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.

core-java/src/test/java/com/baeldung/collection/WhenComparingTreeMapVsHashMap.java

@@ -0,0 +1,60 @@
+package com.baeldung.collection;
+
+import java.util.ConcurrentModificationException;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.TreeMap;
+
+import org.hamcrest.Matchers;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class WhenComparingTreeMapVsHashMap {
+
+    @Test
+    public void whenInsertObjectsTreeMap_thenNaturalOrder() {
+        Map<Integer, String> treemap = new TreeMap<>();
+        treemap.put(3, "TreeMap");
+        treemap.put(2, "vs");
+        treemap.put(1, "HashMap");
+        Assert.assertThat(treemap.keySet(), Matchers.contains(1, 2, 3));
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void whenInsertNullInTreeMap_thenException() {
+        Map<Integer, String> treemap = new TreeMap<>();
+        treemap.put(null, "NullPointerException");
+    }
+
+    @Test
+    public void whenInsertObjectsHashMap_thenRandomOrder() {
+        Map<Integer, String> hashmap = new HashMap<>();
+        hashmap.put(3, "TreeMap");
+        hashmap.put(2, "vs");
+        hashmap.put(1, "HashMap");
+        Assert.assertThat(hashmap.keySet(), Matchers.containsInAnyOrder(1, 2, 3));
+    }
+
+    @Test
+    public void whenInsertNullInHashMap_thenInsertsNull() {
+        Map<Integer, String> hashmap = new HashMap<>();
+        hashmap.put(null, null);
+        Assert.assertNull(hashmap.get(null));
+    }
+
+    @Test
+    public void givenHashMapAndTreeMap_whenputDuplicates_thenOnlyUnique() {
+        Map<Integer, String> treeMap = new HashMap<>();
+        treeMap.put(1, "Baeldung");
+        treeMap.put(1, "Baeldung");
+
+        Assert.assertTrue(treeMap.size() == 1);
+
+        Map<Integer, String> treeMap2 = new TreeMap<>();
+        treeMap2.put(1, "Baeldung");
+        treeMap2.put(1, "Baeldung");
+
+        Assert.assertTrue(treeMap2.size() == 1);
+    }
+}

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);
+    }
 }