Java String indexOf() Method

The String.indexOf() method in Java is used to find the index of a specified character or substring within a string. This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. indexOf Method Syntax
3. Examples
   • Finding a Character's Index
   • Finding a Substring's Index
   • Starting Search from a Specific Index
   • Handling Not Found Cases
4. Conclusion

Introduction

The String.indexOf() method is a member of the String class in Java. It allows you to locate the position of a character or substring within a string. This method is particularly useful for searching, parsing, and manipulating strings based on specific characters or patterns.

indexOf Method Syntax

The indexOf method has several overloads:

1. Finding the index of a character:

public int indexOf(int ch)

2. Finding the index of a character starting from a specified index:

public int indexOf(int ch, int fromIndex)

3. Finding the index of a substring:

public int indexOf(String str)

4. Finding the index of a substring starting from a specified index:

public int indexOf(String str, int fromIndex)

Examples

Finding a Character's Index

The indexOf method can be used to find the first occurrence of a specified character within a string.

Example

public class IndexOfExample {
    public static void main(String[] args) {
        String message = "Hello, World!";

        int indexOfH = message.indexOf('H');
        int indexOfW = message.indexOf('W');

        System.out.println("Index of 'H': " + indexOfH);
        System.out.println("Index of 'W': " + indexOfW);
    }
}

Output:

Index of 'H': 0
Index of 'W': 7

Finding a Substring's Index

The indexOf method can be used to find the first occurrence of a specified substring within a string.

Example

public class IndexOfExample {
    public static void main(String[] args) {
        String message = "Hello, World!";

        int indexOfHello = message.indexOf("Hello");
        int indexOfWorld = message.indexOf("World");

        System.out.println("Index of 'Hello': " + indexOfHello);
        System.out.println("Index of 'World': " + indexOfWorld);
    }
}

Output:

Index of 'Hello': 0
Index of 'World': 7

Starting Search from a Specific Index

The indexOf method can be used to start the search from a specified index.

Example

public class IndexOfExample {
    public static void main(String[] args) {
        String message = "Hello, World! Hello, Java!";

        int indexOfHelloFirst = message.indexOf("Hello");
        int indexOfHelloSecond = message.indexOf("Hello", 10);

        System.out.println("Index of first 'Hello': " + indexOfHelloFirst);
        System.out.println("Index of second 'Hello': " + indexOfHelloSecond);
    }
}

Output:

Index of first 'Hello': 0
Index of second 'Hello': 14

Handling Not Found Cases

The indexOf method returns -1 if the specified character or substring is not found.

Example

public class IndexOfExample {
    public static void main(String[] args) {
        String message = "Hello, World!";

        int indexOfJava = message.indexOf("Java");

        System.out.println("Index of 'Java': " + indexOfJava);
    }
}

Output:

Index of 'Java': -1

Conclusion

The String.indexOf() method in Java for locating the position of characters and substrings within a string. By understanding how to use this method, you can efficiently search, parse, and manipulate strings in your Java applications. Whether you are finding the index of a character, a substring, starting the search from a specific index, or handling cases where the character or substring is not found, the indexOf method provides a reliable solution for these tasks.