== Why is this an issue?

There are various `String` operations that take one or more character indexes as arguments and return a portion of the original string.
Indexing in this context is zero-based, meaning that the first character's index is `0`.
As a result, given a string `myString`, its last character is at index `myString.length() - 1`.

The `String` operation methods throw a `StringIndexOutOfBoundsException` when one of their index argument is smaller than 0 (E.G.: -1).
`String::substring` also throws this exception when the `beginIndex` or `endIndex` argument is larger than `myString.length()`, and
`String::charAt` when the `index` argument is larger than `myString.length() - 1`
For instance, it is not possible to use `String::charAt` to retrieve a value before the start or after the end of a string.
Furthermore, it is not possible to use `String::substring` with `beginIndex > endIndex` to reverse the order of characters in a string.

This rule raises an issue when a negative literal or an index that is too large is passed as an argument to the `String::substring`, `String::charAt`, and related methods.
It also raises an issue when the start index passed to `String::substring` is larger than the end index.


== How to fix it

Use non-negative indexes that are smaller than or equal to the length of the string in question with `String::substring` and strictly smaller with `String::charAt`.


=== Code examples


==== Noncompliant code example

[source,java,diff-id=1,diff-type=noncompliant]
----
String speech = "Lorem ipsum dolor sit amet";

String substr1 = speech.substring(-1, speech.length()); // Noncompliant, -1 is out of bounds
String substr2 = speech.substring(speech.length(), 0);  // Noncompliant, the beginIndex must be smaller than or equal to the endIndex
char ch = speech.charAt(speech.length());               // Noncompliant, speech.length() is out of bounds
----


==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
String speech = "Lorem ipsum dolor sit amet";

String substr1 = speech;                                         // Compliant, no string operation used
String substr2 = new StringBuilder(speech).reverse().toString(); // Compliant, the string can be reversed using StringBuilder::reverse()
char ch = speech.charAt(speech.length() - 1);                    // Compliant, speech.length() - 1 is in bounds.
----



ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Refactor this "[substring|charAt]" call; it will result in an "StringIndexOutOfBounds" exception at runtime.


'''
== Comments And Links
(visible only on this page)

=== relates to: S2121

endif::env-github,rspecator-view[]