Java Effective 49 基本类型优于装箱基本类型

1. 同一样比较，a,b 不相等！dentity comparision!

Integer a = new Integer(1);
Integer b=  new Integer(1);

2. 当装箱基本类型引用为null时，进行自动拆箱时，将抛出java.lang.NullPointerException异常。

        // java.lang.NullPointerException
        // if(42==i){
        // System.out.println("Yes");
        // }

3. 装箱，拆箱效率低！如！

 public static void main(String[] args) {
     
        {
            Date start = new Date();
            Long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("box & unbox cost:  " + (end.getTime() - start.getTime()));
        }

        {
            Date start = new Date();
            long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("NO box & unbox cost:  " + (end.getTime() - start.getTime()));
        }
    }

引用

输出：
box & unbox cost:     26233
NO box & unbox cost:  5874

合适场合：
1. Set中的Key, Value.因为不能将基本类型放入集合中！

评论

1 楼 bugmenot 2011-01-31  

用了包装类型当然只能用equals比较相等

acrbb 写道

3. 装箱，拆箱效率低！如！

 public static void main(String[] args) {
     
        {
            Date start = new Date();
            Long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("box & unbox cost:  " + (end.getTime() - start.getTime()));
        }

        {
            Date start = new Date();
            long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("NO box & unbox cost:  " + (end.getTime() - start.getTime()));
        }
    }

引用

输出：
box & unbox cost:     26233
NO box & unbox cost:  5874

楼主大大能把两个测试的顺序反过来再测一次吗，

 public static void main(String[] args) {
     
        {
            Date start = new Date();
            long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("box & unbox cost:  " + (end.getTime() - start.getTime()));
        }

        {
            Date start = new Date();
            Long sum = 0l;
            for (long i = 0; i < Integer.MAX_VALUE; i++) {
                sum += i;
            }
            System.out.println(sum);
            Date end = new Date();
            System.out.println("NO box & unbox cost:  " + (end.getTime() - start.getTime()));
        }
    }