Java - LinkedList is initialized incorrectly

Question

I want to implement a "back" button and for that, before I enter any function I copy the data from the main list in my temporary list. When the user clicks on the "back" button I call the tempList instead of the mainList.

But although I initialized the tempList(just once) with the old value of the mainList, after the function the tempList has the new values of the mainList....

Code:

     ObservableList<List<String>> fnlData;

     List<List<String>> fnlDataTMP;
     .
     .

private void cnvrtColumn() {

        fnlDataTMP = fnlData;

        delWV();//if the mainList(fnlData) has a change in any of this functions, the tmpList also updates the values
        delWM();
        addVN();
        addWV();
        addWM();
        dateFormat();
        changeChar();

        finalTable.getSelectionModel().clearSelection();
        finalTable.getItems().clear();
        finalTable.getColumns().clear();
        createColumns();
        finalTable.getItems().addAll(fnlData);
}

You didn´t copy the array... you just referenced it inside another variable as well — SomeJavaGuy, Jul 29 '15 at 08:45
possible duplicate of [how to make a separated copy of an ArrayList](http://stackoverflow.com/questions/1330288/how-to-make-a-separated-copy-of-an-arraylist) — Murat Karagöz, Jul 29 '15 at 08:54

Astrogat · Answer 1 · 2015-07-29T10:05:45.817

When you do List listA = listB you are just telling listA and listB to point at the same list. If you want listA to be a list with the same values, but without it being the actual same values you must manually copy them.

Since the Lists are mutable you can't just clone the outer list, you must go through it and clone each of the inner lists.

So:

List<List<String>> newList = new ArrayList<List<String>>();
for(List<String> ls : outerList) {
    newList.add(oldArrayList.clone();
}

This will give you a list of cloned lists, and you can freely change all the lists in newList without it affecting anything in the oldList. Since the innerLists contains Strings (and Strings can't be changed after creation) you don't have to worry about just cloning the innerLists.

nutfox · Accepted Answer · 2015-07-29T08:57:43.020

1

You are just adding a reference to the same list.

If you truly want to copy the lists do it this way:

fnlDataTMP = new ArrayList<List<String>>(fnlData);

Assuming that the lists inside the fnlData list are manipulated you would have to do it like this to create a true copy:

fnlDataTMP = new ArrayList<List<String>>();
for (List<String> sublist : fnlData) {
    fnlDataTMP.add(new ArrayList<String>(sublist));
}

edited Jul 29 '15 at 08:57

answered Jul 29 '15 at 08:48

nutfox

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3
13

I did this: fnlDataTMP = new LinkedList>(fnlData); But again the same problem... – ALSTRA Jul 29 '15 at 08:52
Assuming that you are manipulating the lists that are inside your list you would have to iterate through them and add them one by one to your fnlDataTMP list. – nutfox Jul 29 '15 at 08:54
Strings are immutable, so that shouldn't matter. – Astrogat Jul 29 '15 at 09:41
I think the OP wanted to know why the temp list contained the same Strings in the nested lists. It was not a question about whether the Strings were changed. – nutfox Jul 29 '15 at 09:45
Oh, I missed a List! That explains it. – Astrogat Jul 29 '15 at 10:02

Anto · Answer 3 · 2015-07-29T08:55:47.433

1

Just replace it with:

ObservableList<List<String>> fnlData = XCollections.observableArrayList();

And then try:

private void cnvrtColumn() {

    fnlDataTMP = new ArrayList<List<String>>(fnlData);
....

edited Jul 29 '15 at 08:55

answered Jul 29 '15 at 08:51

Anto

4,265
14
63
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in my code I alrdy do this: fnlData = FXCollections.observableList(jdata); – ALSTRA Jul 29 '15 at 08:55

Java - LinkedList is initialized incorrectly

3 Answers3