Java StringBuilder lastIndexOf() Method

The StringBuilder.lastIndexOf() method in Java is used to find the index of the last occurrence of a specified substring within a StringBuilder object. This guide will cover the method’s usage, explain how it works, and provide examples to demonstrate its functionality, including the overloaded methods.

Table of Contents

  1. Introduction
  2. lastIndexOf Method Syntax
  3. Examples
    • Finding the Last Index of a Substring
    • Finding the Last Index of a Substring from a Specified Position
  4. Real-World Use Case
  5. Conclusion

Introduction

The StringBuilder.lastIndexOf() method is a member of the StringBuilder class in Java. It allows you to search for the last occurrence of a specified substring within the StringBuilder object. This method is particularly useful when you need to locate the last appearance of a substring in a mutable sequence of characters.

lastIndexOf() Method Syntax

The StringBuilder class provides two overloaded lastIndexOf methods:

  1. lastIndexOf(String str)
  2. lastIndexOf(String str, int fromIndex)

Method 1: lastIndexOf(String str)

The syntax for the first lastIndexOf method is as follows:

public int lastIndexOf(String str)
  • str: The substring to search for.

Method 2: lastIndexOf(String str, int fromIndex)

The syntax for the second lastIndexOf method is as follows:

public int lastIndexOf(String str, int fromIndex)
  • str: The substring to search for.
  • fromIndex: The index to start the reverse search from.

Examples

Finding the Last Index of a Substring

The first lastIndexOf method can be used to find the index of the last occurrence of a specified substring within a StringBuilder.

Example

public class StringBuilderLastIndexOfExample {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hello, World! Hello, World!");

        // Find the last index of the substring "World"
        int lastIndex = sb.lastIndexOf("World");

        // Print the index
        System.out.println("Last index of 'World': " + lastIndex);
    }
}

Output:

Last index of 'World': 20

Finding the Last Index of a Substring from a Specified Position

The second lastIndexOf method can be used to find the index of the last occurrence of a specified substring, starting the reverse search from a given index.

Example

public class StringBuilderLastIndexOfExample {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hello, World! Hello, World!");

        // Find the last index of the substring "World" before position 15
        int lastIndex = sb.lastIndexOf("World", 15);

        // Print the index
        System.out.println("Last index of 'World' before position 15: " + lastIndex);
    }
}

Output:

Last index of 'World' before position 15: 7

In this example, the reverse search for the substring "World" starts from index 15, and it finds the first occurrence of "World" in the string starting from the end and moving backwards.

Real-World Use Case

Example: Finding the Last Occurrence of a File Extension

In a real-world scenario, you might need to find the last occurrence of a file extension in a file path. Using the lastIndexOf method, you can locate the position of the last dot (.) to extract the file extension.

Example Code

public class FileExtensionFinder {
    public static void main(String[] args) {
        StringBuilder filePath = new StringBuilder("C:/Users/JohnDoe/Documents/Report.final.docx");

        // Find the last index of the dot (.) to locate the file extension
        int lastIndex = filePath.lastIndexOf(".");

        // Extract the file extension
        String fileExtension = filePath.substring(lastIndex + 1);

        // Print the file extension
        System.out.println("File extension: " + fileExtension);
    }
}

Output:

File extension: docx

Conclusion

The StringBuilder.lastIndexOf() method in Java is used for locating the last occurrence of substrings within a StringBuilder object. By understanding how to use the overloaded methods, you can efficiently search for substrings either from the end of the StringBuilder or from a specified position. Whether you need to find the last index of a substring or start the reverse search from a particular index, the lastIndexOf methods provide a reliable solution for these tasks.

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