Java之创建对象>5.Avoid creating unnecessary objects详解编程语言

根据生日来判断是否是婴儿潮时期出生的，isBabyBoomer()是一个糟糕的设计，每次调用这个方法都会创建Calendar,TimeZone以及2个Date对象实例，当此方法被频繁调用时将会非常地影响性能。

public class Person { 

 private final Date birthDate; 

 // DONT 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; 

}

这个是优化后的方法，The improved version of the Person class creates Calendar, TimeZone, and Date instances only once

这些对象都是初始化后不会再被修改

class Person { 

 private final Date birthDate; 

 /** 

 * 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; 

}

autoboxing(自动装箱)也会带来非常大的性能影响

 // 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); 

}

原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/15969.html