Java之建立對象>5.Avoid creating unnecessary objects

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String s = new String("stringette"); // DON‘T DO THIS!

The improved version is simply the following:

String s = "stringette";

根據生日來判斷是否是嬰兒潮時期出生的，isBabyBoomer()是一個糟糕的設計，每次調用這個方法都會建立Calendar,TimeZone以及2個Date對象執行個體，當此方法被頻繁調用時將會非常地影響效能。

public class Person {    private final Date birthDate;      // DON‘T 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;    }}

這個是最佳化後的方法，The improved version of the Person class creates Calendar, TimeZone, and Date instances only once

這些對象都是初始化後不會再被修改

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(自動裝箱)也會帶來非常大的效能影響

 // 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);}

 

Java之建立對象>5.Avoid creating unnecessary objects