一个静态初始化器的应用--避免创造不必要的对象

来源：互联网发布：vim设置python语法高亮编辑：程序博客网时间：2024/05/02 02:32

亲测

package com.zxm;import java.text.SimpleDateFormat;import java.util.Calendar;import java.util.Date;import java.util.TimeZone;/** * Creator:zxm * Date:2017/2/23 * Description: */public class Person {    private final Date birthDate = new Date(1991,11,28,0,0,0);    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;    }    public static void main(String[] args) {        Date start = new Date();        Person p = new Person();        for (int i = 0; i < 10000000; i++) {            p.isBabyBoomer();        }        Date end = new Date();        long sec = end.getTime() - start.getTime();        System.out.println(sec);    }}

需要5743ms

package com.zxm;import java.util.Calendar;import java.util.Date;import java.util.TimeZone;/** * Creator:zxm * Date:2017/2/23 * Description: */public class PersonAno {    private final Date birthDate = new Date(1991,11,28,0,0,0);    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;    }    public static void main(String[] args) {        Date start = new Date();        PersonAno p = new PersonAno();        for (int i = 0; i < 10000000; i++) {            p.isBabyBoomer();        }        Date end = new Date();        long sec = end.getTime() - start.getTime();        System.out.println(sec);    }}

只需要7ms ！！！

Person类中，isBabyBoomer每次被调用的时候，都会新建一个Canlendar，一个TimeZone和两个Date实例，这是不必要的。

PersonAno用了一个静态的初始化器，避免了Person类中的效率低下的情况。

PersonAno类只在初始化的时候创建Calendar，TimeZone和Date实例一次，而不是在每次调用isBabyBoomer的时候都创建这些实例。

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