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