move unmodifiable set code to java 9 module

core-java-9/src/test/java/com/baeldung/java9/SetExamplesUnitTest.java

@@ -1,5 +1,7 @@
 package com.baeldung.java9;
 
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.Set;
 import org.junit.Test;
 
@@ -23,4 +25,14 @@ public class SetExamplesUnitTest {
         Set<Integer> intSet = Set.of(intArray);
         assertEquals(intSet.size(), intArray.length);
     }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void testUnmodifiableSet() {
+        Set<String> set = new HashSet<>();
+        set.add("Canada");
+        set.add("USA");
+
+        Set<String> unmodifiableSet = Collections.unmodifiableSet(set);
+        unmodifiableSet.add("Costa Rica");
+    }
 }