About string immutability

from ： Micro reading   https://www.weidianyuedu.com/content/2217491421632.html

One 、 Explain the background

I have a system here , Provide a RPC Interface to send SMS . To call my interface externally, you need to pass in the phone number and other parameters , I'm in charge of parsing these parameters 、 Do some business processing , Then call the SMS channel to send SMS .

When the interface of SMS channel provider is called , I will store the records sent this time （ Deposit in MySQL in ）, In the same way, SMS channel will return the receipt sent or failed to me , I'll also put it in storage （ Deposit in MySQL in ）.

On that day , Someone came to me , Say a cell phone number can't receive a message , Users don't block SMS （ In arrears 、 To turn it off ） Wait a minute , I just can't get messages .

So I went to check , First of all, I'll go first DB Look inside for the corresponding sending record , Find out that this record exists , And in DB I can't see anything unusual .

• therefore , This precludes the operation being intercepted in the middle of the way （ Because it's in storage , Must have called the SMS operator's interface ）

Later, I went to the journal , Take a look at the call SMS operator's return Result What is the object's information , Then I asked the SMS operators why they might have this problem . The reply over there is ：“ If it's part of the mobile phone number that has this situation , Is your cell phone number not trim ah ？”

therefore , I went to the journal again , I found that there was a space behind the cell phone number （ heartache , I couldn't see ）. It's very easy to deal with this problem , All I have to do is to check my cell phone number in the entrance trim Just fine .

Two 、 Write code

My side is for Send the same message to multiple cell phone numbers , So my cell phone number is HashSet To receive . Carry out... On the mobile phone number trim I wrote the following code ：

//  explain ：Task object   There is one  key attribute , This key The type of property is HashSetif (task.getKey() != null && task.getKey().size() > 0) {    for (String s : task.getKey()) {        s.trim();    }}

The code is simple , I just do two steps ：

• Judge whether it is null, Not for null Value to traverse the phone number set

• For each cell phone number trim

Is there something wrong with the code above ？ There must be a problem , No problem. What else do I write .

Write a little Demo, We will find that ： In the code of 11 Call on line trim() After the method , stay 12 Line re output , There will still be spaces .

Code example

2.1 Why is there such an illusion ？

Actually , We are at the beginning of learning Java When , I'm sure to learn String class . It is also clear when studying String yes immutable Of , But there's always a feeling that we put String The object has been changed , Why? ？

Immutable String

I think the first point is ： We operate on mutable objects , Some properties of the object have been changed , We think it has been changed . Consider the following code ：

HashSet<Student> students = getStudent();for (Student s1 : students) {    s1.setName("Java3y");}

After execution , We think it's in HashSet Inside Student All the names of are changed to Java3y 了 , And it's actually the same .

I think the second point is ： We usually operate String object , It's all about passing the results of the operation directly to , It looks like the original object has been modified . For example, the following similar code ：

//  duplicate removal String phone = "   137888888888    ";sendPhone(phone.trim());//  Convert to uppercase and output System.out.println(phone.toUpperCase());// ...  wait 

---

Back to a new String object

2.2 How to change

Now the problem is known ,String Object is immutable , Yes String Object to operate on ,“ It looks like ” Original String The object has changed , In fact, a new String object .

Back to my question , It's also very easy to solve , hold trim A good phone number is set to HashSet That's it