java split，String regex, int limit 的使用

项目中遇到了这样一个问题，对 String str = ",," 调用 split(",")方法，预期结果是返回一个长度为 3 的String数组，且每一个元素都为空字符串 ""。但实际结果返还的是一个空数组，长度为 0 。

百度之，原来java中还有 split(String regex, int limit)这中用法，String[] java.lang.String.split(String regex, int limit)，其中regex为分割正则表达式，limit为分割次数限制，官方文档这样解释：

1. The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter.

当 limit > 0的时候，str将被分割 limit - 1次：

1 String str = ",,,,";
2 String [] strLst = str.split(",", 2);                
3 System.out.println(strLst.length);
4 
5 for (String e : strLst) {
6      System.out.println(e);
7 }

输出为：

2

,,,

2. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length.

当 limit < 0 的时候，str将被尽可能多的分割：

1    
2 String str = ",,,,";
3 String [] strLst = str.split(",", -1);                
4 System.out.println(strLst.length);
5 
6 for (String e : strLst) {
7      System.out.println(e);
8 }

输出为：

5

3. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded

当 limt = 0 的时候，str被尽可能多的分割，但是尾部的空字符串会被抛弃：

 1         String str = ",,,,";
 2         String [] strLst = str.split(",", 0);                
 3         System.out.println(strLst.length);
 4         for (String e : strLst) {
 5             System.out.println(e);
 6         }
 7         
 8         str = "a,b,,,";
 9         strLst = str.split(",", 0);
10         System.out.println(strLst.length);
11         for (String e : strLst) {
12             System.out.println(e);
13         }

输出为：

0
2
a
b

对于 没有 limit 参数的 split函数， 官方解释如下：

String[] java.lang.String.split(String regex)

This method works as if by invoking the two-argument split method with the given expression and a limit argument of zero. Trailing empty strings are therefore not included in the resulting array.

也就是等价于 limt = 0 的情况，尾部的空字符串被舍弃掉：

1         String str = "a,b,,,";
2         String [] strLst = str.split(",");                
3         System.out.println(strLst.length);
4         for (String e : strLst) {
5             System.out.println(e);
6         }

输出为：

2
a
b