Java String indexOf() example

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In this guide, you will learn about the String indexOf() method in Java programming and how to use it with an example.

1. String indexOf() Method Overview

Definition:

The indexOf() method returns the position of the first occurrence of a specified character(s) in a string.

Syntax:

1. str.indexOf(int ch)
2. str.indexOf(int ch, int fromIndex)
3. str.indexOf(String substring)
4. str.indexOf(String substring, int fromIndex)

Parameters:

- ch: a character (Unicode code point).

- fromIndex: the index to start the search from.

- substring: a string to be searched.

Key Points:

- Returns the index of the first occurrence of the character or substring in the string.

- Returns -1 if the character or substring is not found.

- The search starts at the specified fromIndex and moves towards the end of this string.

- The method throws no exception if fromIndex is negative or greater than this string's length; it just returns -1.

2. String indexOf() Method Example

public class IndexOfExample {
    public static void main(String[] args) {
        String sample = "JavaProgramming";

        // Searching for a character
        int index1 = sample.indexOf('a');
        System.out.println("First occurrence of 'a': " + index1);

        // Searching from a specific index
        int index2 = sample.indexOf('a', 2);
        System.out.println("Occurrence of 'a' after index 2: " + index2);

        // Searching for a substring
        int index3 = sample.indexOf("Prog");
        System.out.println("First occurrence of 'Prog': " + index3);

        // Searching for a non-existent substring
        int index4 = sample.indexOf("Hello");
        System.out.println("Searching for 'Hello': " + index4);
    }
}

Output:

First occurrence of 'a': 1
Occurrence of 'a' after index 2: 3
First occurrence of 'Prog': 4
Searching for 'Hello': -1

Explanation:

In the example:

1. We first search for the character 'a' in the string "JavaProgramming". It returns the index of the first occurrence.

2. Next, we search for the character 'a' starting from index 2. It finds the next occurrence after that index.

3. Then, we search for the substring "Prog" and it returns the index of its first occurrence.

4. Lastly, we search for a non-existent substring "Hello". Since it's not in the string, -1 is returned.

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