CODEWITHSUNDEEP
javalinked-list
anton1980
anton1980

Reputation: 1009

How to get previous element in a LinkedList?

I am using a LinkedList and I want to get the previous (and the next) element, but not sure how to approach that.

My linked list:

LinkedList<Transaction> transactions = transactionRepository.findAll();

I am searching for this transaction:

Transaction targetTransaction = new Transaction("admin", new Date(), 5);

What I want to do:

for (Transaction transaction : transactions) {
    if (transaction.equals(targetTransaction)) {
        System.out.println("Previous transaction: " + transaction.getPrev());
    }
}

The transaction.getPrev() part does not work, because my Transaction object does not have such method.

Question: how to correctly obtain the "previous" object from the LinkedList?

Upvotes: 6

Views: 23821

Answers (3)

Luiggi Mendoza
Luiggi Mendoza

Reputation: 85779

Enhanced for loop uses Iterator behind the scenes, and this interface doesn't provide any method to go to the previous element. Use LinkedList#listIterator instead:

ListIterator<Transaction> li = transactions.listIterator(0);
while (li.hasNext()) {
    //your logic goes here

    //if you need to go to the previous place
    if (li.hasPrevious()) {
        li.previous();
        //further logic here...
    }
}

Upvotes: 19

resueman
resueman

Reputation: 10613

Keep track of what the last element was manually.

Transaction last = null;
for (Transaction transaction : transactions) {
    if (transaction.equals(targetTransaction)) {
        System.out.println("Previous transaction: " + last);
    }
    last = transaction;
}

Upvotes: 1

Jashaszun
Jashaszun

Reputation: 9270

Keep track of the previous Transaction as you go through the list.

Transaction prev = null;
for (Transaction transaction : transactions) {
    if (transaction.equals(targetTransaction)) {
        System.out.println("Previous transaction: " + (prev = null ? "[none]" : prev));
    }
    prev = transaction;
}

Upvotes: 1

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