To remove null values while traversing arraylist

Question

I am trying with this code to replace null values in arraylist. I am getting null values in a tag in my xml file. Values in that tag are coming from arraylist. I want to remove null from tag and put nothing in place of it. My code is something like this:

    for(String s:a.getList){
here I setting values in tag by create tag and than appending child nodess using DOM parser.
}

where a=object that contains list

output is like this:

    <value>1</value>
<value>2</value>
<value>null</value>
<value>3</value>
<value>4</value>
<value>null</null>

. . .and so on

Expected output:

    <value>1</value>
<value>2</value>
<value/>
<value>3</value>
<value>4</value>
<value/>

null should be removed and tag should look something like this

code I am trying is:

  for(String s:a.list){
if(s.equals("null")){
s.replace("null","");
my code;

}

Always getting null pointer exception and don't know if this runs what will be output. Please help..

try this condition if(null==s ||s.equals("null")) if you are checking only the value i.e (1,2,null..) — Nidhi257, Sep 01 '17 at 05:54
How about: https://stackoverflow.com/questions/4819635/how-to-remove-all-null-elements-from-a-arraylist-or-string-array — tomas, Sep 01 '17 at 06:03
saw this question and tried with it too..getting same exception — zaib7777, Sep 01 '17 at 06:04
You are "updating" the `String` that is immutable... you need to change the list, so change the value at the current index (need to change the loop) — AxelH, Sep 01 '17 at 06:06

AxelH · Answer 1 · 2017-09-01T06:10:12.660

1

You are not updating the list, you are creating a new String instance since String are immutable. Just set the value you want if the current value is "null"

for(int i = 0; i < list.size(); ++i){
     if("null".equals(list.get(i)){
         list.set(i, "");
     }
}

The condition won't fail for null value, but if you want to replace those, you need to add the condition because for now, it will only update "null".

edited Sep 01 '17 at 06:10

answered Sep 01 '17 at 06:07

AxelH

14,325
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This is btw a nice use-case for a [`ListIterator`](https://docs.oracle.com/javase/7/docs/api/java/util/ListIterator.html) due to [`ListIterator#set(E)`](https://docs.oracle.com/javase/7/docs/api/java/util/ListIterator.html#set(E)). – Izruo Sep 01 '17 at 06:09
@Izruo it is, indeed, I don't use it enough (in my head, iterator are always read-only) ! You are welcome to post it has an answer if you like ! – AxelH Sep 01 '17 at 06:11

Arjun Nagathankandy · Answer 2 · 2017-09-01T11:34:10.380

1

The best way to approach using array list is iterate from last to first if you want to remove concurrently.

for (int i = list.size()-1; i >= 0; i--) {
        if ("null".equalsIgnoreCase(list.get(i))) {
            list.remove(i);
        }
    }

"null".equalsIgnoreCase(list.get(i)) will avoid null pointer exception

For removing and printing value

for (String str  :  abc) {
        if ("null".equalsIgnoreCase(str)) {
            System.out.println("<value/>");
        } else {
            System.out.println("<value>"+str+"</value>");
        }
    }

edited Sep 01 '17 at 11:34

answered Sep 01 '17 at 08:48

Arjun Nagathankandy

705
8
15

Unfortunatly, it is still printing an XML node in the expected output. – AxelH Sep 01 '17 at 10:37
@AxelH could you please give me a sample of arraylist values and what is your expecting output. Hope null in your arraylist is coming as a string. I mean "null" not null. If it is not a string then you have to use null == lis.get (i) for removing condition – Arjun Nagathankandy Sep 01 '17 at 11:18
check OP question for that... He is printint his list in an XML formatted value. (assuming it is adding the content of the list in a node. But a `null` should output `` not simply removing it. – AxelH Sep 01 '17 at 11:22
@AxelH I was more concentrating on the removal on null string from arraylist concurrently. If you want something like which you have mentioned above I think second part of my answer will help you – Arjun Nagathankandy Sep 01 '17 at 11:31
I don't want anything... Check the names ;) – AxelH Sep 01 '17 at 11:50

tomas · Answer 3 · 2017-09-01T06:14:39.927

0

To make it clearer

public static void main(String... args) {
    ArrayList<String> a = new ArrayList<String>();
    a.add("one");
    a.add(null);
    a.add("two");
    a.removeAll(Collections.singleton(null));
    for(String value : a) {
        System.out.println(value);
    }
}

Output

one
two

edited Sep 01 '17 at 06:14

answered Sep 01 '17 at 06:11

tomas

830
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"_For better understanding_" ? – AxelH Sep 01 '17 at 06:13
hope my java is better than my english – tomas Sep 01 '17 at 06:14
`removeAll` is nice but it will become a problem since OP want to remove `"null"`, if those become `"NULL"`, not sure you can `removeAllIgnoreCase` (not sur there is an equivalent for lambda) – AxelH Sep 01 '17 at 06:15
My point is : "There is no explanation" ... you need to explain what you have done – AxelH Sep 01 '17 at 06:15
He said " saw this question and tried with it too..getting same exception" so i wanted to show him a working example....I think the explanation in the original question is good enough – tomas Sep 01 '17 at 06:17
And he said he is getting NPE....So i don't think he has "null" or "NULL" strings.... – tomas Sep 01 '17 at 06:20
Then you should add at least the source of the answer used, and explain that this is a working example. This, for the moment, is an incomplete answer (not really answering the question by the way, but showing an alternative) – AxelH Sep 01 '17 at 06:49

To remove null values while traversing arraylist

3 Answers3