Removing name if case matches from ArrayList not working

Question

I am trying to write a program that will remove names from an ArrayList if the name has specific value, but I am unable to do this.

class UserNamesTest {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("John");
        names.add("Jerry");
        names.add("Jim");
        names.add("Todd");

        for(String name : names) {
            if(name.equals("John")) {
                System.out.println("true");
                names.remove(names.indexOf(name));
            }
        }
    }
}

When I execute this code, I am getting an exception:

Exception in thread "main" java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
        at java.util.ArrayList$Itr.next(Unknown Source)
        at UserNamesTest.main(UserNamesTest.java:13)

How can I accomplish this?

Does this answer your question? [Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop](https://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re) — OH GOD SPIDERS, Mar 20 '20 at 15:17
[Use this method](https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html#removeIf-java.util.function.Predicate-) to avoid having to iterate within your own code. — Andrew S, Mar 20 '20 at 15:21

score 3 · Answer 1 · answered Mar 20 '20 at 15:15

Use the iterator not to operate on List you're iterating over:

Iterator<String> iterator = names.iterator();
while(iterator.hasNext()) {
    String name iterator.next();
    if(name.equals("John")) {
        System.out.println("true");
        iterator.remove();
    }
}

manikant gautam · Answer 2 · 2020-03-20T15:36:57.437

Because this exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible java doc. Use removeIf over your list.

        ArrayList<String> names = new ArrayList<>();
        names.add("John");
        names.add("Jerry");
        names.add("Jim");
        names.add("Todd");
        names.removeIf(name->name.equals("John"));

Or Simply use Iterator

        Iterator<String> iter = names.iterator();
        while (iter.hasNext()) {
            String str = iter.next();
            if (str.equals("John"))
                iter.remove();
        }

Note that Iterator.remove() is the only safe way to modify a collection during iteration.more about iterator

Vikas · Answer 3 · 2020-03-20T15:27:54.533

0

Apart from using Iterator, with java-8, you could do as below,

names = names.stream()
             .collect(Collectors.partitioningBy(e->e.equals("John")))
             .get(false);

Or more readable with filter,

names = names.stream()
             .filter(e->e.equals("John"))
             .collect(Collectors.toList());

edited Mar 20 '20 at 15:27

answered Mar 20 '20 at 15:24

Vikas

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1

I'd rather use filter, this way is unreadable. Besides there is an overhead in creating the map holding list of elements that we want and the rest. – Andronicus Mar 20 '20 at 15:26

score 0 · Answer 4 · answered Mar 20 '20 at 15:29

Using Java8 or 11 you could:

class UserNamesTest {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("John");
        names.add("Jerry");
        names.add("Jim");
        names.add("Todd");

        List<String> namesWithoutJohn = names.stream()
                .filter(((Predicate<String>)("John"::equals)).negate()) // Java 8
//                .filter(Predicate.not("John"::equals)) // Java 11
                .collect(Collectors.toList());
    }

}

WJS · Answer 5 · 2020-03-20T15:41:44.920

Be careful when iterating over a list using an index to remove an item. Because items get shifted after the move, you can miss some of them. Here is an example:

       ArrayList<String> names = new ArrayList<>();
       names.add("Jerry");
       names.add("Jim");
       names.add("John");
       names.add("John");
       names.add("Todd");

       for(int i = 0; i < names.size(); i++) {
           String name = names.get(i);
           if(name.equals("John")) {
               names.remove(i);
           }
       }

       names.forEach(System.out::println);

Prints

Jerry
Jim
John  <-- wasn't removed
Todd

If you are going to to it that way, start with the last item since removing one doesn't affect subsequent positions.

       names = new ArrayList<>();
       names.add("Jerry");
       names.add("Jim");
       names.add("John");
       names.add("John");
       names.add("Todd");

       for (int i = names.size() -1; i >= 0; i--) {
           String name = names.get(i);
           if(name.equals("John")) {
               names.remove(i);
           }
       }

       names.forEach(System.out::println);

Prints

Jerry
Jim
Todd

But my preferred way would be using the following:

names.removeIf(name->name.equals("John"));

It gets all of them with one statement.

Removing name if case matches from ArrayList not working

5 Answers5