public class A {
      public static void main(String[] args) {
          //定义的字符串         String s = "Hello World"; //        通过length方法获取字符串长度         for (int i = 0;i < s.length();i++){
              //charAt是获取字符串第i个字符             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));     } }