Java Final关键字与HashMap

参考链接： Java中的final关键字

Why final is not applied for Map ?  Prabhat Ranjan Ranch Hand Joined: Oct 04, 2006Posts: 374  

       posted 2010年11月16日 GMT+8下午8:00:19 

      Hi, Map hm= new HashMap(); hm.put("hash","test"); ok final Map hm= new HashMap(); hm.put("hash","test");// this should not allow here as Map i have declared as final. ko but Map hms = Collections.unmodifiable(hm) is working fine. Matthew Brown Bartender Joined: Apr 06, 2010Posts: 4115      

           4

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       posted 2010年11月16日 GMT+8下午8:02:20 

      Prabhat Ranjan wrote:final Map hm= new 

      HashMap(); 

      hm.put("hash","test");// this should not allow here as Map i have declared as final. 

      But hm is still pointing at the same object. final means that the reference can't be altered, not the object itself. Paul Sturrock Bartender Joined: Apr 14, 2004Posts: 10336 

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       posted 2010年11月16日 GMT+8下午8:09:12 

      hm.put("hash","test");// this should not allow here as Map i have declared as final. [ 

      Final does not imply unmodifiable, unless the object is immutable. If you want an unmodifiable map have a look at the Collections class, or alternatively wrap the map in your own custom immutable class.

 如果要实现HashMap不可被修改，即如果不可变类的成员变量是可变的（譬如HashMap），我们需要使用深拷贝(deep copy)或者克隆来防止成员变量被更改： 

  private 

  final 

  HashMap testMap; 

  ……

  ……

  ……

  /** 

  * 可变对象的访问方法 

  */ 

  public 

  HashMap getTestMap() { 

  //return testMap; 

  return 

  (HashMap) testMap.clone(); 

  }