How to Properly Split a String with a Dot Delimiter in Java

Why does using `split()` with a dot (`.`) delimiter in Java cause an ArrayIndexOutOfBoundsException?

String filename = "D:/some folder/001.docx";
String extensionRemoved = filename.split(".")[0];

In Java, the `String.split()` method is used to divide a string into an array of substrings based on a specified delimiter. However, using a dot (`.`) as a delimiter can lead to unexpected behavior due to regular expressions' interpretation. Here’s why your code throws an `ArrayIndexOutOfBoundsException` and how to fix it, along with a working approach.

String filename = "D:/some folder/001.docx";
String[] parts = filename.split("\\."); // Correctly splitting by dot
String extensionRemoved = parts[0]; // Accessing the base filename

Causes

• The dot character in regular expressions matches any character except line terminators, therefore `filename.split(".")` effectively splits the string into each character rather than by the dot.
• If the filename does not contain a dot (.), the resulting array will be of size 1, and trying to access the first element (index 0) is not an issue, but if the dot is at the beginning or missing, no valid index for non-existent substrings is accessible.

Solutions

• To split the string by a literal dot, escape the dot with a double backslash: `filename.split("\\.")` which will correctly interpret it as a literal dot in the string.
• Alternatively, you can use `StringUtils` from Apache Commons Lang which simplifies the splitting of strings without dealing with regex quirks.

Mistake: Using an unescaped dot as a delimiter in split causing incorrect array length.

Solution: Always escape special characters when using them as delimiters in regex functions.

Mistake: Not checking the resultant array's length before accessing an index.

Solution: Always check array length to prevent accessing an invalid index.