Java StringBuilder indexOf() Method

The StringBuilder.indexOf() method in Java is used to find the index of the first occurrence of a specified substring within a StringBuilder object. This guide will cover both overloaded versions of the method, explain how they work, and provide examples to demonstrate their functionality. We will also cover a real-world use case to show how StringBuilder.indexOf() can be used effectively.

Table of Contents

1. Introduction
2. indexOf Method Syntax
3. Examples
   • Finding the Index of a Substring
   • Finding the Index of a Substring from a Specific Position
4. Real-World Use Case
   • Example: Finding the Position of a Keyword in a String
5. Conclusion

Introduction

The StringBuilder.indexOf() method is a member of the StringBuilder class in Java. It allows you to find the index of the first occurrence of a specified substring within a StringBuilder object. There are two overloaded versions of this method: one that starts the search from the beginning of the StringBuilder and another that starts the search from a specified index.

indexOf Method Syntax

indexOf(String str)

The syntax for the indexOf(String str) method is as follows:

public int indexOf(String str)

• Parameters:
  • str: The substring to search for.
• Returns: The index of the first occurrence of the specified substring, or -1 if there is no such occurrence.

indexOf(String str, int fromIndex)

The syntax for the indexOf(String str, int fromIndex) method is as follows:

public int indexOf(String str, int fromIndex)

• Parameters:
  • str: The substring to search for.
  • fromIndex: The index from which to start the search.
• Returns: The index of the first occurrence of the specified substring starting from the specified index, or -1 if there is no such occurrence.

Examples

Finding the Index of a Substring

The indexOf(String str) method can be used to find the index of the first occurrence of a substring within a StringBuilder object.

Example

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

        // Finding the index of the substring "world"
        int index = sb.indexOf("world");

        // Printing the index
        System.out.println("Index of 'world': " + index);
    }
}

Output:

Index of 'world': 7

Finding the Index of a Substring from a Specific Position

The indexOf(String str, int fromIndex) method can be used to find the index of the first occurrence of a substring starting from a specified index within a StringBuilder object.

Example

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

        // Finding the index of the substring "Hello" starting from index 10
        int index = sb.indexOf("Hello", 10);

        // Printing the index
        System.out.println("Index of 'Hello' from index 10: " + index);
    }
}

Output:

Index of 'Hello' from index 10: 14

Real-World Use Case

Example: Finding the Position of a Keyword in a String

A common real-world use case for StringBuilder.indexOf() is finding the position of a keyword within a string.

Example

public class KeywordSearch {
    public static void main(String[] args) {
        StringBuilder text = new StringBuilder("The quick brown fox jumps over the lazy dog.");

        // Finding the position of the keyword "fox"
        int keywordPosition = text.indexOf("fox");

        // Printing the position of the keyword
        if (keywordPosition != -1) {
            System.out.println("The keyword 'fox' is found at index: " + keywordPosition);
        } else {
            System.out.println("The keyword 'fox' is not found.");
        }
    }
}

Output:

The keyword 'fox' is found at index: 16

In this example, StringBuilder.indexOf() is used to find the position of the keyword "fox" within a string, demonstrating how it can be useful for text searching and processing tasks.

Conclusion

The StringBuilder.indexOf() method in Java provides a way to find the index of the first occurrence of a specified substring within a StringBuilder object. By understanding how to use both overloaded versions of this method, you can efficiently search for substrings in your Java applications. The method allows you to perform various string search operations, making it a versatile tool for string manipulation and text processing in various scenarios.