新手求助：父类与子类对象的转换

public class SuperClass{
        .....
   }
   public class SubClass extends SuperClass{
       ......
       SuperClass sup1;
       SubClass sub1;
       sub1=(SubClass)sup1;      //怎么理解？？
       sup1=（SuperClass)sub1;   //怎么理解？？
       ......
   }
      " sub1=(SubClass)sup1;  "的这种形势 与     
      " SuperClass sup1 = new SubClass(); "有什么区别？？