This PR is related to BAEL-7556 (#15900)

* This commit is related to BAEL-7556

This commit aims to add a test class "FindUniqueEmailsUnitTest.java".

* Update FindUniqueEmailsUnitTest.java

core-java-modules/core-java-string-operations-8/src/test/java/com/baeldung/finduniqueemails/FindUniqueEmailsUnitTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.finduniqueemails;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FindUniqueEmailsUnitTest {
+    String[] emailList = {"user@example.com", "user@example.com", "user@gmail.com", "admin@example.com", "USER@example.com"};
+    Set<String> expectedUniqueEmails = new HashSet<>();
+
+    FindUniqueEmailsUnitTest() {
+        expectedUniqueEmails.add("user@example.com");
+        expectedUniqueEmails.add("user@gmail.com");
+        expectedUniqueEmails.add("admin@example.com");
+    }
+
+    @Test
+    public void givenEmailList_whenUsingStringManipulation_thenFindUniqueEmails() {
+        Set<String> uniqueEmails = new HashSet<>();
+        for (String email : emailList) {
+            uniqueEmails.add(email.toLowerCase());
+        }
+
+        assertEquals(expectedUniqueEmails, uniqueEmails);
+    }
+
+    @Test
+    public void givenEmailList_whenUsingJavaStreams_thenFindUniqueEmails() {
+        Set<String> uniqueEmails = Arrays.stream(emailList)
+                .map(String::toLowerCase)
+                .collect(Collectors.toSet());
+
+        assertEquals(expectedUniqueEmails, uniqueEmails);
+    }
+}