避免创建重复对象

迟一步 — Mon, 08 Jan 2007 08:55:00 GMT

作为一个极端的反面例子,如下
String s = new String("silly"); //don't do this!
"silly"本身就是一个String实例一般改进为String s = "silly";

第二个例子
public class Person{
     prvate final Date birthDate; //Other fields omitted
     public Person(Date birthDate){
        this.birthDate = birthDate;
    }

    public boolean isBabyBoomer() {
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1981,Calendar.JANUARY,1,0,0,0);
        Date boomStart = gmtCal.getTime();
        gmtCal.set(1991,Calendar.JANUARY,1,0,0,0);
        Date boomEnd = gmtCal.getTime();
        return birthDate.compareTo(boomStart) >= 0 && birthDate.compareTo(boomEnd) < 0;
    }
}
上面的例程一个bean,
只要调用isBabyBoomer就会创建Caledar,TimeZone等等对象,
如果方法被频繁调用消耗就很大了每调用100W次,本程序消耗3600ms 我们改进一下
public calss Person{
     private static final Date BOOM_START;
     private static final Date BOOM_END;
     static{
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1981,Calendar.JANUARY,1,0,0,0);
        BOOM_START = gmtCal.getTime();
        gmtCal.set(1991,Calendar.JANUARY,1,0,0,0);
        BOOM_END = gmtCal.getTime();
      }
      prvate final Date birthDate; //Other fields omitted
      public Person(Date birthDate){
           this.birthDate = birthDate;
      }
      public boolean isBabyBoomer() {
            return birthDate.compareTo(BOOM_START) >= 0 && birthDate.compareTo(BOOM_END) < 0;
      }
}
本版本每运行100W次消耗时间为370ms比上个版本快100倍.
明显看出少创建对象的好处不过第2个版本代价比较昂贵

迟一步 2007-01-08 16:55 发表评论

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避免创建重复对象