test: add test for Tombstone::equals

Changed signature of HashSet::tombstone to allow for testing.

Author: euchangxian

src/main/java/dataStructures/hashSet/openAddressing/HashSet.java

@@ -318,7 +318,7 @@ private boolean isLoadFactorExceeded() {
         return this.size() >= this.capacity() * LOAD_FACTOR;
     }
 
-    private static <T> T tombstone() {
+    public T tombstone() {
         // It is safe to cast Tombstone to T, because methods retrieving elements (HashSet::get) from the HashSet
         // should, and will check whether the item is a Tombstone object, returning null in-place of the Tombstone.
         @SuppressWarnings("unchecked")

src/test/java/dataStructures/hashSet/openAddressing/HashSetTest.java

@@ -2,6 +2,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertTrue;
 
 import java.util.List;
@@ -137,4 +138,20 @@ public void testAdd_afterRemove() {
 
         assertEquals(expectedList, actualList);
     }
+
+    @Test
+    public void testTombstoneEquals() {
+        HashSet<Integer> integerHashSet = new HashSet<>();
+        assertNotEquals(Integer.valueOf(5), integerHashSet.tombstone());
+        assertNotEquals(null, integerHashSet.tombstone());
+
+        HashSet<Boolean> booleanHashSet = new HashSet<>();
+        assertNotEquals(Boolean.TRUE, booleanHashSet.tombstone());
+        assertNotEquals(Boolean.FALSE, booleanHashSet.tombstone());
+        assertNotEquals(null, booleanHashSet.tombstone());
+
+        HashSet<Object> objectHashSet = new HashSet<>();
+        assertNotEquals(new Object(), objectHashSet.tombstone());
+        assertNotEquals(null, objectHashSet.tombstone());
+    }
 }