java split(String regex, int limit) 的使用

項目中遇到了這樣一個問題，對 String str = ",," 調用 split(",")方法，預期結果是返回一個長度為 3 的String數組，且每一個元素都為空字符串 ""。但實際結果返還的是一個空數組，長度為 0 。

百度之，原來java中還有 split(String regex, int limit)這中用法，String[] java.lang.String.split(String regex, int limit)，其中regex為分割正則表達式，limit為分割次數限制，官方文檔這樣解釋：

 

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.

當 limit  > 0的時候，str將被分割 limit - 1次：

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

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

輸出為：

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. 

當 limit < 0 的時候，str將被盡可能多的分割：

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

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

輸出為：

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

當 limt = 0 的時候，str被盡可能多的分割，但是尾部的空字符串會被拋棄：

        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);
        }

輸出為：

0
2
a
b

 

對於 沒有 limit 參數的 split函數， 官方解釋如下：

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. 

也就是等價於 limt = 0 的情況，尾部的空字符串被舍棄掉：

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

輸出為：

2
a
b