ArrayList little doubt

Looking head first java in Chapter XVI , the compiler in this column I encountered this problem :

have an Animal class ( abstract class ) , a Dog class . Dog obviously inherited the Animal class .

Then there are :

ArrayList ani = new ArrayList ();
ArrayList dog = new ArrayList ();

These two certainly correct .

but the following two :

ArrayList ani = new ArrayList (); (1)
ArrayList dog = new ArrayList (); (2)

I tried it , eclipse is being given . I doubt obtained from the following reasoning :

Animal ani = new Dog (); ; ( 3 )
Dog dog = new Animal (); ; ( 4 )

The first obvious is right, because you can use a subclass to initialize the parent class ; second right because there is no Animal object.
back to the previous question:

doubt one : ArrayList ani = new ArrayList (); I think is right. Is not that ( 3 ) a variant of it ? Nothing is mounted in an ArrayList container.
doubts II: The second error is because ArrayList can not be instantiated in Animal caused it ? After trying , I put Animal to a non- abstract class , but not one of the above errors disappear . eclipse tips are type mismatch : can not convert ArrayList into ArrayList .

hope to understand how this is going to be available to answer , thank you ~
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this has nothing to do with the transformation .
ArrayList ani = new ArrayList (); This is certainly not enough .
you declare a collection can be kept Animal, so Zhu cat ah dogs, as long as both can be inherited Animal entered survival.
behind the new collection and you can only save dog, pigs and cats can deposit into it ?
So in this way will not work
So, in the use of generics , as long as both sides are used, it must be the same on both sides
In order to maintain the compatibility , but only on one side there is nothing wrong with generics .

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supplementary question : ArrayList only initialize it with the T ? Of course , in addition ArrayList so used ( this can be achieved generic words , but why not just use an ArrayList such a way that it ? It's not more convenient for you ? )
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ArrayList and ArrayList not like the Dog and Animal class as class inheritance relationship , the relationship between them should be parallel , so you can not use ArrayList ani = new ArrayList (); such a similar way to do polymorphism .
I suggest you look up and down the transformation of knowledge transformation , it should be almost the same.
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Oh transformation of knowledge , good . Thank you ~
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add:
if you use wildcards , although it seems that the wording is not the same on both sides , in fact, essentially the same
like this is possible
ArrayList al = new ArrayList ();
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upstairs +1 ....
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generics are not require both types are the same as you ?
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as Java, generics are not covariant .