有趣的String类型




















运行结果如下:
strA = 123
strB = 123
修改字符串strB的值:strB = "1234"
strB = 1234
strA = 123
string类型的引用类型,根据引用类型的特点,strA、strB都是指向同一个内存地址A,修改了strB的值就是修改了内存地址A中的值,此时strA的值应该也变为"1234",但为什么strA的值还是"123"呢? 在网上搜了一下才知道string类型原来是特殊分子!以下内容完全引用自TerryLee的文章http://www.cnblogs.com/Terrylee/archive/2005/12/26/304876.aspx:
概述
String在任何语言中,都有它的特殊性,在.NET中也是如此。它属于基本数据类型,也是基本数据类型中唯一的引用类型。字符串可以声明为常量,但是它却放在了堆中。希望通过本文能够使大家对.NET中的String有一个深入的了解。
不可改变对象
在.NET中String是不可改变对象,一旦创建了一个String对象并为它赋值,它就不可能再改变,也就是你不可能改变一个字符串的值。这句话初听起来似乎有些不可思议,大家也许马上会想到字符串的连接操作,我们不也可以改变字符串吗?看下面这段代码:

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

运行的结果:




看起来我们似乎已经把MyStr的值从“1234”改为了“12345678”。事实是这样的吗?实际上并没有改变。在第5行代码中创建了一个String对象它的值是“1234”,MyStr指向了它在内存中的地址;第七行代码中创建了一个新的String对象它的值是“12345678”,MyStr指向了新的内存地址。这时在堆中其实存在着两个字符串对象,尽管我们只引用了它们中的一个,但是字符串“1234”仍然在内存中驻留。
引用类型
前面说过String是引用类型,这就是如果我们创建很多个相同值的字符串对象,它在内存中的指向地址应该是一样的。也就是说,当我们创建了字符串对象a,它的值是“1234”,当我们再创建一个值为“1234”的字符串对象b时它不会再去分配一块内存空间,而是直接指向了a在内存中的地址。这样可以确保内存的有效利用。看下面的代码:

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

运行结果:



做一个小改动,注意Change(ref string s)

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

运行结果:



字符串的比较
在.NET中,对字符串的比较操作并不仅仅是简单的比较二者的值,= =操作首先比较两个字符串的引用,如果引用相同,就直接返回True;如果不同再去比较它们的值。所以如果两个值相同的字符串的比较相对于引用相同的字符串的比较要慢,中间多了一步判断引用是否相同。看下面这段代码:

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

执行的结果(运行的结果可能有些不同):



由此我们看出值相同时的比较用= =比引用相同时的比较慢了好多。这里仅仅是一个测试,因为做这样的比较并没有任何实际的意义。
有一点需要明确的是,.NET中==跟Equals()内部机制完全是一样的,==是它的一个重载。

2

3

4

5


2

3

4

5

6

7

8

9

10

11

12

13

字符串驻留
看一下这段代码:

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

执行的结果:



在这段代码中,比较这两个对象发现它的引用并不是一样的。如果要想是它们的引用相同,可以用Intern()函数来进行字符串的驻留(如果有这样的值存在)。
StringBuilder对象
通过上面的分析可以看出,String类型在做字符串的连接操作时,效率是相当低的,并且由于每做一个连接操作,都会在内存中创建一个新的对象,占用了大量的内存空间。这样就引出StringBuilder对象,StringBuilder对象在做字符串连接操作时是在原来的字符串上进行修改,改善了性能。这一点我们平时使用中也许都知道,连接操作频繁的时候,使用StringBuilder对象。但是这两者之间的差别到底有多大呢?来做一个测试:

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

运行结果:



通过上面的分析,可以看出用String来做字符串的连接时效率非常低,但并不是所任何情况下都要用StringBuilder,当我们连接很少的字符串时可以用String,但当做大量的或频繁的字符串连接操作时,就一定要用StringBuilder。
【推荐】编程新体验,更懂你的AI,立即体验豆包MarsCode编程助手
【推荐】凌霞软件回馈社区,博客园 & 1Panel & Halo 联合会员上线
【推荐】抖音旗下AI助手豆包,你的智能百科全书,全免费不限次数
【推荐】博客园社区专享云产品让利特惠,阿里云新客6.5折上折
【推荐】轻量又高性能的 SSH 工具 IShell:AI 加持,快人一步