effictive--避免创建不必要的对象

demo01：

package com.xiaohao.effective;import java.util.Calendar;import java.util.Date;import java.util.TimeZone;/** * 避免创建不必要的对象Calendar，BOOM_START，BOOM_END * @author Administrator * */public class Demo01 {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 static boolean isBabyBoomer(Date birthDate) {return birthDate.compareTo(BOOM_START) >= 0 &&birthDate.compareTo(BOOM_END) < 0;}}

demo02：

package com.xiaohao.effective;import java.util.Date;/** * countNum01方法中每执行一次sum +=1,实际上是sum = sum + 1,但是sum是对象，对象不支持“+”运算，所以sum * 会进行自动拆箱和装箱，sum = sum.intValue() + 1; Long sum = new Long(sum); * 就会创建一个Long类型对象，非常耗性能 , 而countNum02并不会如此。 * 注意：要优先使用基本类型而不是装箱基本类型，要当心无意识的自动装箱 * @author Administrator * */public class Demo02 {private static Demo02 d= new Demo02();public static void countNum01(){Long sum = 0L;long t1 = new Date().getTime();for(long i=0;i<Integer.MAX_VALUE;i++) {sum += i;}long t2 = new Date().getTime();System.out.println(sum + " :case " + (t2-t1));}public static void countNum02(){long sum = 0L;long t1 = new Date().getTime();for(long i=0;i<Integer.MAX_VALUE;i++) {sum += i;}long t2 = new Date().getTime();System.out.println(sum + " :case " + (t2-t1));}public static Demo02 getInstance() {if(d == null) {d = new Demo02();}return d;}}

测试demo02：

package com.xiaohao.test;import org.junit.Test;import com.xiaohao.effective.Demo02;public class TestEffective {@Testpublic void testDemo02() {Demo02.getInstance().countNum01();Demo02.getInstance().countNum02();}}

output：

2305843005992468481 :case 7277
2305843005992468481 :case 655