当提供正则表达式时，String.split()方法在Java中到底是如何工作的？

Question

我正在准备OCPJP考试，我遇到了下面的例子：

class Test {
   public static void main(String args[]) {
      String test = "I am preparing for OCPJP";
      String[] tokens = test.split("\\S");
      System.out.println(tokens.length);
   }
}

这段代码打印16。我期望的是像no_of_characters + 1这样的东西。有人能解释一下吗，在这种情况下，split()方法实际上是做什么的？我只是不明白..。

Answer 1

它在正则表达式引擎中代表\S非空格字符的每个"\\S"上拆分。

因此，让我们尝试在非空格(\S)上拆分"x x"。因为这个正则表达式可以由一个字符匹配，所以让我们迭代它们来标记拆分的位置(我们将使用管道|来实现)。

• 是否是'x'非空格？是的，那么让我们将其标记为| x
• is ' '非空格？不是，所以我们把它保留为
• is 'x'非空格？是的，所以让我们将其标记为| |

因此，我们需要在开头和结尾拆分字符串，这最初会给出结果数组

["", " ", ""]
   ^    ^ - here we split

但由于删除了尾随的空字符串，因此结果将为

[""," "]     <- result
        ,""] <- removed trailing empty string

所以split返回数组["", " "]，它只包含两个元素。

顺便说一句。要关闭删除最后一个空字符串的功能，您需要像split("\\S",-1)一样使用带有负值limit的split(regex,limit)。

现在让我们回到您的示例。如果您的数据被拆分到每个

I am preparing for OCPJP
| || ||||||||| ||| |||||

这意味着

 ""|" "|""|" "|""|""|""|""|""|""|""|""|" "|""|""|" "|""|""|""|""|""

所以这表示这个数组

[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]  

但由于尾随的空字符串被删除(如果它们的存在是由Confusing output from String.split上的"" - more信息引起的)

[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]  
                                                     ^^ ^^ ^^ ^^ ^^

您将获得结果数组，其中只包含此部分：

[""," ",""," ","","","","","","","",""," ","",""," "]  

恰好是16个元素。