Added ConcurrentModificationException test

core-java/src/test/java/com/baeldung/java/concurrentmodificationexception/ConcurrentModificationUnitTest.java

@@ -0,0 +1,54 @@
+package com.baeldung.java.concurrentmodificationexception;
+
+import org.assertj.core.api.Assertions;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.ConcurrentModificationException;
+import java.util.Iterator;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.util.Lists.newArrayList;
+
+public class ConcurrentModificationUnitTest {
+    @Test(expected = ConcurrentModificationException.class)
+    public void whilstRemovingDuringIteration_shouldThrowException() throws InterruptedException {
+
+        ArrayList<Integer> integers = newArrayList(1, 2, 3);
+
+        for (Integer integer : integers) {
+            integers.remove(1);
+        }
+    }
+
+    @Test
+    public void whilstRemovingDuringIteration_shouldNotThrowException() throws InterruptedException {
+
+        ArrayList<Integer> integers = newArrayList(1, 2, 3);
+
+        for (Iterator<Integer> iterator = integers.iterator(); iterator.hasNext();) {
+            Integer integer = iterator.next();
+            if(integer == 2) {
+                iterator.remove();
+            }
+        }
+
+        assertThat(integers).containsExactly(1, 3);
+    }
+
+    @Test
+    public void whilstRemovingDuringForEach_shouldNotThrowException() throws InterruptedException {
+
+        ArrayList<Integer> integers = newArrayList(1, 2, 3);
+        ArrayList<Integer> toRemove = newArrayList();
+
+        for (Integer integer : integers) {
+            if(integer == 2) {
+                toRemove.add(integer);
+            }
+        }
+        integers.removeAll(toRemove);
+
+        assertThat(integers).containsExactly(1, 3);
+    }
+}