From 1a288b591186c7b8699a096b4ef5ac40b9080c4d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Melo?= <jmelo@lyncode.com>
Date: Mon, 10 Apr 2017 11:44:16 +0100
Subject: [PATCH] Code for Avoiding ConcurrentModificationException when
 iterating and removing (#1617)

---
 core-java/README.md                           |  2 +
 .../ConcurrentModificationExceptionTest.java  | 82 +++++++++++++++++++
 2 files changed, 84 insertions(+)
 create mode 100644 core-java/src/test/java/com/baeldung/java/collections/ConcurrentModificationExceptionTest.java

diff --git a/core-java/README.md b/core-java/README.md
index e75701717f..a7c8b0e855 100644
--- a/core-java/README.md
+++ b/core-java/README.md
@@ -88,5 +88,7 @@
 - [Java Primitive Conversions](http://www.baeldung.com/java-primitive-conversions)
 - [Java Money and the Currency API](http://www.baeldung.com/java-money-and-currency)
 - [Guide to Java 8 Comparator.comparing()](http://www.baeldung.com/java-8-comparator-comparing)
+- [Avoiding ConcurrentModificationException when iterating and removing](http://www.baeldung.com/avoiding-concurrentmodificationexception-when-iterating-and-removing)
 - [Removing all nulls from a List in Java](http://www.baeldung.com/java-remove-nulls-from-list)
 - [Removing all duplicates from a List in Java](http://www.baeldung.com/java-remove-duplicates-from-list)
+
diff --git a/core-java/src/test/java/com/baeldung/java/collections/ConcurrentModificationExceptionTest.java b/core-java/src/test/java/com/baeldung/java/collections/ConcurrentModificationExceptionTest.java
new file mode 100644
index 0000000000..100d25ab8d
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/java/collections/ConcurrentModificationExceptionTest.java
@@ -0,0 +1,82 @@
+package com.baeldung.java.collections;
+
+import org.junit.Test;
+
+import java.util.*;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import static java.util.Arrays.asList;
+import static org.testng.Assert.assertEquals;
+
+public class ConcurrentModificationExceptionTest {
+    @Test
+    public void changingContentWithSetDoesNotThrowConcurrentModificationException() throws Exception {
+        ArrayList<Object> array = new ArrayList<>(asList(0, "one", 2, "three"));
+
+        for (Object item : array) {
+            array.set(3, 3);
+        }
+    }
+
+    @Test
+    public void removingElementUsingIteratorAPI() throws Exception {
+        List<String> originalList = new ArrayList<>(asList("zero", "one", "two", "three"));
+
+        Iterator<String> iterator = originalList.iterator();
+
+        while (iterator.hasNext()) {
+            String next = iterator.next();
+            if (Objects.equals(next, "one")) iterator.remove();
+        }
+
+        assertEquals(originalList, asList("zero", "two", "three"));
+    }
+
+    @Test
+    public void modifyingContentAndIteratingUsingListIteratorAPI() throws Exception {
+        List<String> originalList = new ArrayList<>(asList("zero", "one", "two", "three"));
+
+        ListIterator<String> iterator = originalList.listIterator();
+
+        while (iterator.hasNext()) {
+            String next = iterator.next();
+            if (Objects.equals(next, "one")) {
+                iterator.set("another");
+            }
+
+            if (Objects.equals(next, "two")) {
+                iterator.remove();
+            }
+
+            if (Objects.equals(next, "three")) {
+                iterator.add("four");
+            }
+        }
+
+        assertEquals(originalList, asList("zero", "another", "three", "four"));
+    }
+
+    @Test
+    public void removingElementUsingCopyAndListAPI() throws Exception {
+        List<String> originalList = new ArrayList<>(asList("zero", "one", "two", "three"));
+
+        List<String> listCopy = new ArrayList<>(originalList);
+
+        for (String next : listCopy) {
+            if (Objects.equals(next, "one")) originalList.remove(originalList.indexOf(next) - 1);
+        }
+
+        assertEquals(originalList, asList("one", "two", "three"));
+    }
+
+    @Test
+    public void copyOnWriteList() throws Exception {
+        List<String> originalList = new CopyOnWriteArrayList<>(asList("zero", "one", "two", "three"));
+
+        for (String next : originalList) {
+            if (Objects.equals(next, "one")) originalList.remove(originalList.indexOf(next) - 1);
+        }
+
+        assertEquals(originalList, asList("one", "two", "three"));
+    }
+}