我们常常用String的split()方法去分割字符串，有两个地方值得注意：
1. 当分隔符是句号时(".")，需要转义：
由于String.split是基于正则表达式来分割字符串，而句号在正则表达式里表示任意字符。
//Wrong:
//String[] words = tmp.split("."); //Correct:
String[] words = tmp.split("\\.");
所以，假设分隔符在正则表达式里有一定的意义时，需要格外留心，必须将它们转义才能达到分割的效果。
2. 假设字符串最后有连续多个分隔符，且这些分隔符都需要被分割的话，需要调用split(String regex,int limit)这个方法：
String abc = "a,b,c,,,";
String[] str = abc.split(","); System.out.println(Arrays.toString(str)+" "+str.length); String[] str2 = abc.split(",",-1); System.out.println(Arrays.toString(str2)+" "+str2.length);
输出如下：
[a, b, c] 3
[a, b, c, , , ] 6
需要输出csv文件的时候，尤其需要注意。
3. 假设需要快速分割字符串，split()并不是最有效的方法。在split()方法内，有如下的实现：
public String[] split(String regex, int limit) {
return Pattern.compile(regex).split(this, limit);
}
频繁调用split()会不断创建Pattern这个对象，因此可以这样去实现，减少Pattern的创建：
//create the Pattern object outside the loop
Pattern pattern = Pattern.compile(" "); for (int i = 0; i < 1000000; i++)
{
String[] split = pattern.split("Hello World", 0);
list.add(split);
}
另外split()也往往比indexOf()+subString()这个组合分割字符串要稍慢，详情可看这个帖子。
我在本机做过测试，感觉indexOf()+subString()比split()快一倍：
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
for (int i = 100000; i < 100000 + 60; i++)
sb.append(i).append(' ');
String sample = sb.toString();
int runs = 100000;
for (int i = 0; i < 5; i++) {
{
long start = System.nanoTime();
for (int r = 0; r < runs; r++) {
StringTokenizer st = new StringTokenizer(sample);
List<String> list = new ArrayList<String>();
while (st.hasMoreTokens())
list.add(st.nextToken());
}
long time = System.nanoTime() - start;
System.out.printf("StringTokenizer took an average of %.1f us%n", time / runs
/ 1000.0);
}
{
long start = System.nanoTime();
Pattern spacePattern = Pattern.compile(" ");
for (int r = 0; r < runs; r++) {
List<String> list = Arrays.asList(spacePattern.split(sample, 0));
}
long time = System.nanoTime() - start;
System.out.printf("Pattern.split took an average of %.1f us%n", time / runs
/ 1000.0);
}
{
long start = System.nanoTime();
for (int r = 0; r < runs; r++) {
List<String> list = new ArrayList<String>();
int pos = 0, end;
while ((end = sample.indexOf(' ', pos)) >= 0) {
list.add(sample.substring(pos, end));
pos = end + 1;
}
}
long time = System.nanoTime() - start;
System.out
.printf("indexOf loop took an average of %.1f us%n", time / runs / 1000.0);
}
}
}
在jdk1.7测试后，结果如下：
StringTokenizer took an average of 7.2 us
Pattern.split took an average of 7.9 us
indexOf loop took an average of 3.5 us
------------------------------------------
StringTokenizer took an average of 6.8 us
Pattern.split took an average of 5.4 us
indexOf loop took an average of 3.1 us
------------------------------------------
StringTokenizer took an average of 6.0 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.1 us
------------------------------------------
StringTokenizer took an average of 5.9 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.1 us
------------------------------------------
StringTokenizer took an average of 6.4 us
Pattern.split took an average of 5.5 us
indexOf loop took an average of 3.2 us
本文完