Question

I am reading the Drools Planner examples and I came across code like this a lot:

List<Column> columnList = new ArrayList<Column>(n);

As far as I get it, it is supposed to initialize a list of length n which stores the Column datatype.

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But what is the deal with having different collection datatypes on either side of the expression?

If ArrayList<Column> type-matches with List<Column>, what makes it different from doing:

List<Column> columnList = new List<Column>(n);

Answer 1

List is an interface. You can’t create instances of interfaces.

ArrayList is a class which implements List, you can create one.

An interface just defines behavior. A class that implements and interface implements that behavior.

You will note that if you look at the API, many different classes implement the List interface. That’s because they all provide implementations for the methods that List defines. Moreover, those implementations are likely mostly different, because and ArrayList works differently than, say, a LinkedList.

It is preferable to use the form

List list = new ListType()

because you can change the list implementation later, if you want to, without affecting your code. The reason for this is if you do

ListType list = new ListType()

the type of list is ListType, which only an instance of ListType and subclasses have.

If you do the preferred assignment shown above, then you can assign anything that implements List to list.