StringBuilder的实现与技巧
在上一篇进一步了解String 中,发现了string的不便之处,而string的替代解决方案就是StringBuilder的使用..它的使用也很简单System.Text.StringBuilder sb = new System.Text.StringBuilder();这样就初始化了一个StringBuilder ..之后我们可以通过Append()来追加字符串填充到sb中..在你初始化一个StringBuilder 之后,它会自动申请一个默认的StringBuilder 容量(默认值是16),这个容量是由Capacity来控制的.并且允许,我们根据需要来控制Capacity的大小,也可以通过Length来获取或设置StringBuilder 的长度..
先来看Length的用法:
1
System.Text.StringBuilder sb = new System.Text.StringBuilder();
2sb.Append( "123456789" );//添加一个字符串
3sb.Length = 3;//设置容量为3
4Console.WriteLine( sb.ToString() );//这里输出:123
5
6sb.Length = 30;//重新设置容量为30
7Console.WriteLine( sb.ToString() + ",结尾");//这里在原来字符串后面补齐空格,至到Length的为30
8Console.WriteLine( sb.Length );//这里输出的长度为30
System.Text.StringBuilder sb = new System.Text.StringBuilder();2
sb.Append( "123456789" );//添加一个字符串3
sb.Length = 3;//设置容量为34
Console.WriteLine( sb.ToString() );//这里输出:1235
6
sb.Length = 30;//重新设置容量为307
Console.WriteLine( sb.ToString() + ",结尾");//这里在原来字符串后面补齐空格,至到Length的为308
Console.WriteLine( sb.Length );//这里输出的长度为30
通过上面的代码,我们可以看出如果StringBuilder 中的字符长度小于Length的值,则StringBuilder 将会用空格硬填充StringBuilder ,以满足符合长度的设置..如果StringBuilder 中的字符长度大于Length的值,则StringBuilder 将会截取从第一位开始的Length个字符..而忽略超出的部分..
再来看看最重要的部分Carpacity的用法:
1System.Text.StringBuilder sb = new System.Text.StringBuilder();//初始化一个StringBuilder
2Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
3Console.WriteLine( "\t Length:" + sb.Length );
4
5sb.Append( '1',17 );//添加一个字符串,这里故意添加17个字符,是为了看到Capacity是如何被扩充的
6Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
7Console.WriteLine( "\t Length:" + sb.Length );
8
9sb.Append( '2',32 );//添加一个字符串
10Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
11Console.WriteLine( "\t Length:" + sb.Length );
12
13sb.Append( '3',64 );//添加一个字符串
14Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
15Console.WriteLine( "\t Length:" + sb.Length );
16
17//注意这里:如果你取消Remove这步操作,将会引发ArgumentOutOfRangeException异常,因为当前容量小于
18
19//Length,这在自己控制StringBuilder的时候务必要注意容量溢出的问题
20
21sb.Remove(0,sb.Length);//移出全部内容,再测试
22sb.Capacity = 1;//重新定义了容量
23sb.Append( 'a',2 );
24Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
25Console.WriteLine( "\t Length:" + sb.Length );
26
27sb.Append( 'b',4 );
28Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
29Console.WriteLine( "\t Length:" + sb.Length );
30
31sb.Append( 'c',6 );
32Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大
33Console.WriteLine( "\t Length:" + sb.Length
System.Text.StringBuilder sb = new System.Text.StringBuilder();//初始化一个StringBuilder2
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大3
Console.WriteLine( "\t Length:" + sb.Length );4
5
sb.Append( '1',17 );//添加一个字符串,这里故意添加17个字符,是为了看到Capacity是如何被扩充的6
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大7
Console.WriteLine( "\t Length:" + sb.Length );8
9
sb.Append( '2',32 );//添加一个字符串10
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大11
Console.WriteLine( "\t Length:" + sb.Length );12
13
sb.Append( '3',64 );//添加一个字符串14
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大15
Console.WriteLine( "\t Length:" + sb.Length );16
17
//注意这里:如果你取消Remove这步操作,将会引发ArgumentOutOfRangeException异常,因为当前容量小于18
19
//Length,这在自己控制StringBuilder的时候务必要注意容量溢出的问题20
21
sb.Remove(0,sb.Length);//移出全部内容,再测试22
sb.Capacity = 1;//重新定义了容量23
sb.Append( 'a',2 );24
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大25
Console.WriteLine( "\t Length:" + sb.Length );26
27
sb.Append( 'b',4 );28
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大29
Console.WriteLine( "\t Length:" + sb.Length );30
31
sb.Append( 'c',6 );32
Console.Write( "Capacity:" + sb.Capacity );//这里的Capacity会自动扩大33
Console.WriteLine( "\t Length:" + sb.Length上面的代码输出的结果:
1Capacity:16 Length:0 //输出第一次,默认的Capacity是16
2Capacity:32 Length:17 //第二次,我们故意添加了17个字符,于是Capacity=Capacity*2
3Capacity:64 Length:49 //继续超出,则Capacity=Capacity*2
4Capacity:128 Length:113
5Capacity:3 Length:2 //清空内容后,设置Capacity=1,重新添加了字符
6Capacity:7 Length:6 //后面的结果都类似
7Capacity:14 Length:12
Capacity:16 Length:0 //输出第一次,默认的Capacity是162
Capacity:32 Length:17 //第二次,我们故意添加了17个字符,于是Capacity=Capacity*23
Capacity:64 Length:49 //继续超出,则Capacity=Capacity*24
Capacity:128 Length:1135
Capacity:3 Length:2 //清空内容后,设置Capacity=1,重新添加了字符6
Capacity:7 Length:6 //后面的结果都类似7
Capacity:14 Length:12从上面的代码和结果可以说明StringBuilder中容量Capacity是如何增加的:创建一个StringBuilder之后,默认的Capacity初始化为16,接着我们添加17个字符,以方便看到Capacity的扩充后的值..大家在修改Capacity的时候,一定要注意21行的注释,一定要确保Capacity >= Length,否则会引发ArgumentOutOfRangeException异常...看完结果,就可以推断出Capacity的公式:
if ( Capacity < Length && Capacity > 0 ){
Capacity *= 2;
}
OK..看到公式就明白了..StringBuilder是以当前的Capacity*2来扩充的..所以,在使用StringBuilder需要特别注意,尤其是要拼接或追加N多字符的时候,要注意技巧的使用,可以适当的,有预见性的设置Capacity的值,避免造成过大内存的浪费,节约无谓的内存空间..例如,下列代码就可以根据情况自动的扩展,而避免了较大的内存浪费.
