创建和销毁对象
2016-05-20 23:44
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考虑用静态工厂方法代替构造器
静态工厂方法与构造器第一大优势在于,静态工厂方法有名称,当有多个构造器时,常常因为构造器名字相同而不知道调用那个构造器第二大优势在于静态工厂方法不用每次创建一个对象
第三大优势在于可以返回原返回类型的子类型的对象
第四大优势在于,创建参数化实例类时,他们使代码变得更简介,而在调用参数化类的构造器时,你必须提供两次类型参数(参数化类,即带泛型的类,用类型来传递参数类型)
//参数化类构造器 Map<String,List<String>> m=new HashMap<String,List<String>>(); //假设该类提供如下方法 public static<K,V> HashMap<K,V> newInstance(){ return new HashMap<K,V>(); } //是不是以下代码就简洁多了 Map<String,List<String>> m=HashMap.newInstance();
静态工厂方法也有如下缺点
类不含有公有的或受保护的(protected)构造器,就不能被子类化静态工厂方法和其他静态方法名字区别不大,但可通过定义命名规范解决
遇到多个构造器参数时要考虑构建器
当有多个参数时,重叠构造器模式可行,但是会很难编写,难以阅读javaBean也是一种方法(即提供一个无参构造器,用set方法来设置参数),但是javaBean,把阻止了把类变成不可变的可能,需要确保线程安全
BUilder模式(构建器模式)
public class NutritionFacts { private final int servingSize;//食用量 private final int servings;//可供食用人数 private final int calories;//卡路里 private final int fat; private final int sodium;//钠 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 carbohydrate = 0; private int sodium = 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 carbohydrate(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; 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(); } }
用私有构造器和枚举类强化 Singleton(单例)属性
私有构造器枚举强化
public enum Elvis { INSTANCE; public void leaveTheBuilding() { System.out.println("Whoa baby, I'm outta here!"); } // This code would normally appear outside the class! public static void main(String[] args) { Elvis elvis = Elvis.INSTANCE; elvis.leaveTheBuilding(); } }
###避免创建不必要的对象
String str=new String("aa")每次执行都会创建对象,最好用String str="sdsd"
用static来加载只需加载一次的资源
class Person { private final Date birthDate; public Person(Date birthDate) { // Defensive copy - see Item 39 this.birthDate = new Date(birthDate.getTime()); } // Other fields, methods /** * The starting and ending dates of the baby boom. */ private static final Date BOOM_START; private static final Date BOOM_END; static { Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT")); gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0); BOOM_START = gmtCal.getTime(); gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0); BOOM_END = gmtCal.getTime(); } public boolean isBabyBoomer() { return birthDate.compareTo(BOOM_START) >= 0 && birthDate.compareTo(BOOM_END) < 0; } }
消除过期对象
有些过期对象不会被回收器回收,这时候需要你手动置为null
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