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Java String indexOf()

The String indexOf() method returns the index of the first occurrence of the specified character/substring within the string.

The syntax of the String indexOf() method either

string.indexOf(int ch, int fromIndex)

or

string.indexOf(String str, int fromIndex)

Here, string is an object of the String class.


indexOf() Parameters

To find the index of a character, indexOf() takes these two parameters:

  • ch - the character whose starting index is to be found
  • fromIndex (optional) - if fromIndex is passed, the ch character is searched starting from this index

To find the index of the specified substring within the string, indexOf() takes these two parameters:

  • str - the string whose starting index is to be found
  • fromIndex (optional) - if fromIndex is passed, the str string is searched starting from this index

indexOf() Return Value

  • returns the index of the first occurrence of the specified character/string
  • returns -1 if the specified character/string is not found.

Example 1: Java String indexOf()

// Java String indexOf() with only one parameter
class Main {
  public static void main(String[] args) {
    String str1 = "Learn Java";
    int result;

    // getting index of character 'J'
    result = str1.indexOf('J');
    System.out.println(result); // 6

    // the first occurrence of 'a' is returned
    result = str1.indexOf('a');
    System.out.println(result); // 2

    // character not in the string
    result = str1.indexOf('j');
    System.out.println(result); // -1

    // getting the index of "ava"
    result = str1.indexOf("ava");
    System.out.println(result); // 7

    // substring not in the string
    result = str1.indexOf("java");
    System.out.println(result); // -1

    // index of empty string in the string
    result = str1.indexOf("");
    System.out.println(result); // 0
  }
}

Notes:

  • The character 'a' occurs multiple times in the "Learn Java" string. The indexOf() method returns the index of the first occurrence of 'a' (which is 2).
  • If the empty string is passed, indexOf() returns 0 (found at the first position. It is because the empty string is a subset of every substring.

Example 2: indexOf() With fromIndex Parameter

class Main {
  public static void main(String[] args) {
    String str1 = "Learn Java programming";
    int result;

    // getting the index of character 'a'
    // search starts at index 4
    result = str1.indexOf('a', 4);
    System.out.println(result); // 7

    // getting the index of "Java"
    // search starts at index 8
    result = str1.indexOf("Java", 8);
    System.out.println(result); // -1
  }
}

Notes:

  • The first occurrence of 'a' in the "Learn Java programming" string is at index 2. However, the index of second 'a' is returned when str1.indexOf('a', 4) is used. It is because the search starts at index 4.
  • The "Java" string is in the "Learn Java programming" string. However, str1.indexOf("Java", 8) returns -1 (string not found). It is because the search starts at index 8 and there is no "Java" in "va programming".

Recommended Reading: Java String lastIndexOf()