我正在上大学的Java课程，并且正在处理一些给定的任务。这是我为它编写的代码。

public class String
{
 public static void main(String[] args)
 {
  String city = "San Francisco";
  int stringLength = city.length();
  char oneChar = city.charAt(0);
  String upperCity = city.toUpperCase();
  String lowerCity = city.toLowerCase();

  System.out.println(city);
  System.out.println(stringLength);
  System.out.println(oneChar);
  System.out.println(upperCity);
  System.out.println();
  }
 }

产生了这些结果

C:\Users\sam\Documents\Java>javac String.java
String.java:8: error: incompatible types: java.lang.String cannot be 
converted to String
          String city = "San Franciso";
                        ^
String.java:9: error: cannot find symbol
            int stringLength = city.length();
                                   ^
symbol:   method length()
location: variable city of type String
String.java:10: error: cannot find symbol
            char oneChar = city.charAt(0);
                               ^
symbol:   method charAt(int)
location: variable city of type String
String.java:11: error: cannot find symbol
            String upperCity = city.toUpperCase();
                                   ^
symbol:   method toUpperCase()
location: variable city of type String
String.java:12: error: cannot find symbol
            String lowerCity = city.toLowerCase();
                                   ^
symbol:   method toLowerCase()
location: variable city of type String
5 errors

我试过寻找答案，但我真的没有找到任何有帮助的东西。任何帮助都很感激，谢谢。