String字符串

public class StringClassTest {
   
    public static void main(String[] args) {
//遍历字符串
        String str = "Hello world";
        for (int i = 0; i < str.length(); i++) {
          char ch = str.charAt(i);
            System.out.print(ch+" ");
        }
        System.out.println();
//在字符串里查找子串并且返回其字串的第一个字母的小标，么找到返回-1
        System.out.println(str.indexOf("llo"));
        System.out.println(str.indexOf("all"));
//在字符串查找某个字符，找到返回下标，么找到返回-1       
        System.out.println(str.indexOf(‘l‘));
        System.out.println(str.indexOf(‘a‘));
//给出要输出的字符串的起始小标和最终下标，输出字符串
        String sub = str.substring(0,str.length());
        System.out.println(sub);    }
}