Adding objects to list?

Question

I just want to make sure I'm doing the right thing. I have a subclass Account and then two subclasses SavingsAccount and CreditAccount and i want to store them in an arraylist, but when I do like this I get errors

List<Account> accountList = new ArrayList<Account>();
// creditaccount
Account account = new CreditAccount();
list.add(account);

// savingsaccount
Account account = new SavingsAccount();
list.add(account);

I thought I could just add them like that, but I guess there must be a unique name like this...

Account account1 = new SavingsAccount();
list.add(account1);

...or am I misunderstanding it? I'm new to this, so help i preciated! Thanks!

home · Accepted Answer · 2012-01-02 10:01:29Z

5

You're correct, variable names are unique in its given scope (local in-method vs. instance variable). Nevertheless, as this is object-oriented you can reuse the variable as it only references a given object:

List<Account> accountList = new ArrayList<Account>();
// creditaccount
Account account = new CreditAccount();
list.add(account); // <-- adds the creditAccount to the list

// savingsaccount
account = new SavingsAccount(); // <-- reuse account
list.add(account); // <-- adds the savingsAccount to the list

Personally, I do not like that approach and rather use self-explanatory names like:

Account creditAccount = new CreditAccount();
Account savingsAccount = new SavingsAccount();
...
list.add(creditAccount);
list.add(savingsAccount);

UPDATE 1: If you do not have to further initialize the account objects you could just do this:

list.add(new CreditAccount());
list.add(new SavingsAccount());

UPDATE 2: I forgot to mention that there is a 'more advanced' approach using anonymous inner blocks, enabling you to declare a variable more than once inside a method:

void someMehtod() {
    List<Account> accountList = new ArrayList<Account>();

    { // <-- start inner block
        Account account = new CreditAccount();
        accountList.add(account);
    } // <-- end inner block

    {
        Account account = new SavingsAccount();
        accountList.add(account);
    }

    account.toString() // compile time error as account is not visible!

}

edited Jan 2, 2012 at 10:01

answered Jan 2, 2012 at 9:31

home

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6 Comments

3D-kreativ Over a year ago

So if I first declare like this: Account account; and then: account = new CreditAccount(); and account = new SavingsAccount(); I can use the same variable account

3D-kreativ Over a year ago

Reply to your last edit: Then I have to make a declaration for each variable to account like: Account creditAccount; and Account savingsAccount; ?

home Over a year ago

Yes, but you must work with the former account object, otherwise you will override the reference when assigning a new object. If you're not familar with this approach or object-orientation in general, I recommend to do some simple tests to understand the concept by e.g. using a class like java.lang.String. (answer to your first comment).

home Over a year ago

Second comment: yes, you need two declarations. It all depends on your application an whether you have to further initialize the objects (see my update).

3D-kreativ Over a year ago

Hmm, I guess I only need to initialize the account object only once and then add several creditAccount and savingsAccount into the list like you just wrote in your last update?

|

user1111538 · Accepted Answer · 2012-01-02 09:32:05Z

3

Account account = new SavingsAccount();
This will give a compile time error. You cannot declare account twice. Change above statement to account = new SavingsAccount();

answered Jan 2, 2012 at 9:32

user1111538

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