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Java addAll(collection) vs new ArrayList(collection)

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我寻月下人不归
我寻月下人不归 2021-01-08 00:08

Why do i get different behaviors with:

  1. Collection col2 = new ArrayList(col);

  2. Collection col2 = new ArrayList();

3条回答
  • 渐次进展
    渐次进展 (楼主)
    2021-01-08 01:02

    This code works:

    public void updateCollection(Collection col) {
    this.objectCollection.clear();
    this.objectCollection.addAll(col);
}



    But this introduces problems:
    


    public void updateCollection(Collection col) {
    this.objectCollection=new ArrayList(col);
}



    I suspect that this variation on your first method would introduce identical problems:
    


    public void updateCollection(Collection col) {
    this.objectCollection = new ArrayList();
    this.objectCollection.clear();
    this.objectCollection.addAll(col);
}



    Why?  Evidently you have another reference to objectCollection in use somewhere.  Somewhere in your code, another object is saying (for instance):
    


    myCopyOfObjectCollection = theOtherObject.objectCollection;
    


    If you're using a getter, that doesn't change the underlying behavior - you are still keeping another reference around.
    


    So if on initial assignment, say, the collection contained {1, 2, 3}, you start out with:
    


      

    • this.objectCollection : {1, 2, 3}
      • 

    • that.copyOfObjectCollection: {1, 2, 3}
      • 

    


    When you assign a new ArrayList to this.objectCollection, and populate it with, say, {4, 5, 6}, you get this:
    


      

    • this.objectCollection : {4, 5, 6}
      • 

    • that.copyOfObjectCollection: {1, 2, 3}
      • 

    


    "that" is still pointing to the original ArrayList.  
    
    
    
             
    
                                                        
    
            
            
    
              
                
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