This post will discuss how to remove elements from a mutable list in Java that satisfies the given condition within a loop or iterator.
It is not recommended adding or removing elements from a list within a loop as an index of its elements, and the length of the list is changed. This might lead to the incorrect output, or java.util.IndexOutOfBoundsException or java.util.ConcurrentModificationException will be thrown to avoid non-deterministic behavior at later stage.
Issues with removing elements from a list in Java/Kotlin within a loop
There are several workarounds to deal with this problem. These are discussed below:
1. Iterating backwards
We have seen that moving forward in the list using a for-loop and removing elements from it might cause us to skip a few elements. One workaround is to iterate backward in the list, which does not skip anything.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; class Main { // Generic method to remove elements from a list in Java public static void filterList[List list, Predicate condition] { for [int i = list.size[] - 1; i >= 0 ; i--] { if [condition.test[list.get[i]]] { list.remove[i]; } } } public static void main[String[] args] { List colors = new ArrayList[Arrays.asList["BLUE", "RED", "RED", "YELLOW"]]; filterList[colors, i -> i.equals["RED"]]; System.out.println[colors]; // [BLUE, YELLOW] } } |
DownloadRun Code
2. Decrementing index
The idea is to iterate forward in the list and decrement the loop index whenever an element is removed. Now no elements would be skipped.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; class Main { // Generic method to remove elements from a list in Java public static void filterList[List list, Predicate condition] { for [int i = 0; i i.equals["RED"]]; System.out.println[colors]; // [BLUE, YELLOW] } } |
DownloadRun Code
3. Using Iterator.remove[] method
We have seen that a ConcurrentModificationException will be thrown if we try to modify a list while iterating over it. The solution is to use the iterators own remove method, which removes the last element returned by the iterator.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | import java.util.ArrayList; import java.util.Arrays; import java.util.Iterator; import java.util.List; import java.util.function.Predicate; class Main { // Generic method to remove elements from a list in Java public static void filterList[List list, Predicate condition] { Iterator itr = list.iterator[]; while [itr.hasNext[]] { T t = itr.next[]; if [condition.test[t]] { itr.remove[]; } } } public static void main[String[] args] { List colors = new ArrayList[Arrays.asList["BLUE", "RED", "RED", "YELLOW"]]; filterList[colors, i -> i.equals["RED"]]; System.out.println[colors]; // [BLUE, YELLOW] } } |
DownloadRun Code
4. Use Another Collection
Instead of removing elements as moving forward in the list, we create a collection of such elements and delete them later. Depending upon the deletion condition used, this approach might fail if the list contains duplicate elements.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | import java.util.ArrayList; import java.util.Arrays; import java.util.HashSet; import java.util.List; import java.util.function.Predicate; class Main { // Generic method to remove elements from a list in Java public static void filterList[List list, Predicate condition] { HashSet toRemove = new HashSet[]; for [T item: list] { if [condition.test[item]] { toRemove.add[item]; } } list.removeAll[toRemove]; } public static void main[String[] args] { List colors = new ArrayList[Arrays.asList["BLUE", "RED", "RED", "YELLOW"]]; filterList[colors, i -> i.equals["RED"]]; System.out.println[colors];// [BLUE, YELLOW] } } |
DownloadRun Code
Thats all about removing elements from a list while iterating over it in Java.
Suggested Read:
Remove elements from a List that satisfies a given predicate in Java