adding elements while iterating [duplicate]

Question

I have a list of concrete objects. While iterating over this list, I'm trying to update an object from it by adding values and I'm getting of course a ConcurentModificationException: What are my alternatives? Thank you and appreciate the help. I'm using Java 11.

import lombok.Data;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Objects;

public class Test {

    public static void main(String[] args) throws IOException {

        List<Person> myList = new ArrayList<>();
        List<Hobby> hobbies = new ArrayList<>();
        Hobby h1 = new Hobby("SKI");
        Hobby h2 = new Hobby("reading");
        hobbies.add(h1);
        hobbies.add(h2);
        Person p = new Person("R", hobbies);

        Person p1 = new Person("M", hobbies);

        myList.add(p);
        myList.add(p1);
        myList
                .forEach(currentElement -> {
                    if (Objects.isNull(currentElement.getHobbies())) {
                        currentElement.setHobbies(Collections.singletonList(new Hobby("NOTHING")));

                    } else {
                        currentElement.getHobbies()
                                .forEach(hobby -> {
                                    if (hobby.getMyHobby().equals("SKI")) {
                                        currentElement.getHobbies().add(new Hobby("SAILING"));
                                    } else {
                                        hobby.getMyHobby().toLowerCase();
                                    }
                                });
                    }
                });
    }

    @Data
    static
    class Person {
        String name;
        List<Hobby> hobbies;

        public Person(String name, List<Hobby> hobbies) {
            this.name = name;
            this.hobbies = hobbies;
        }
    }

    @Data
    static class Hobby {
        String myHobby;

        public Hobby(String myHobby) {
            this.myHobby = myHobby;
        }
    }
}

I would question the decision of using forEach(...). The lambda constructs make the code hard to read. — Turing85
– Turing85, Commented Oct 13, 2021 at 18:26
Java doesn't like modification of list elements within foreach loops like that, you would want to use a ListIterator. — Cade Weiskopf
– Cade Weiskopf, Commented Oct 13, 2021 at 18:28
@freecoderw If SKI hobby is found; do you want to add SAILING and keep both or replace SKI with SAILING — navnath
– navnath, Commented Oct 13, 2021 at 18:46

shmosel · Accepted Answer · 2021-10-13 18:34:00Z

0

You can iterate over the indexes instead:

for (int i = 0; i < currentElement.getHobbies().size(); i++) {
    Hobby hobby = currentElement.getHobbies().get(i);
    if (hobby.getMyHobby().equals("SKI")) {
        currentElement.getHobbies().add(new Hobby("SAILING"));
    } else {
        hobby.getMyHobby().toLowerCase();  // sic
    }
}

answered Oct 13, 2021 at 18:34

shmosel

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3 Comments

freecoderw Over a year ago

I wouldn't wanna do the old fashion way if there is another possibility using streams...

shmosel Over a year ago

Streams generally shouldn't modify their data source. And you're not using streams anyway.

freecoderw Over a year ago

true, just hope to be able to use

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