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  • 五、避免创建不必要的对象

    一般来说,对于java的对象来说,可以重用对象的情况下:尽量不要在需要的时候就创建一个相同功能的对象

    1、String

    @Test
        public void test02() {
            btime=System.currentTimeMillis();
            for(long i=0;i<1000000000;i++){
                String a=new String("a");
            }
            etime=System.currentTimeMillis();
            System.out.println(etime-btime);
            //这种方式创建String对象10亿次花了9574
        
        }
        
        @Test
        public void test03() {
            btime=System.currentTimeMillis();
            for(long i=0;i<1000000000;i++){
                String a="a";
            }
            etime=System.currentTimeMillis();
            System.out.println(etime-btime);
            //这种方式花了2337
        }

    2、尽量用静态初始化一些相同功能的对象

    class Person231 {
        private final Date birth = new Date();
    
        public boolean isBabyBoomer() {
            Calendar g = Calendar.getInstance();
            g.set(1946, Calendar.JANUARY, 1, 0, 0);
            Date start = g.getTime();
            g.set(1965, Calendar.JANUARY, 1, 0, 0);
            Date end = g.getTime();
            return birth.before(end) && birth.after(start);
    
        }
    }
    @Test
        public void test21() {
            Person231 p=new Person231();
            
            btime=System.currentTimeMillis();
            for(long i=0;i<10000000;i++){
                p.isBabyBoomer();
            }
            etime=System.currentTimeMillis();
            System.out.println(etime-btime);
        }//调用一千万次,花了27585

    改进后:

    class Person231 {
        
        private final static Date birth = new Date();
        private static Calendar g;
        private static Date start;
        private static Date end;
        static{
            g= Calendar.getInstance();
            g.set(1946, Calendar.JANUARY, 1, 0, 0);
            start=g.getTime();
            g.set(1965, Calendar.JANUARY, 1, 0, 0);
            end=g.getTime();
        }
        public boolean isBabyBoomer() {
            
            return birth.before(end) && birth.after(start);
        }
    }
           @Test
        public void test21() {
            Person231 p=new Person231();
            
            btime=System.currentTimeMillis();
            for(long i=0;i<10000000;i++){
                p.isBabyBoomer();
            }
            etime=System.currentTimeMillis();
            System.out.println(etime-btime);
        }//调用一千万次,花了148    

    3、基本类型及其包装类---优先使用基础类

    //使用包装类,每使用一次创建一个对应类
    @Test
        public void test20() {
            int i = Integer.MAX_VALUE;
            System.out.println(i);
            Long sum = 0L;
            btime = System.currentTimeMillis();
            for (int j = 0; j < i; j++) {
                sum = sum + j;
            }
            etime = System.currentTimeMillis();
            System.out.println(etime - btime);
        }// 调用2147483647次,花了28551
    @Test
        public void test20() {
            //使用基本类型
            int i = Integer.MAX_VALUE;
            System.out.println(i);
            long sum = 0L;
            btime = System.currentTimeMillis();
            for (int j = 0; j < i; j++) {
                sum = sum + j;
            }
            etime = System.currentTimeMillis();
            System.out.println(etime - btime);
        }// 调用2147483647次,花了5151
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  • 原文地址:https://www.cnblogs.com/wangyang108/p/5880849.html
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