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Java Generics: List, List<Object>, List<?>

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你的背包 2020-11-22 17:08

Can someone explained, as detailed as possible, the differences between the following types?

List
List
List



Let me m         


        
      
      
      

        

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                 失恋的感觉 
              
              
    
                
                   失恋的感觉
                                             
                
                
                (楼主)
            
              
                  
    
                2020-11-22 17:44
              
    

            
    
            
    
              
              

    In my own simple terms:
    


    
  
    List
    

    


    Would declare an ordinary collection, can hold any type, and will always return Object.
    


    
  
    List
    



    Will create a list that can hold any type of object, but can only get assigned a another List 
    


    For instance this doesn't work;
    


    List l = new ArrayList();



    Of course you can add anything but only can pull Object.
    


    List l = new ArrayList();

l.add( new Employee() );
l.add( new String() );

Object o = l.get( 0 );
Object o2 = l.get( 1 );



    Finally 
    


    
  
    List
    

    


    Will let you assign any type, including 
    


    List  l = new ArrayList(); 
List  l2 = new ArrayList();

    


    This would be called collection of unknown and since the common denominator of unknown is Object you will be able to fetch Objects ( a coincidence ) 
    


    The importance of unknown comes when its used with subclassing:
    


    List l = new ArrayList(); // compiles

List l = new ArrayList(); // doesn't,
// because String is not part of *Collection* inheritance tree. 

    


    I hope using Collection as the type doesn't create confusion, that was the only tree that came to my mind.
    


    The difference here, is that l is a collection of unknow that belongs to the Collection hierarchy.
    
    
    
             
    
                                                        
    
            
            
    
              
                
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