In depth analysis of the source code, why is the string class immutable? (hit me before you understand)

One 、 What is immutable ？

String Immutability is actually very simple , As shown in the figure below , For an existing string s=“abcd”, Assign it a second time to "abcdel", This operation will not be in the original memory address （ namely “abcd” Memory address of ） Modify the data on , Instead, it points to a new memory address （ namely “abcdel”）. Therefore, it is not what we generally think that the dove occupies the magpie's nest ——“abcdel” To replace the “abcd”, Occupy “abcd” Memory location for , Lead to “abcd” homeless , Disappear from this world .

Two 、String The immutable nature

Check out jdk in String Class source code ：
（ Only the first three lines are listed here , If you are interested, you can check the complete source code by yourself ）

We found that
（ One ）String Class is a final class .
String Class by final modification , We all know when final When modifying a class , This class cannot be inherited by other classes , natural String Class has no subclasses , There is also no saying that methods are overridden by subclasses , So this guarantees The outside world cannot inherit String class , To achieve the right String The destruction of immutability .
（ Two ）String The bottom is through a char[] To store data , And the char[] from private final modification .
The value The array is final modification , We know By final Variables of modified reference types can no longer point to other objects , in other words value An array can only point to its own array in the heap , You can no longer point to other arrays . But this does not prevent us from changing the contents of the array it points to , For example, let's give a random example ：

Then the external saboteurs may think , Since you won't let me change value The point of the array , Then I can modify the contents of it , Hey
Then he thought about visiting this value Array to modify the values inside , But when operating, he found , He . Don't come out of this value Array , He wondered , How can you not visit ？

In fact, this time is private work , With private Naturally, the outside world can't visit me value Array , Naturally, I can't modify the data inside .
Okay , Now the outside wants to destroy us String The road of non variability is blocked , It also has to ensure that there is no internal “ spy ” ah , Don't make jokes, it's not ！

（ 3、 ... and ）String Class does not provide or expose anything that can be changed value Array method .
Looking at the source code , We found that String The interior of the class does not provide or expose anything that can be changed value Method of array contents , Therefore, the internal safety is guaranteed , In this way, the exterior and interior are impeccable . This has reached us String The ultimate goal of class immutability ！

3、 ... and 、 summary

Guarantee String Class immutability depends on the following three points ：
（ One ）String Class quilt final Modification causes it not to be inherited , Thus avoiding subclass destruction String Immutability .
（ Two ） Save string value The array is final Decorated and private .
（ 3、 ... and ）String Class does not provide or expose modification of this value Array method .