How to Use Negative Indexing in Java's String `indexOf` Method to Count from the End?

How can I effectively use negative indexing in Java's `indexOf` method to count positions from the end of a string?

String str = "Hello World";
int index = str.length() - 1 - str.indexOf("o"); // Thinking from the end

In Java, the `indexOf` method is used to find the position of a character or substring within a string. However, it does not support negative indexing natively, which implies counting positions from the end of the string. We can achieve this by combining the `length()` method and the `indexOf` method to determine the position relative to the end of the string.

// Example of using length() to find position from the end:
String str = "Hello World";
int positionFromEnd = str.length() - str.indexOf("o") - 1; // 7 (for 'o' found at index 4) 
System.out.println("Position from end: " + positionFromEnd);

Causes

• Misunderstanding the behavior of the `indexOf` method which returns the first occurrence of a character or substring starting from the beginning of the string.
• Assuming `indexOf` supports negative indices, leading to confusion about string indexing in Java.

Solutions

• Use the `length()` method to obtain the length of the string and subtract the result of `indexOf` from this length to emulate negative indexing.
• Calculate the position from the end by manipulating the return value of `indexOf` based on string length.

Mistake: Confusing `indexOf` with negative indexing which is common in languages like Python.

Solution: Remember that Java does not support negative indexes; instead, calculate positions manually.

Mistake: Using `indexOf` assuming it will return -1 for negative notations.

Solution: Always check if the result of `indexOf` is -1 to handle cases where the substring is not found.