One : What is immutable object ?

java Language requires us to comply with the following 5 Rules

* All fields within the class are final Decorated .
* All fields within the class are private , That is to say private modification .
* Class cannot be integrated and extended .
* Class cannot provide external methods that can modify internal state ,setter No way .
* If the field inside the class is a reference , That is, it can point to mutable objects , Then we programmers can't get this app
Two :String How to design an immutable object

* Doubt one

We said it at the beginning of the article ,String Object is immutable , here a= Zhang San , then a= Li Si , This is consistent with String The immutability of ? The answer is, of course .

We can see from the picture above , For the first time String
a=" Zhang San " When , The same object was created in the heap “ Zhang San ”. And then we were doing it a=" Li Si " And then an object is created in memory “ Li Si ”. Which means our a Just changed the reference a It's just the address

  2. Source code explanation confusion

The two main fields above :value and hash. We are here mainly to see value array ,hash It's not about the subject, so it's not going to be covered here , I have a special introduction hash.

ours String Objects are just characters inside and stored here value In the array . however value No external setValue Method of , So the whole String Objects look immutable on the outside . Let's draw a picture to explain the above doubts

Now I see , That is to say, what really changes the quotation is value, because value Also an array reference . It's easy to explain the next question .

   3, Doubt two

Since our String Is immutable , There seems to be a lot more inside substring, replace,
replaceAll Methods of these operations . It's all right String Object changed , It's easy to explain , Every time we do replace These operations , In fact, a new object is created in the heap memory . And then our value Point to different objects .

During the interview, we just explained that the above reasons are not so perfect , Want to get a better raise and pretend to be forced , We need further answers .

Three , What can be changed String

Now that it's titled . There must be a way , Don't forget our reflex mechanism , In general , He can do things that violate the principles of language design . It's also a technique , Every time an interviewer asks questions that violate language design principles , You can counter him with a reflex . Let's take a look :

We can change it by reflection String.

Now we know its principle and usage , We also know that it can be changed by reflection String, There's another problem we haven't figured out , You can ask him back during the interview , In order to further improve their own situation .

Four ,JAVA Why does language put String Type is designed to be immutable

Here are a few features .

first : stay Java In process String Type is the most used , This involves a lot of additions, deletions, modifications and searches , Every time before adding, deleting, or correcting jvm Need to check this String Security of objects , Is through hashcode, When designed as immutable , It ensures that every time you add, delete, modify or search hashcode Uniqueness of , It can be operated safely .

second : Network connection address URL, File path path In general, it is String Type save ,
If String Not fixed , All kinds of potential safety hazards will be caused . It's like our password can't String Type save for ,, If you save the password as a string in clear text , Then it will stay in memory , Until the garbage collector clears it . Because strings are placed in the string buffer pool for reuse , So it could be in memory for a long time , And this will lead to security risks

third : String values are reserved in the constant pool , That is, if the string object allows changes , It will lead to various logical errors

OK, That's all String Why type objects are immutable









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