Understanding the 'String Index Out of Range' Error in Java when Using Substrings

What causes the 'String index out of range' error in Java when using the substring method?

String str = "Hello World!";
String subStr = str.substring(0, 12); // This will throw an exception!

The 'String index out of range' error occurs in Java when you attempt to access a character in a string using an invalid index with the substring method. This error often arises when the specified start or end index exceeds the string's length, leading to an IllegalArgumentException.

String str = "Hello World!";
int startIndex = 0;
int endIndex = 5;
try {
    String subStr = str.substring(startIndex, endIndex);
    System.out.println(subStr); // Outputs: Hello
} catch (StringIndexOutOfBoundsException e) {
    System.out.println("Invalid substring indices: " + e.getMessage());
}

Causes

• The provided start index is greater than the string's length.
• The end index provided exceeds the length of the string.
• The start index is negative.
• The end index is less than the start index.

Solutions

• Always verify that the start and end indices are within the range of the string's length.
• Use the length() method to check the size of the string before calling substring.
• Handle exceptions using a try-catch block to manage index errors gracefully.

Mistake: Assuming the end index is equal to the string length instead of less than the length.

Solution: Remember that the end index is exclusive; always use indices that are strictly less than the string length.

Mistake: Not validating indices before using them in the substring method.

Solution: Always validate and manage indices, especially when dynamically generating them.