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。

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。

我真的很喜欢SCJP Sun认证程序员Java 5学习指南的解释。

为了提高Java的内存效率，JVM留出了一个特殊的内存区域，称为“字符串常量池”。当编译器遇到String字面值时，它会检查池，看看是否已经存在相同的String。如果找到匹配，则对新字面值的引用将指向现有的String，并且不会创建新的String字面值对象。

In large applications its common for string literals to occupy large bits of memory. So to efficiently handle the memory, the JVM allocates an area called "String constant pool".(Note that in memory even an unreferenced String carries around a char[], an int for its length, and another for its hashCode. For a number, by contrast, a maximum of eight immediate bytes is required) When complier comes across a String literal it checks the pool to see if there is an identical literal already present. And if one is found, the reference to the new literal is directed to the existing String, and no new 'String literal object' is created(the existing String simply gets an additional reference). Hence : String mutability saves memory... But when any of the variables change value, Actually - it's only their reference that's changed, not the value in memory(hence it will not affect the other variables referencing it) as seen below....

字符串s1 = "旧字符串";

//s1 variable, refers to string in memory
        reference                 |     MEMORY       |
        variables                 |                  |

           [s1]   --------------->|   "Old String"   |

字符串s2 = s1;

//s2 refers to same string as s1
                                  |                  |
           [s1]   --------------->|   "Old String"   |
           [s2]   ------------------------^

s1 = "New String";

//s1 deletes reference to old string and points to the newly created one
           [s1]   -----|--------->|   "New String"   |
                       |          |                  |
                       |~~~~~~~~~X|   "Old String"   |
           [s2]   ------------------------^

原来的字符串'in memory'没有改变，但是 引用变量已被更改，以便它引用新字符串。 如果我们没有s2, Old String仍然在内存中，但是 我们无法访问它…

不可变对象是指创建后不能修改的对象。一个典型的例子是字符串字面量。

越来越流行的D编程语言通过“不变”关键字具有“不变性”的概念。查看Dr.Dobb关于它的文章http://dobbscodetalk.com/index.php?option=com_myblog&show=Invariant-Strings.html&Itemid=29。它完美地解释了这个问题。

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)。