1. 最好能重用物件而不是在每次需要的時候就建立一個相同功能的新物件
2. 對於同時提供了靜態工廠方法和構造器的不可變類，通常可以使用靜態工廠方法而不是構造器，以避免建立不必要的物件。例如：Boolean.valueOf(String)幾乎走勢優先於構造器Boolean(String)
3. 除了重用不可變的物件之外，也可以重用那些已知不會被改變的可變物件 建立了不必要物件的Person1類

import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class Person1 {
    private Date birthDate;

    public Person1(Date birthDate) {
        this.birthDate = birthDate;
    }

    /**
     * 不要這麼做，一定
     * 
     * @return
     */
    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;
    }
}
 

重用了不變物件的Person2類

import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class Person2 {
    private Date birthDate;

    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 Person2(Date birthDate) {
        this.birthDate = birthDate;
    }

    public boolean isBabyBoomer() {
        return birthDate.compareTo(BOOM_START) >= 0
                && birthDate.compareTo(BOOM_END) < 0;
    }
}
 

測試方法，並且列印耗時，以及耗時倍數

import java.util.Date;

public class PersonMain {
    public static void main(String[] args) {
        int time = (int) (System.currentTimeMillis() / 1000);
        int testTimes = 10000000;

        long person1Start = System.currentTimeMillis();
        for (int i = time - testTimes; i < time; i++) {
            Person1 p = new Person1(new Date(i * 1000L));
            p.isBabyBoomer();
        }
        long person1End = System.currentTimeMillis();
        long cost1 = person1End - person1Start;
        System.out.println(cost1);

        long person2Start = System.currentTimeMillis();
        for (int i = 0; i < testTimes; i++) {
            Person2 p = new Person2(new Date(i * 1000L));
            p.isBabyBoomer();
        }
        long person2End = System.currentTimeMillis();
        long cost2 = person2End - person2Start;
        System.out.println(cost2);
        System.out.println(cost1 / cost2);
    }
}
 

java提供的自動裝箱功能提供了方便，但是也建立了不必要的物件，所以要優先使用基本型別而不是裝箱基本型別，要當心無意識的自動裝箱

public class AutoBoxing {
    public static void main(String[] args) {
        long start1 = System.currentTimeMillis();
        Long sum1 = 0L;
        for (long i = 0; i < Integer.MAX_VALUE; i++) {
            sum1 += i;
        }
        System.out.println(sum1);
        long end1 = System.currentTimeMillis();
        long cost1 = end1 - start1;
        System.out.println(cost1);
        long start2 = System.currentTimeMillis();
        long sum2 = 0L;
        for (long i = 0; i < Integer.MAX_VALUE; i++) {
            sum2 += i;
        }
        System.out.println(sum2);
        long end2 = System.currentTimeMillis();
        long cost2 = end2 - start2;
        System.out.println(cost2);
        System.out.println(cost1 / cost2);
    }
}

通過維護自己的物件池（object pool）來避免建立物件並不是一種好的做法，除非池中的物件是非常重量級的。真正正確使用物件池的典型物件示例就是資料庫連線池。