Java – How to remove items from a List while iterating?

In Java, if we remove items from a List while iterating it, it will throw java.util.ConcurrentModificationException. This article shows a few ways to solve it.

Table of contents

• 1. java.util.ConcurrentModificationException
• 2. Java 8 Collection#removeIf
  • 2.1 removeIf examples
  • 2.2 removeIf uses Iterator
• 3. ListIterator example
• 4. Filter and Collect example
• 5. References

P.S Tested with Java 11.

1. java.util.ConcurrentModificationException

If we remove an item from an ArrayList while iterating it, the list.remove(s) will throws java.util.ConcurrentModificationException.

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class IteratorApp1 {

  public static void main(String[] args) {

      List<String> list = new ArrayList<>();
      list.add("A");
      list.add("B");
      list.add("C");

      for (String s : list) {
          if ("A".equals(s)) {
              // throws java.util.ConcurrentModificationException
              list.remove(s);
          }
      }

      System.out.println(list);

  }

}

Output

Exception in thread "main" java.util.ConcurrentModificationException

at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1042)
at java.base/java.util.ArrayList$Itr.next(ArrayList.java:996)
at com.mkyong.basic.IteratorApp.main(IteratorApp.java:16)

2. Java 8 Collection#removeIf

In Java 8, we can use the Collection#removeIf API to remove items from a List while iterating it.

2.1 removeIf examples

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.List;

public class IteratorApp2A {

  public static void main(String[] args) {

      List<String> list = new ArrayList<>();
      list.add("A");
      list.add("B");
      list.add("C");

      // remove if item is "A"
      list.removeIf("A"::equals);

      System.out.println(list);

  }

}

Output

[B, C]

Another removeIf example.

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.List;

public class IteratorApp2B {

  public static void main(String[] args) {

      List<Integer> list = new ArrayList<>();
      list.add(1);
      list.add(2);
      list.add(3);

      // remove if item is 1 or 3
      list.removeIf(x -> x == 1 || x == 3);

      System.out.println(list);

  }

}

Output

[2]

2.2 removeIf uses Iterator

Review the Java 8 Collection#removeIf method signature, and the API uses Iterator to remove the item while iterating it.

package java.util;

public interface Collection<E> extends Iterable<E> {

  //...

  default boolean removeIf(Predicate<? super E> filter) {
      Objects.requireNonNull(filter);
      boolean removed = false;
      final Iterator<E> each = iterator();
      while (each.hasNext()) {
          if (filter.test(each.next())) {
              each.remove();
              removed = true;
          }
      }
      return removed;
  }

  //...
}

3. ListIterator example

Before Java 8, we can use ListIterator to remove the items from a List while iterating it.

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class IteratorApp3 {

    public static void main(String[] args) {

        List<String> list = new ArrayList<>();
        list.add("A");
        list.add("B");
        list.add("C");

        ListIterator<String> iter = list.listIterator();
        while(iter.hasNext()){
            if("A".equals(iter.next())){
                iter.remove();
            }
        }

        System.out.println(list);

    }

}

Output

[B, C]

4. Filter and Collect example

4.1 We also can create a new List to store the filtered result.

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class IteratorApp4A {

  public static void main(String[] args) {

      List<String> list = new ArrayList<>();
      list.add("A");
      list.add("B");
      list.add("C");

      List<String> result = new ArrayList<>();
      for (String s : list) {
          if (!"A".equals(s)) {
              result.add(s);
          }
      }

      System.out.println(result);

  }

}

Output

[B, C]

4.2 In Java 8, we can use a stream to filter and collect.

package com.mkyong.basic;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class IteratorApp4B {

  public static void main(String[] args) {

      List<String> list = new ArrayList<>();
      list.add("A");
      list.add("B");
      list.add("C");

      // Java 8
      List<String> result = list
              .stream()
              .filter(x -> !"A".equals(x))
              .collect(Collectors.toList());

      System.out.println(result);

  }

}

Output

[B, C]

5. References

• Collection.html#removeIf JavaDoc
• ConcurrentModificationException JavaDoc
• Java 8 Predicate Examples
• Java Iterator examples
• How to loop a List in Java