Java Final关键字与HashMap

 

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
	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;

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    /**
     * 可变对象的访问方法
     */
    public HashMap getTestMap() {
        //return testMap;
        return (HashMap) testMap.clone();
    }