This commit is related to BAEL-7854 (#16489)

This commit aims to add a test class named "HashMapCharacterCountUnitTest".

core-java-modules/core-java-string-operations-9/src/test/java/com/baeldung/hashmapcharactercount/HashMapCharacterCountUnitTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.hashmapcharactercount;
+
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static java.util.stream.Collectors.toMap;
+import static org.junit.Assert.assertEquals;
+
+public class HashMapCharacterCountUnitTest {
+    String str = "abcaadcbcb";
+
+    @Test
+    public void givenString_whenUsingStreams_thenVerifyCounts() {
+        Map<Character, Integer> charCount = str.chars()
+                .boxed()
+                .collect(toMap(
+                        k -> (char) k.intValue(),
+                        v -> 1,
+                        Integer::sum));
+
+        assertEquals(3, charCount.get('a').intValue());
+    }
+
+    @Test
+    public void givenString_whenUsingLooping_thenVerifyCounts() {
+        Map<Character, Integer> charCount = new HashMap<>();
+        for (char c : str.toCharArray()) {
+            charCount.merge(c,
+                    1,
+                    Integer::sum);
+        }
+        assertEquals(3, charCount.get('a').intValue());
+    }
+
+
+}