Java generics - ArrayList initialization

Question

It is known that arraylist init. should be like this

ArrayList a = new ArrayList();
ArrayList a = new ArrayList

        

Answer 1

The key lies in the differences between references and instances and what the reference can promise and what the instance can really do.

ArrayList a = new ArrayList();

Here a is a reference to an instance of a specific type - exactly an array list of As. More explicitly, a is a reference to an array list that will accept As and will produce As. new ArrayList() is an instance of an array list of As, that is, an array list that will accept As and will produce As.

ArrayList a = new ArrayList(); 

Here, a is a reference to exactly an array list of Integers, i.e. exactly an array list that can accept Integers and will produce Integers. It cannot point to an array list of Numbers. That array list of Numbers can not meet all the promises of ArrayList a (i.e. an array list of Numbers may produce objects that are not Integers, even though its empty right then).

ArrayList a = new ArrayList(); 

Here, declaration of a says that a will refer to exactly an array list of Numbers, that is, exactly an array list that will accept Numbers and will produce Numbers. It cannot point to an array list of Integers, because the type declaration of a says that a can accept any Number, but that array list of Integers cannot accept just any Number, it can only accept Integers.

ArrayList a= new ArrayList