Java String lastIndexOf() Method

The String.lastIndexOf() method in Java is used to find the index of the last occurrence of a specified character or substring within a string.

Table of Contents

1. Introduction
2. lastIndexOf Method Syntax
3. Examples
   • Finding the Last Occurrence of a Character
   • Finding the Last Occurrence of a Substring
   • Finding with a Start Index
   • Handling Cases Where the Character or Substring is Not Found
   • Real-World Use Case
4. Conclusion

Introduction

The String.lastIndexOf() method is a member of the String class in Java. It allows you to find the position of the last occurrence of a character or substring within a string. If the character or substring is found, the method returns the index of its last occurrence. If it is not found, the method returns -1.

lastIndexOf() Method Syntax

The syntax for the lastIndexOf method is as follows:

Finding the Last Occurrence of a Character

public int lastIndexOf(int ch)

Finding the Last Occurrence of a Character from a Specified Index

public int lastIndexOf(int ch, int fromIndex)

Finding the Last Occurrence of a Substring

public int lastIndexOf(String str)

Finding the Last Occurrence of a Substring from a Specified Index

public int lastIndexOf(String str, int fromIndex)

Examples

Finding the Last Occurrence of a Character

The lastIndexOf method can be used to find the last occurrence of a character within a string.

Example

public class LastIndexOfCharExample {
    public static void main(String[] args) {
        String str = "Welcome to Java Programming";
        int index = str.lastIndexOf('a');
        System.out.println("Last index of 'a': " + index);
    }
}

Output:

Last index of 'a': 23

Finding the Last Occurrence of a Substring

The lastIndexOf method can be used to find the last occurrence of a substring within a string.

Example

public class LastIndexOfSubstringExample {
    public static void main(String[] args) {
        String str = "Welcome to Java Programming";
        int index = str.lastIndexOf("Java");
        System.out.println("Last index of 'Java': " + index);
    }
}

Output:

Last index of 'Java': 11

Finding with a Start Index

The lastIndexOf method can also be used to find the last occurrence of a character or substring, starting the search from a specified index.

Finding the Last Occurrence of a Character from a Specified Index

public class LastIndexOfCharFromIndexExample {
    public static void main(String[] args) {
        String str = "Welcome to Java Programming";
        int index = str.lastIndexOf('a', 10);
        System.out.println("Last index of 'a' from index 10: " + index);
    }
}

Output:

Last index of 'a' from index 10: 9

Finding the Last Occurrence of a Substring from a Specified Index

public class LastIndexOfSubstringFromIndexExample {
    public static void main(String[] args) {
        String str = "Welcome to Java Programming, Welcome to Learning";
        int index = str.lastIndexOf("Welcome", 30);
        System.out.println("Last index of 'Welcome' from index 30: " + index);
    }
}

Output:

Last index of 'Welcome' from index 30: 0

Handling Cases Where the Character or Substring is Not Found

If the character or substring is not found, the lastIndexOf method returns -1.

Example

public class LastIndexOfNotFoundExample {
    public static void main(String[] args) {
        String str = "Welcome to Java Programming";
        int index = str.lastIndexOf('z');
        System.out.println("Last index of 'z': " + index);
    }
}

Output:

Last index of 'z': -1

Real-World Use Case

Example: Finding the Last Occurrence of a File Extension

One common use case for lastIndexOf is finding the position of the last occurrence of a file extension in a file path.

public class FileExtensionExample {
    public static void main(String[] args) {
        String filePath = "/home/user/documents/report.pdf";
        int index = filePath.lastIndexOf('.');
        if (index != -1) {
            String extension = filePath.substring(index);
            System.out.println("File extension: " + extension);
        } else {
            System.out.println("No file extension found.");
        }
    }
}

Output:

File extension: .pdf

In this example, the lastIndexOf method is used to find the position of the last occurrence of the dot (.) character to extract the file extension.

Conclusion

The String.lastIndexOf() method in Java is used for finding the index of the last occurrence of a character or substring within a string. It provides various overloads to accommodate different use cases, including starting the search from a specific index. This method is particularly useful for searching, validation, and manipulation tasks in various applications. By understanding and utilizing the lastIndexOf method, you can efficiently manage string searches in your Java programs.