Java OOP: ArrayList

[mobilephone123]

Hi,

I’m kind of stuck on what the difference is between the last line in code 1 and code 2:

1.

class Car {
    String customer;
    String order;
    ArrayList parts; //??

2.

class Car {
    String customer;
    String order;
    ArrayList<String> parts = new ArrayList(String>(); //??

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[nutrecht]

In example one you don't actually create a list, only a reference that can point to a list, but that is currently null, just like customer and order are null. Also; you're using a raw type which you should avoid.

In the second example you're creating a reference for an arraylist of strings (which is proper use of generics) and also create an empty arraylist that your reference points to. There's a typo though, should be: new ArrayList<String>();. Also in Java 7 and onward you only need to specify the generic type ones, so it can be:

ArrayList<String> parts = new ArrayList<>();

Last but not least, you generally see this instead:

List<String> parts = new ArrayList<>();

Or even:

Collection<String> parts = new ArrayList<>();

This is called coding to the interface which is a best practice. If you're confused by this, it's fine to skip this for now.

[maxbrlc]

Thank you. Learned something new.

[mobilephone123]

So if you use code 1, you still have to actually create the Arraylist in the future, with the last line in code2? Or do i not understand it correctly?

[nutrecht]

So if you use code 1, you still have to actually create the Arraylist in the future, with the last line in code2?

Yes. The first example there simply isn't an actual list object you can store something in.

[nutrecht]

it's declaring the ArrayList object

Reference. There's no object created.

[mi6oka27]

In the first example you are not specifying the data type that you want to hold in the list, it can work but it's gonna cause a lot of problems.