不可变的对象在创建后不能改变其状态。

尽可能使用不可变对象有三个主要原因，所有这些都将有助于减少你在代码中引入的错误数量:

It is much easier to reason about how your program works when you know that an object's state cannot be changed by another method Immutable objects are automatically thread safe (assuming they are published safely) so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code, so they can be used as the keys in a HashMap (or similar). If the hash code of an element in a hash table was to change, the table entry would then effectively be lost, since attempts to find it in the table would end up looking in the wrong place. This is the main reason that String objects are immutable - they are frequently used as HashMap keys.

当你知道一个对象的状态是不可变的时，你还可以在代码中做一些其他的优化——例如缓存计算的哈希——但这些都是优化，因此没有那么有趣。

一旦实例化，就不能更改。考虑一个类，它的实例可能用作哈希表或类似的键。查看Java最佳实践。

String s1="Hi";
String s2=s1;
s1="Bye";

System.out.println(s2); //Hi  (if String was mutable output would be: Bye)
System.out.println(s1); //Bye

s1="Hi":创建一个包含"Hi"值的对象s1。

S2 =s1:引用s1对象创建对象S2。

s1="Bye":之前的s1对象的值不会改变，因为s1有String类型，而String类型是一个不可变的类型，相反，编译器创建了一个新的String对象与"Bye"值和s1引用它。在这里，当我们打印s2值时，结果将是“Hi”而不是“Bye”，因为s2引用了前一个具有“Hi”值的s1对象。

不可变意味着一旦对象被创建，它的所有成员都不会改变。字符串是不可变的，因为你不能改变它的内容。 例如:

String s1 = "  abc  ";
String s2 = s1.trim();

在上面的代码中，字符串s1没有改变，使用s1创建了另一个对象(s2)。

不可变的对象在创建后不能改变其状态。

尽可能使用不可变对象有三个主要原因，所有这些都将有助于减少你在代码中引入的错误数量:

It is much easier to reason about how your program works when you know that an object's state cannot be changed by another method Immutable objects are automatically thread safe (assuming they are published safely) so will never be the cause of those hard-to-pin-down multithreading bugs Immutable objects will always have the same Hash code, so they can be used as the keys in a HashMap (or similar). If the hash code of an element in a hash table was to change, the table entry would then effectively be lost, since attempts to find it in the table would end up looking in the wrong place. This is the main reason that String objects are immutable - they are frequently used as HashMap keys.

当你知道一个对象的状态是不可变的时，你还可以在代码中做一些其他的优化——例如缓存计算的哈希——但这些都是优化，因此没有那么有趣。

其中一个含义与值如何存储在计算机中有关，例如，对于一个。net字符串，它意味着内存中的字符串不能被更改，当你认为你正在更改它时，你实际上是在内存中创建了一个新的字符串，并将现有的变量(这只是一个指向其他地方的实际字符集合的指针)指向新的字符串。