java中隐藏对象引用问题

//代码如下： 

class Bar(){ 

   int barNum = 28; 

} 

class Foo(){ 

   Bar myBar = new Bar(); 

   void changeIt(Bar myBar){ 

      myBar.barNum = 99; 

      System.out.println("myBar.barNum in changeIt is " + myBar.barNum); 

      myBar = new Bar(); 

      myBar.barNum = 420; 

      System.out.println("myBar.barNum in changIt is now " + myBar.barNum); 

} 

   public static void main(String[] args){ 

      Foo f = new Foo(); 

      System.out.println("f.myBar.barNum is " + f.myBar.barNum); 

      f.changeIt(f.myBar); 

      System.out.println("f.myBar,barNum after changeIt is " + f.myBar.barNum); 

} 

} 

  

//代码输出结果如下： 

f.myBar.barNum is 28 

myBar.barNum in changeIt is 99 

myBar.barNum in changeIt is now 420     //不理解 

f.myBar.barNum after changeIt is 99       //不理解