BAEL-1438: TreeMap vs HashMap (#3265)

* BAEL-1438: Final code.

* BAEL-1438: Temporary commit.

* BAEL-1438: New Changes.

* BAEL-1438: New test case to support Java 8 features.

testing-modules/junit-5/src/test/java/com/baeldung/ExceptionUnitTest.java

@@ -3,7 +3,12 @@ package com.baeldung;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.util.ConcurrentModificationException;
+import java.util.HashMap;
+import java.util.Map;
+
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
 
 public class ExceptionUnitTest {
 
@@ -22,4 +27,14 @@ public class ExceptionUnitTest {
             Integer.valueOf(str);
         });
     }
+
+    @Test
+    public void whenModifyMapDuringIteration_thenThrowExecption() {
+        Map<Integer, String> hashmap = new HashMap<>();
+        hashmap.put(1, "One");
+        hashmap.put(2, "Two");
+
+        Executable executable = () -> hashmap.forEach((key, value) -> hashmap.remove(1));
+        assertThrows(ConcurrentModificationException.class, executable);
+    }
 }