建对象,主要有以下优点:
- 静态工厂方法有自己的方法名
- 静态工厂方法不必在每次被调用时都创建新对象
- 静态工厂方法可以返回本类的任意子类型
- 静态工厂方法可以减少创建范型实例时的复杂性
Map<String, List<String>> m = new HashMap<String, List<String>>();
可以用如下方式代替(HashMap.java中无此方法,只为举例):
public static <K, V> HashMap<K, V> newInstance() {
return new HashMap<K, V>();
}
注 现在来看并不需要这种方法,从Java 7开始,范型实例初始化可直接省略实例中的范型声明,即:
Map<String, List<String>> m = new HashMap<>();
同时有以下缺点:
- 静态工厂方法的使用一般都会将构造方法设为private,导致该类无法被继承
- 静态工厂方法和该类的其他静态方法容易混淆,不好区分
// There is no default constructor available in 'java.util.Collections'
class SubCollections extends Collections {
}
常用静态工厂方法名称:valueOf
, of
, getInstance
, newInstance
, getType
, newType
一个类如果有多个参数,如何构造?两种方式:
- 使用“望远镜”型构造函数(构造函数中赋值成员变量的个数依次增加)
- 使用JavaBeans依次调用成员变量的set方法
“望远镜”型构造函数的缺点是可读性太差;JavaBeans#set方法的缺点是在多线程情况下可能存在线程安全问题。此时非常适合通过Builder创建实例:
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;
private 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;
}
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;
private Builder(int servingSize, int servings) {
this.servingSize = servingSize;
this.servings = servings;
}
public Builder calories(int calories) {
this.calories = calories;
return this;
}
public Builder fat(int fat) {
this.fat = fat;
return this;
}
public Builder sodium(int sodium) {
this.sodium = sodium;
return this;
}
public Builder carbohydrate(int carbohydrate) {
this.carbohydrate = carbohydrate;
return this;
}
public NutritionFacts build() {
return new NutritionFacts(this);
}
}
public static Builder builder(int servingSize, int servings) {
return new Builder(servingSize, servings);
}
}
注 lombok可以通过注解方式自动生成类对应的builder:
@lombok.Builder
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;
}
Builder模式有一个缺点是在创建类实例时,需要先创建一个Builder实例,在某些对性能要求极高的场景下可能会存在问题。
补充:抽象工厂(泛型)
public interface Builder<T> {
T build();
}
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() {}
public static Elvis getInstance() {
return INSTANCE;
}
public void leaveTheBuilding() {}
}
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() {}
public static Elvis getInstance() {
return INSTANCE;
}
}
貌似是不需要的现在,JIT会做调优