遇到Public Class XXX Should Be in File怎么解决？java报错

遇到Public Class XXX Should Be in File怎么解决？java报错

在编写java的样例代码时出现这种问题。这种情况应该如何解决和避免那？

package javaapplication3;  
  public class Robot {  
        int xlocation;  
        int ylocation;  
        String name;  
        static int ccount = 0;  
        public Robot(int xxlocation, int yylocation, String nname) {  
            xlocation = xxlocation;  
            ylocation = yylocation;  
            name = nname;  
            ccount++;         
        } 
  }
  public class JavaApplication1 { 
    public static void main(String[] args) {  
        robot firstRobot = new Robot(34,51,"yossi");  
        System.out.println("numebr of robots is now " + Robot.ccount);  
    }
  }

原因是什么，为什么会发生异常，以及下面到底发生了什么。如果有人能帮忙？提前谢谢