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。

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。

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。

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…

不可变对象是内部字段(或者至少是影响其外部行为的所有内部字段)不能被更改的对象。

不可变字符串有很多优点:

性能:执行如下操作:

String substring = fullstring.substring(x,y);

substring()方法的底层C可能是这样的:

// Assume string is stored like this:
struct String { char* characters; unsigned int length; };

// Passing pointers because Java is pass-by-reference
struct String* substring(struct String* in, unsigned int begin, unsigned int end)
{
    struct String* out = malloc(sizeof(struct String));
    out->characters = in->characters + begin;
    out->length = end - begin;
    return out;
}

注意，没有一个字符必须被复制!如果String对象是可变的(字符可以在以后更改)，那么您将不得不复制所有字符，否则对子字符串中的字符的更改将在以后反映到另一个字符串中。

并发性:如果一个不可变对象的内部结构是有效的，那么它将总是有效的。不同的线程不可能在该对象中创建无效状态。因此，不可变对象是线程安全的。

垃圾收集:垃圾收集器更容易对不可变对象做出逻辑决策。

然而，不可变性也有缺点:

性能:等等，我记得你说过性能是不变性的好处!有时候是这样，但不总是这样。取以下代码:

foo = foo.substring(0,4) + "a" + foo.substring(5);  // foo is a String
bar.replace(4,5,"a"); // bar is a StringBuilder

The two lines both replace the fourth character with the letter "a". Not only is the second piece of code more readable, it's faster. Look at how you would have to do the underlying code for foo. The substrings are easy, but now because there's already a character at space five and something else might be referencing foo, you can't just change it; you have to copy the whole string (of course some of this functionality is abstracted into functions in the real underlying C, but the point here is to show the code that gets executed all in one place).

struct String* concatenate(struct String* first, struct String* second)
{
    struct String* new = malloc(sizeof(struct String));
    new->length = first->length + second->length;

    new->characters = malloc(new->length);
    
    int i;

    for(i = 0; i < first->length; i++)
        new->characters[i] = first->characters[i];

    for(; i - first->length < second->length; i++)
        new->characters[i] = second->characters[i - first->length];

    return new;
}

// The code that executes
struct String* astring;
char a = 'a';
astring->characters = &a;
astring->length = 1;
foo = concatenate(concatenate(slice(foo,0,4),astring),slice(foo,5,foo->length));

注意，concatenate被调用两次，这意味着整个字符串必须循环!将其与bar操作的C代码进行比较:

bar->characters[4] = 'a';

可变字符串操作显然要快得多。

总结:在大多数情况下，您需要一个不可变的字符串。但是如果你需要在一个字符串中做大量的追加和插入，你就需要可变性来提高速度。如果你想要并发安全性和垃圾收集的好处，关键是保持你的可变对象本地的方法:

// This will have awful performance if you don't use mutable strings
String join(String[] strings, String separator)
{
    StringBuilder mutable;
    boolean first = true;

    for(int i = 0; i < strings.length; i++)
    {
        if(first) first = false;
        else mutable.append(separator);

        mutable.append(strings[i]);
    }

    return mutable.toString();
}

因为可变对象是一个本地引用，所以不必担心并发安全性(只有一个线程接触过它)。由于它没有在其他任何地方被引用，所以它只在堆栈上分配，所以函数调用一结束它就会被释放(您不必担心垃圾收集)。你可以同时获得可变性和不可变性的性能优势。

实际上，如果你使用上面建议的维基百科定义，String不是不可变的。

字符串的状态改变后构造。看一下hashcode()方法。String将hashcode值缓存在本地字段中，但直到第一次调用hashcode()才计算它。这种对hashcode的惰性求值将String置于一个有趣的位置，作为状态发生变化的不可变对象，但如果不使用反射，就无法观察到它发生了变化。

所以也许不可变的定义应该是一个不能被观察到已经改变的对象。

如果一个不可变对象在创建后状态发生了变化，但是没有人可以看到它(没有反射)，这个对象仍然是不可变的吗?