Java Effective 49 基本门类优于装箱基本类型

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()));        }    }

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())); } }

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())); } }

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