Java ArrayList Adding objects --> Throw out old ones

Question

I have a class called "Transaction.java", and I have an ArrayList with the newest five transactions. So the size of this ArrayList should always be between 0 and 5.

When there's a new transaction, I have to rotate all the entries in this ArrayList. Entry 4 becomes entry 3, entry 3 becomes entry 2, entry 2 becomes entry 1, entry 1 becomes entry 0 and entry 0 becomes the new transaction. The oldest one (the object with the index 4) is overwritten.

When trying to code this, I'm in trouble with ArrayIndexOutOfBoundsExceptions or NPEs because I do transactions.set(4, transactions.get(3));, even if there's nothing in transaction.get(3), and transaction.get(4) is empty as well!

This is a little snippet of my code:

for (int i =  4; i >= 0; i--) {
    try {
        if (i == 0) {
            latestTransactions.set(i, newTransaction);
        }
        else {
            latestTransactions.set(i, latestTransactions.get(i - 1));
        }
    } catch (Exception e) { // NPE / AIOOBE
        e.printStackTrace();
    }

}

The exception is thrown 5 times (that's how big the for-loop is) and nothing's set in the ArrayList.

Please try to keep calm and understandable, I'm new in the buiseness ^^

`latestTransactions.get(i - 1)` - what will happen in the last iteration where `i` is 0? — Maroun, Jun 02 '15 at 16:23
You should have a look at Queue interface and implementation: http://stackoverflow.com/questions/9580457/fifo-class-in-java — Gary SEBASTIANI, Jun 02 '15 at 16:23
Any particular reason you are using `ArrayList` over bounded queues? They will be less prone to errors of sizing and easier to manage elements in your use case — hitz, Jun 02 '15 at 16:26
An `ArrayList` is a very inefficient data structure for what you are trying to do with it. You'd be better off using a `LinkedList` instead. — neuronaut, Jun 02 '15 at 16:28

Eran · Accepted Answer · 2015-06-02T16:27:56.840

You can do it without the loop.

Assuming the ArrayList already contains 5 elements :

oldestElement = latestTransactions.remove(0); // this will remove the element at the 0 index
                                              // and decrement the index of all the others

Then

latestTransactions.add(newElement); // will add the new element at the end (index 4)

If the ArrayList can have less than 5 elements, you can write :

if (latestTransactions.size () >= 5) {
    oldestElement = latestTransactions.remove(0);
}
latestTransactions.add(newElement);

This will only remove the oldest element if the ArrayList already contains 5 elements, and add the new element at the end.

I have to assume that there's 0 up to 5 elements in the ArrayList. It does not always contain 5 elements though. — Baustein, Jun 02 '15 at 16:27

score -1 · Answer 2 · answered Jun 02 '15 at 16:30

-1

Queue queue = new LinkedList();
queue.push(newTransaction);
setLatestTransaction(queue.pop());

answered Jun 02 '15 at 16:30

sirolf2009

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Java ArrayList Adding objects --> Throw out old ones

2 Answers2