java中的getMessage()和toString()有什么区别？

lass ZiDingYiChang 
    {
        public static void main(String[] args) 
        {
                  Demo a =  new Demo();
                 try
                 {
                            int b=a.div(4,-2);
                         System.out.println(b);
                       }
                    catch (FuShuException e)
                 {
                            System.out.println(e.toString());//为什么说这里的toString()方法自动调用下面的getMessage()方法？
                   }
                 System.out.println("Hello World!");
              }
}
class FuShuException extends Exception
  {
      private String str;
      FuShuException(String str)
         {
         this.str=str;
        }

         public String getMessage()
               {
              return str;
               }
    }
 class Demo
  {
        int div(int x,int y) throws FuShuException
          {
          if (y<0)
            {
                 throw new FuShuException("除数为负数，运算失败"); 
             }
      return x/y;
   }
}