split on the AP CS A Exam

Unlike the other String methods featured on the AP CS A Exam, usage of split is explicitly limited. The split method accepts a regular expression as a parameter. Regular expressions are not featured on the AP CS A Exam and are rarely covered in introductory programming courses.

In AP CS A materials, split is shown as String[] split(String del). In Java documentation, split is shown as String[] split(String regex).

Calls to the split method on the AP CS A Exam will be with basic strings as delimeters, such as

not with regular expressions.

When writing code that calls split it is important to avoid accidentally passing a regular expression as an argument when the intent is to pass a basic string.

split example 1 (works)

String str = "Brandon Brenda Evan";
String[] parts = str.split(" ");

After the code segment above executes, parts contains:

["Brandon", "Brenda", "Evan"]

The split method extracts the individual strings "Brandon", "Brenda", and "Evan" from the implicit parameter ("Brandon Brenda Evan"). The method extracts strings separated by a space (" ") (a space) because the space is passed as an argument.

The split method returns a String array (String[]) containing the extracted strings.

split example 2 (works)

String str = "Brandon Horn, Brenda Horn, Evan Monster";
String[] parts = str.split(", ");

After the code segment above executes, parts contains:

["Brandon Horn", "Brenda Horn", "Evan Monster"]

The argument is a comma followed by a space (", "). This is treated as a basic string with no special meaning.

split example with accidental regular expression

String str = "Brandon.Brenda.Evan";
String[] parts = str.split(".");

After the code segment above executes, parts is an empty array ([]).

The programmer may have intended for the call to split to return ["Brandon", "Brenda", "Evan"] but it does not.

Many characters have special meaning when included in a regular expression, including a dot (".").

Calling split

Each character in the string "<([{\^-=$!|]})?*+.>" has a special meaning when used in a regular expression. A backslash followed by a character (ex: "\d") also has special meaning.

Avoid including these in strings passed as arguments to split (unless your intent is to write a regular expression).

Additional split examples

The examples below demonstrate specific behaviors that may not be obvious. The AP CS A Exam is unlikely to trick students with respect to the behavior shown in these examples; however, students may encounter these in their own code.

With a word

String str = "onefishtwofishredfishbluefish";
String[] parts = str.split("fish");

After the code segment above executes, parts contains:

["one", "two", "red", "blue"]

With spaces

String str = "one fish two fish red fish blue fish";
String[] parts = str.split("fish");

After the code segment above executes, parts contains:

["one ", " two ", " red ", " blue "]

Note the spaces surrounding each extracted string, except the first.

With spaces 2

String str = "one fish two fish red fish blue fish";
String[] parts = str.split(" fish ");

After the code segment above executes, parts contains:

["one", "two", "red", "blue fish"]

Note the last extracted string.

With delimeter at beginning

String str = " Brandon Brenda Evan ";
String[] names = str.split(" ");

After the code segment above executes, names contains:

["", "Brandon", "Brenda", "Evan"]

Note the empty string ("") at the beginning and the lack of one at the end.

Documentation

The 2025 AP CS A Course Description documents split as used on the AP CS A Exam (PDF page 114, page number 107 on bottom).

The Java API documentation for split(String) references the Java API documentation for split(String, int). The 2 parameter split method is not featured on the AP CS A Exam. The Java API documentation describes functionality that is not used on the AP CS A Exam.

Additional resources

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