Java Final关键字与HashMap

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本文探讨了Java中final修饰符的应用，特别是针对HashMap的使用限制。解释了final修饰符只限制引用不可改变，并不影响其指向的对象状态变化。同时介绍了如何通过深拷贝或克隆方法实现HashMap的不可变性。

Why final is not applied for Map ?
Prabhat Ranjan Ranch Hand Joined: Oct 04, 2006 Posts: 374	posted 2010年11月16日 GMT+8下午8:00:19
Prabhat Ranjan Ranch Hand Joined: Oct 04, 2006 Posts: 374	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, 2010 Posts: 4115 4 I like...	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, 2004 Posts: 10336 I like...	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();

}

Why final is not applied for Map ?