Java indexOf()

1. Java 문자열 indexOf()

Java의 indexOf() 메소드는 주어진 문자열 내에서 문자 또는 하위 문자열이 처음 나타나는 위치를 반환한다. 문자나 부분 문자열을 찾을 수 없으면 메서드는 -1을 반환한다.

public int indexOf(int ch)
public int indexOf(int ch, int fromIndex)
public int indexOf(String str)
public int indexOf(String str, int fromIndex)

2. 문자열에서 문자 위치 찾기

문자열에서 문자 ‘o’의 위치를 ​​찾으려면 다음 코드를 사용한다.

1) indexOf(int ch)

• 예제

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

    int position = str.indexOf('o');
    System.out.println("Position of 'o' in the string is: " + position);
}

• 결과

Position of 'o' in the string is: 4

2) indexOf(int ch, int fromIndex)

• 예제

public static void main(String[] args) {
    String str = "Hello, world!";
    
    int position = str.indexOf('o');

    while (position >= 0) {
        System.out.println("'o' found at position " + position);
        position = str.indexOf('o', position + 1);
    }
}

• 결과

'o' found at position 4
'o' found at position 8

3) indexOf(String str)

• 예제

public static void main(String[] args) {
    String str = "Java is a popular programming language";
    
    int position = str.indexOf("programming");
    System.out.println("Position of 'programming' in the string is: " + position);
}

• 결과

Position of 'programming' in the string is: 18

4) indexOf(String str, int fromIndex)

• 예제

public static void main(String[] args) {
    String str = "Java is a popular programming language. Java is widely used in web development.";

    int position = -1;


    while ((position = str.indexOf('a', position + 1)) != -1) {
        System.out.println("Found 'a' at position: " + position);
    }

    while ((position = str.indexOf("Java", position + 1)) != -1) {
        System.out.println("Found 'Java' at position: " + position);
    }
}

• 결과

Found 'a' at position: 1
Found 'a' at position: 3
Found 'a' at position: 8
Found 'a' at position: 15
Found 'a' at position: 23
Found 'a' at position: 31
Found 'a' at position: 35
Found 'a' at position: 41
Found 'a' at position: 43
Found 'Java' at position: 0
Found 'Java' at position: 40

[출처 및 참고]

• https://codegym.cc/ko/groups/posts/ko.1064.–indexof