Java string intercepts a string that specifies the location of the subscript

public class A {
    public static void Main (string[] args) {
        The defined string
        String s = "Hello world";
        get the length of a string by the length method
        for (int i = 0;i < S.length (); i++) {
            Charat is a string that gets the I-character
            System.out.println (S.charat (i));
        }

        String a = S.replaceall (S.substring (7,s.length ()), "");
        System.out.println (a);

        System.out.println (s.substring (2,7));
    }
}

Java string intercepts a string that specifies the location of the subscript