1System.Text.StringBuilder sb = new System.Text.StringBuilder();
2int i = 0;
3long StartTime = DateTime.Now.Ticks;
4
while ( i < 100000 ) {
5
sb.Append( i.ToString() );
6i++;
7
}
8long EndTime = DateTime.Now.Ticks;
9
10Console.WriteLine( "时间:" + ( EndTime-StartTime ) + "\t Capacity:"+ sb.Capacity + "\t Length:"
11
12+ sb.Length);
13
14System.Text.StringBuilder sb1 = new System.Text.StringBuilder();
15i = 0;
16StartTime = DateTime.Now.Ticks;
17while ( i < 100000 )
18{
19if ( sb1.Capacity <= sb1.Length )//先判断是否>Length
20sb1.Capacity += 7;//这里一定要根据情况的增加容量,否则会有性能上的消耗
21sb1.Append( i.ToString() );
22i++;
23}
24EndTime = DateTime.Now.Ticks;
25
26Console.WriteLine( "时间:" + ( EndTime-StartTime ) + "\t Capacity:"+ sb1.Capacity + "\t
27
28Length:" + sb1.Length);
需要特别说明的一点是,自动增加的容量,一定要根据实际预见的情况而改变,否则不但起不到优化的作用,反而会影响到程序的性能..System.Text.StringBuilder sb = new System.Text.StringBuilder();2
int i = 0;3
long StartTime = DateTime.Now.Ticks;4
while ( i < 100000 ) {
5
sb.Append( i.ToString() );6
i++;7
}8
long EndTime = DateTime.Now.Ticks;9
10
Console.WriteLine( "时间:" + ( EndTime-StartTime ) + "\t Capacity:"+ sb.Capacity + "\t Length:" 11
12
+ sb.Length);13
14
System.Text.StringBuilder sb1 = new System.Text.StringBuilder();15
i = 0;16
StartTime = DateTime.Now.Ticks;17
while ( i < 100000 ) 18
{19
if ( sb1.Capacity <= sb1.Length )//先判断是否>Length20
sb1.Capacity += 7;//这里一定要根据情况的增加容量,否则会有性能上的消耗21
sb1.Append( i.ToString() );22
i++;23
}24
EndTime = DateTime.Now.Ticks;25
26
Console.WriteLine( "时间:" + ( EndTime-StartTime ) + "\t Capacity:"+ sb1.Capacity + "\t 27
28
Length:" + sb1.Length);另外,如果有时间的话,可以测试一下下面的代码,用string和StringBuilder拼接字符串的区别..你会吓到的!!
1System.Text.StringBuilder sb = new System.Text.StringBuilder();
2int i = 0;
3long StartTime = DateTime.Now.Ticks;
4while ( i < 100000 ) {
5sb.Append( i.ToString() );
6i++;
7}
8long EndTime = DateTime.Now.Ticks;
9
10Console.WriteLine( "时间:" + ( EndTime-StartTime ) );
11
12string sb1 = null;
13i = 0;
14StartTime = DateTime.Now.Ticks;
15while ( i < 100000 )
16{
17sb1 += i;
18i++;
19}
20EndTime = DateTime.Now.Ticks;
21Console.WriteLine( "时间:" + ( EndTime-StartTime ));
System.Text.StringBuilder sb = new System.Text.StringBuilder();2
int i = 0;3
long StartTime = DateTime.Now.Ticks;4
while ( i < 100000 ) {5
sb.Append( i.ToString() );6
i++;7
}8
long EndTime = DateTime.Now.Ticks;9
10
Console.WriteLine( "时间:" + ( EndTime-StartTime ) );11
12
string sb1 = null;13
i = 0;14
StartTime = DateTime.Now.Ticks;15
while ( i < 100000 ) 16
{17
sb1 += i;18
i++;19
}20
EndTime = DateTime.Now.Ticks;21
Console.WriteLine( "时间:" + ( EndTime-StartTime ));
