Java里String.split需要注意的用法

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