In Java, the **`indexOf()` function** is used to find the index of a specific character or substring.

In case the function successfully locates the character or substring, the starting index is returned. If not, -1 is returned. 

There are four variants of the `indexOf()` function. Let's look at each of them in detail.

# 1. indexOf(ch)
This returns the index of the first occurrence of the character `ch` in the given string.


## Parameter
* `ch` - the character to search for

## Example

class HelloWorld {
    public static void main( String args[] ) {
        String str = "Hello!";
        System.out.printf("o is located at index: %d", str.indexOf('o'));
    }
}

# 2. indexOf(ch, searchFrom)
This variant of the indexOf() function searches for the first occurrence of the character `ch` starting from the specified index, `searchFrom`, of the string.

## Parameter
* `ch` - the character to search for
* `searchFrom` - the index where the search will begin from

## Example


class HelloWorld {
    public static void main( String args[] ) {
        String str = "Hello!";
        System.out.printf("l is located at index: %d", str.indexOf('l',3));
    }
}

# 3. indexOf(str)
This variant of the indexOf() function returns the index at which the first occurrence of the String str starts.

## Parameter
* `str` - the string to search for

## Example


class HelloWorld {
    public static void main( String args[] ) {
        String str = "Image!";
        System.out.printf("age is located at index: %d", str.indexOf("age"));
    }
}

# 4. indexOf(str, searchFrom)
This variant of the indexOf() function returns the starting index of the first occurrence of the String str, starting the search from the specified index, searchFrom, of the string.
## Parameter
* `str` - the string to search for
* `searchFrom` - the index where the search will begin from


## Example


As the substring *age* is not located ahead of the third index, the function will return -1.

class HelloWorld {
    public static void main( String args[] ) {
        String str = "Image!";
        System.out.printf("age is located at index: %d", str.indexOf("age", 3));
    }
}

How to use indexOf() in Java

The indexOf() function in Java locates characters or substrings, returning their starting index or -1 if not found. It has four variants for different use cases.