Java String indexOf() Method

Java String indexOf() method is used to find the index of a specified character or a substring in a given String. Here are the different variations of this method in String class:

Finding the index of a character:

int indexOf(int ch)

It returns the index of first occurrence of character ch in the given string. If the character is not found, it returns -1.

String str = "Hello, world!";
int index = str.indexOf('o'); // returns 4

Finding the index of a character starting from a specific position:

int indexOf(int ch, int fromIndex)

It returns the index of first occurrence of character ch in the given string, starting the search from specified index fromIndex. If the character is not found, it returns -1.

String str = "Hello, world!";
int index = str.indexOf('o', 5); // returns 8

Finding the index of a substring:

int indexOf(String str)

It returns the index of the first occurrence of the specified substring. If the substring is not found, it returns -1.

String str = "Hello, world!";
int index = str.indexOf("world"); // returns 7

Finding the index of a substring starting from a specific position:

int indexOf(String str, int fromIndex)

It returns the index of string str in the given string after the specified index “fromIndex”. If the substring is not found, it returns -1.

String str = "Hello, world!";
int index = str.indexOf("world", 8); // returns -1

Example 1:

public class StringIndexOfExample {
    public static void main(String[] args) {
        String str = "Hello, world!";

        // Find the index of a character
        int index1 = str.indexOf('o');
        System.out.println("Index of 'o': " + index1); // Output: 4

        // Find the index of a character starting from the given index
        int index2 = str.indexOf('o', 5);
        System.out.println("Index of 'o' from position 5: " + index2); // Output: 8

        // Find the index of a substring
        int index3 = str.indexOf("world");
        System.out.println("Index of \"world\": " + index3); // Output: 7

        // Find the index of a substring starting from the given index
        int index4 = str.indexOf("world", 8);
        System.out.println("Index of \"world\" from position 8: " + index4); // Output: -1
    }
}

Example 2:

Let’s take a simple example with a short string where we are finding the indexes of given chars and substring using the indexOf() method.

public class JavaExample {  
   public static void main(String[] args) {  
	String str = "Java String";
	char ch = 'J';
	char ch2 = 'S';
	String subStr = "tri";
	int posOfJ = str.indexOf(ch);
	int posOfS = str.indexOf(ch2);
	int posOfSubstr = str.indexOf(subStr);
	System.out.println(posOfJ);
	System.out.println(posOfS);
	System.out.println(posOfSubstr);
   }  
}

Output:

0
5
6