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：

String str = ",,,,";
String [] strLst = str.split(",", 2);                
System.out.println(strLst.length);

for (String e : strLst) {
     System.out.println(e);
}

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：

   
String str = ",,,,";
String [] strLst = str.split(",", -1);                
System.out.println(strLst.length);

for (String e : strLst) {
     System.out.println(e);
}

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：

        String str = ",,,,";
        String [] strLst = str.split(",", 0);                
        System.out.println(strLst.length);
        for (String e : strLst) {
            System.out.println(e);
        }
        
        str = "a,b,,,";
        strLst = str.split(",", 0);
        System.out.println(strLst.length);
        for (String e : strLst) {
            System.out.println(e);
        }

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：

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

输出为：

2
a
b