Changing String element in an ArrayList<List<String>> is changing both Lists in ArrayList

Question

Here is the code I am using

    ArrayList<List<String>> sentanceParts = new ArrayList<List<String>>();

    int i;

    sentanceParts.add(Arrays.asList(sentance.split("\\s+")));
    sentanceParts.add(sentanceParts.get(0));
    System.out.printf( "%s\n", sentanceParts.get(1).get(0));

    for( i = 0; i < sentanceParts.get(0).size(); i++ ){
        sentanceParts.get(0).set(i, 
        sentanceParts.get(0).get(i).replaceAll( "[^aeiou]+", "" ));
        System.out.printf( "%s:%d\n",
                sentanceParts.get(1).get(i),
                sentanceParts.get(0).get(i).length() );
    }

And it is outputting this

Type a sentance for analysis...
test case
e:1
ae:2

which should be

Type a sentance for analysis...
test case
test:1
case:2

Why is my code not doing this? I thought I was setting the sentanceParts(0) not sentanceParts(1)

The debugger is a wonderful thing. By learning how to use it you will have one more tool in your arsenal. — Brian Roach
– Brian Roach, Commented May 9, 2012 at 5:16
Yes I am aware of the debugger but it is too complicated to figure out why a reference was being made in two places as apposed to a copy of the data in memory when I have only been programming in this language for 10 something minutes. — Supernovah
– Supernovah, Commented May 9, 2012 at 7:05

Paul Bellora · Accepted Answer · 2012-05-09 05:15:29Z

4

Because of this line:

sentanceParts.add(sentanceParts.get(0));

sentanceParts is referencing the same List<String> twice.

If you want to add a copy of the List at element 0, then write:

sentanceParts.add(new ArrayList<String>(sentanceParts.get(0)));

answered May 9, 2012 at 5:15

Paul Bellora

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1 Comment

Bananeweizen Over a year ago

You already got the answer for duplication above: The constructor "new ArrayList(otherList)" makes a copy for you.

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