《Effective Java》笔记 ：（一）创建和销毁对象
一 .考虑用静态工厂方法代替构造器
1. 静态工厂方法与设计模式中的工厂方法模式不同，注意不要混淆
例子：
public static Boolean valueOf(boolean b){
return b ? Boolean.TRUE : Boolean.FALSE;
}
2. 采用静态工厂方法比构造器方法的优势
①有名称
② 不必在每次调用他们的时候都创建一个新的对象
③可以返回原类型的任何子类型的对象
④在创建参数化实例的时候，使代码更加简洁
比如
Map<String,List<String>> m = new HashMap<String,List<String>>();
当参数越多的时候会越复杂，可以考虑用静态工厂方法
public static<K,V> HashMap<K,V> newInstance(){
return new HashMap<K,V>();
}
Map<String,List<String>> m = HashMap.newInstance();
3.采用静态工厂方法的缺点：类如果不含公有的或者受保护的构造器，就不能被子类化；与其他的静态方法实质上没区别
静态工厂方法惯用名词：valueOf/of/getInstance/newInstance/getType/newType
二.遇到多个构造器参数时考虑用构建器（建造器Builder）
重构构造器模式可行，但是参数很多的时候，客户端代码很难编写，容易出错，且较难阅读维护
替代办法一： 使用JavaBeans模式，调用无参构造器创建对象，然后调用setter设置参数
public class NutritionFacts {
// Parameters initialized to default values (if any)
private int servingSize = -1; // Required; no default value
private int servings = -1; // " " " "
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);
}
}
这种办法有缺陷：① 构造过程被分到了几个调用中，在构造过程中JavaBean可能处于不一致的状态 ②JavaBeans模式阻止了把类做成不可变的可能
替代办法二：使用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 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();
}
}
三.用私有构造器或者枚举类型强化Singleton属性
单元素的枚举类型已经成为实现Singleton的最佳方法
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(); } }
四.通过私有构造器强化不可实例化的能力
企图通过将类做成抽象类来强制该类不可实例化，是行不通的 --可以子类化，子类可以实例化
让这个类包含私有构造器，就不能实例化了
public class UtilityClass { // Suppress default constructor for noninstantiability private UtilityClass() { throw new AssertionError(); } }
五.避免创建不必要的对象
String s = new String("hello")------->两个实例
换成
String s = "hello"; ------> 一个实例
优先使用静态工厂方法，而不是构造器，避免创建不必要的对象
public class Person {
private final Date birthDate;
public Person(Date birthDate) {
// Defensive copy - see Item 39
this.birthDate = new Date(birthDate.getTime());
}
// Other fields, methods omitted
// DON'T DO THIS!
public boolean isBabyBoomer() {
// Unnecessary allocation of expensive object
Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
Date boomStart = gmtCal.getTime();
gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
Date boomEnd = gmtCal.getTime();
return birthDate.compareTo(boomStart) >= 0
&& birthDate.compareTo(boomEnd) < 0;
}
}
//上面的程序每次被调用都会创建Date ,Calendar,TimeZone没必要
//下面的程序使用了一个静态初始化器，避免了效率低下
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;
}
}
优先使用基本类型而不是装箱基本类型，当心无意识的自动装箱
//声明成Long而不是long，意味着构造了很多Long实例。应该改成long
public class Sum {
// Hideously slow program! Can you spot the object creation?
public static void main(String[] args) {
Long sum = 0L;
for (long i = 0; i < Integer.MAX_VALUE; i++) {
sum += i;
}
System.out.println(sum);
}
}
六.消除过期的对象引用
//下面的例子存在着内存泄漏的可能，因为过期引用并没有被回收
public class Stack {
private Object[] elements;
private int size = 0;
private static final int DEFAULT_INITIAL_CAPACITY = 16;
public Stack() {
elements = new Object[DEFAULT_INITIAL_CAPACITY];
}
public void push(Object e) {
ensureCapacity();
elements[size++] = e;
}
public Object pop() {
if (size == 0)
throw new EmptyStackException();
return elements[--size];
}
/**
* Ensure space for at least one more element, roughly doubling the capacity
* each time the array needs to grow.
*/
private void ensureCapacity() {
if (elements.length == size)
elements = Arrays.copyOf(elements, 2 * size + 1);
}
}
//修复方法
public Object pop() {
if (size == 0)
throw new EmptyStackException();
Object result = elements[--size];
elements[size] = null;
return result;
}
内存泄漏的另一个常见来源是缓存，处理方法是使用WeakHashMap代表缓存
内存泄漏的第三个常见来源是监听器和其他回调，，处理办法是保存他们的弱引用，例如只将它们保存成WeakHashMap中的键
七.避免使用终结方法
而使用显式终止方法，比如InputStream/OutputStream和java.sql.Connection上的close()方法。
通常与try-finally结合使用，在finally子句中调用显式终止方法。
