Item 4：避免创建重复的对象

 

import java.util.*;

public class Person {
    /**
     *重复使用同一个对象，不要每次需要时就创建一个功能上等价的新对象
     *若一个对象是非可变的（immutable），那么它总是可以被重用
     *对于同时提供了静态工厂方法和构造函数的非可变类，通常可利用前者避免创建重复对象
     *也可以重用那些已知不回修改的可变对象(如下)
     */
    privatefinal Date birthDate;
    public Person(Date birthDate){
       this.birthDate = birthDate;
    }
    //Don't do this!
    /*
    public boolean isBabyBoomer(){
       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;
    }
    */

    //Do like this,该版本的Person类只在初始化时刻创建Calendar,TimeZone,Date实例一次，
    //而不是每次调用isBabyBoomer时创建它们
    //好处：1。性能提升 2。日期被作为常量对待，代码更容易理解。
    //the starting and ending dates of the baby boom.
    privatestaticfinal Date BOOM_START;
    privatestaticfinal 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();
    }
    publicboolean isBabyBoomer(){
       returnbirthDate.compareTo(BOOM_START)>=0 && birthDate.compareTo(BOOM_END)<0;
    }   

    //Entry
    publicstaticvoid main(String[] args) {
        Person p = new Person(new Date());
        long startTime = System.currentTimeMillis();
        for (int i=0; i<1000000; i++)
            p.isBabyBoomer();
        long endTime = System.currentTimeMillis();
        long time = endTime - startTime;
        System.out.println(time+" ms.");
    }
}