Effective Java 第五條:避免建立不必要的物件
阿新 • • 發佈:2019-01-22
反例:
public class Person { private final Date birthDate; public Person(Date birthDate) { this.birthDate = birthDate; } public boolean isBabyBoomer(){ //判斷嬰兒是否在生育高峰期出生 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; } }
對於上面方法中,如果檢查出生嬰兒是否在生育高峰期,每次呼叫此方法就要初始化Calender、Date物件。並且我們知道boomStart和boomEnd值始終是不變的,我們大可不必每次呼叫方法就初始化物件。建議修改如下:
正例:public class PersonNew { private final Date birthDate; private static Date BOOM_START = null; private static Date BOOM_END = null; public PersonNew(Date birthDate) { this.birthDate = birthDate; } 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; } }
以上程式碼來自《Effective Java》,執行程式碼感受一下會理解更深刻喔。