Removing name if case matches from ArrayList not working

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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?

arraylist for-loop list algorithm java
5 Answers

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();
    }
}
user_76067640

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

user_28373590

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());
user_38649170

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());
    }

}
user_25477530

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.

user_15525340
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