Get Previous Element From a List // Java

user_19576909 0
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I am getting data from a csv. If a row in csv doesn't contain values, I will need to update it with the previous row values. The way I am doing it is -

  1. Parse the csv and add all the rows to a linked list.
  2. Iterate through the list and check if a row is empty then fill it with the previous row.

Point #2 is not working as expected. Below is my code snippet:

    ListIterator<Data> li = dataList.listIterator(0);
    while (li.hasNext()) {
        Data prev = null;
        if (li.hasPrevious()) {
            prev = li.previous();
            System.out.println("Previous Node is: "+prev);
        }
        Data data = li.next();
        if (data.getNumber() == null || data.getNumber().trim().isEmpty()) {
            data.setPartType(prev.getPartType());
            data.setNumber(prev.getNumber());
        }

I believe its happening because when we call next() it moves the pointer to the next and hence the previous element will be the current element.

Question - how do I get the actual previous element?

java
1 Answers

From docs listIterator.previous() and listIterator.next() moves the cursor position backwards and forwards respectively. Hence, in your case it'll mostly result in a endless loop at 2nd index.

What you essentially need is previous element without moving cursor. listIterator.previousIndex() can help you with it.

ListIterator<Data> li = dataList.listIterator(0);

while (li.hasNext()) {
    Data prev = null;
    if (li.hasPrevious()) {
        prev = dataList.get(li.previousIndex());
        System.out.println("Previous Node is: "+prev.getNumber());
    }
    Data data = li.next();
    if (data.getNumber() == null || data.getNumber().trim().isEmpty()) {
        data.setPartType(prev.getPartType());
        data.setNumber(prev.getNumber());
    }
}

Although this seems to clutter the readability of code. Better to consider using a simple forEach loop. As -

Data prev = null;
for (Data data: dataList) {
    if ((data.getNumber() == null || data.getNumber().trim().isEmpty()) && prev != null) {
        data.setPartType(prev.getPartType());
        data.setNumber(prev.getNumber());
    }
    prev = data;
}
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