Enhance docs, add more tests in LFUCache (#5949)

src/test/java/com/thealgorithms/datastructures/caches/LFUCacheTest.java

@@ -1,6 +1,8 @@
 package com.thealgorithms.datastructures.caches;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -22,7 +24,7 @@ class LFUCacheTest {
         lfuCache.put(6, 60);
 
         // will return null as value with key 2 is now evicted
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
 
         // should return 60
         assertEquals(60, lfuCache.get(6));
@@ -30,7 +32,7 @@ class LFUCacheTest {
         // this operation will remove value with key as 3
         lfuCache.put(7, 70);
 
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
         assertEquals(70, lfuCache.get(7));
     }
 
@@ -41,7 +43,7 @@ class LFUCacheTest {
         lfuCache.put(2, "Beta");
         lfuCache.put(3, "Gamma");
         lfuCache.put(4, "Delta");
-        lfuCache.put(5, "Eplison");
+        lfuCache.put(5, "Epsilon");
 
         // get method call will increase frequency of key 1 by 1
         assertEquals("Alpha", lfuCache.get(1));
@@ -50,7 +52,7 @@ class LFUCacheTest {
         lfuCache.put(6, "Digamma");
 
         // will return null as value with key 2 is now evicted
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
 
         // should return string Digamma
         assertEquals("Digamma", lfuCache.get(6));
@@ -58,25 +60,79 @@ class LFUCacheTest {
         // this operation will remove value with key as 3
         lfuCache.put(7, "Zeta");
 
-        assertEquals(null, lfuCache.get(2));
+        assertNull(lfuCache.get(2));
         assertEquals("Zeta", lfuCache.get(7));
     }
 
-    /**
-     * test addNodeWithUpdatedFrequency method
-     * @author yuluo
-     */
     @Test
-    void testAddNodeWithUpdatedFrequency() {
+    void testUpdateValueShouldPreserveFrequency() {
         LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
-        lfuCache.put(1, "beijing");
-        lfuCache.put(2, "shanghai");
-        lfuCache.put(3, "gansu");
+        lfuCache.put(1, "A");
+        lfuCache.put(2, "B");
+        lfuCache.put(3, "C");
 
-        assertEquals("beijing", lfuCache.get(1));
+        assertEquals("A", lfuCache.get(1)); // Accessing key 1
+        lfuCache.put(4, "D"); // This should evict key 2
 
-        lfuCache.put(1, "shanxi");
+        assertNull(lfuCache.get(2)); // Key 2 should be evicted
+        assertEquals("C", lfuCache.get(3)); // Key 3 should still exist
+        assertEquals("A", lfuCache.get(1)); // Key 1 should still exist
 
-        assertEquals("shanxi", lfuCache.get(1));
+        lfuCache.put(1, "Updated A"); // Update the value of key 1
+        assertEquals("Updated A", lfuCache.get(1)); // Check if the update was successful
+    }
+
+    @Test
+    void testEvictionPolicyWhenFull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(2);
+        lfuCache.put(1, "One");
+        lfuCache.put(2, "Two");
+
+        assertEquals("One", lfuCache.get(1)); // Access key 1
+        lfuCache.put(3, "Three"); // This should evict key 2 (least frequently used)
+
+        assertNull(lfuCache.get(2)); // Key 2 should be evicted
+        assertEquals("One", lfuCache.get(1)); // Key 1 should still exist
+        assertEquals("Three", lfuCache.get(3)); // Check if key 3 exists
+    }
+
+    @Test
+    void testGetFromEmptyCacheShouldReturnNull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
+        assertNull(lfuCache.get(1)); // Should return null as the cache is empty
+    }
+
+    @Test
+    void testPutNullValueShouldStoreNull() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(3);
+        lfuCache.put(1, null); // Store a null value
+
+        assertNull(lfuCache.get(1)); // Should return null
+    }
+
+    @Test
+    void testInvalidCacheCapacityShouldThrowException() {
+        assertThrows(IllegalArgumentException.class, () -> new LFUCache<>(0));
+        assertThrows(IllegalArgumentException.class, () -> new LFUCache<>(-1));
+    }
+
+    @Test
+    void testMultipleAccessPatterns() {
+        LFUCache<Integer, String> lfuCache = new LFUCache<>(5);
+        lfuCache.put(1, "A");
+        lfuCache.put(2, "B");
+        lfuCache.put(3, "C");
+        lfuCache.put(4, "D");
+
+        assertEquals("A", lfuCache.get(1)); // Access 1
+        lfuCache.put(5, "E"); // Should not evict anything yet
+        lfuCache.put(6, "F"); // Evict B
+
+        assertNull(lfuCache.get(2)); // B should be evicted
+        assertEquals("C", lfuCache.get(3)); // C should still exist
+        assertEquals("D", lfuCache.get(4)); // D should still exist
+        assertEquals("A", lfuCache.get(1)); // A should still exist
+        assertEquals("E", lfuCache.get(5)); // E should exist
+        assertEquals("F", lfuCache.get(6)); // F should exist
     }
 }