Exception handling - a small case that takes you to solve NullPointerException

original   Qianfeng sol

One . Demand background

Recently, Mr. sol told the students about the basic concepts and usage of classes and objects , And I left a little exercise for you , The topic of the exercise is to use an array to store the student information in the class , Including the student number 、 full name 、 Age attribute , Finally, output the students' information . This topic seems to be very simple , So everyone began to knock the code crazily ...........

20 Minutes later , A student's code appears bug......

Two . Implementation code

The following figure is the code written by a classmate , See if there is any problem ？

1. Students

This is the definition of Student Students , The code is as follows ：

2. Student tests

This is the defined student test class , The code is shown in the following figure ：

3. Running results

The above code , After running, the following error prompt appears ：

According to the abnormal information in the above figure , A null pointer exception occurred in the code ！ We know , The reason for the null pointer exception is that an object in the code is null ！

4. Abnormal cause analysis

Null pointer exceptions are common , But why do null pointers appear ？ Let's analyze the specific reasons ......

According to the prompt, we can see , The exception is in StudentTest Class 28 Row occurrence , So let's take a look at , In the 28 Which object in the row may be empty ？ The first 28 The line code is as follows ：

Let's look at number one 28 Line code , Which object is empty ？ This is obviously students[i] This object is empty , This leads to a null pointer exception ！

So why students[i] The object is empty ？ our Students Array is not already new Have you passed ？

Be careful ： In us new Object array , It just opens up memory space of a specified length for the array , But the element in this array defaults to null Oh ！ Next, we can debug it to see if it's like this ！

according to debug As a result, we can see , Sure enough, the elements in the array are null！

5. Exception resolution

Now that we know the cause of the fault , Then how to solve this problem ？

We can be in the 28 before new A student object , And store it in the array , As shown in the figure below ：

Next, let's run the code , Retest , You will find that the problem of null pointer exception has been solved ！ Now you know how null pointer exceptions occur , And how to solve it ？ If you have other questions , You can leave a message for Mr. sol in the comment area , I will reply to you in time after I see it .