refactor: Enhance docs, add tests in GenericHeap

src/main/java/com/thealgorithms/datastructures/heaps/GenericHeap.java

@@ -3,49 +3,90 @@
 import java.util.ArrayList;
 import java.util.HashMap;
 
+/**
+ * A generic implementation of a max heap data structure.
+ *
+ * @param <T> the type of elements in this heap, must extend Comparable.
+ */
 public class GenericHeap<T extends Comparable<T>> {
 
-    ArrayList<T> data = new ArrayList<>();
-    HashMap<T, Integer> map = new HashMap<>();
+    private final ArrayList<T> data = new ArrayList<>();
+    private final HashMap<T, Integer> map = new HashMap<>();
 
+    /**
+     * Adds an item to the heap, maintaining the heap property.
+     *
+     * @param item the item to be added
+     */
     public void add(T item) {
         if (item == null) {
             throw new IllegalArgumentException("Cannot insert null into the heap.");
         }
 
         this.data.add(item);
-        map.put(item, this.data.size() - 1); //
+        map.put(item, this.data.size() - 1);
         upHeapify(this.data.size() - 1);
     }
 
+    /**
+     * Restores the heap property by moving the item at the given index upwards.
+     *
+     * @param ci the index of the current item
+     */
     private void upHeapify(int ci) {
         int pi = (ci - 1) / 2;
-        if (isLarger(this.data.get(ci), this.data.get(pi)) > 0) {
+        if (ci > 0 && isLarger(this.data.get(ci), this.data.get(pi)) > 0) {
             swap(pi, ci);
             upHeapify(pi);
         }
     }
 
+    /**
+     * Displays the contents of the heap.
+     */
     public void display() {
         System.out.println(this.data);
     }
 
+    /**
+     * Returns the number of elements in the heap.
+     *
+     * @return the size of the heap
+     */
     public int size() {
         return this.data.size();
     }
 
+    /**
+     * Checks if the heap is empty.
+     *
+     * @return true if the heap is empty, false otherwise
+     */
     public boolean isEmpty() {
         return this.size() == 0;
     }
 
+    /**
+     * Removes and returns the maximum item from the heap.
+     *
+     * @return the maximum item
+     */
     public T remove() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Heap is empty");
+        }
         this.swap(0, this.size() - 1);
         T rv = this.data.remove(this.size() - 1);
-        downHeapify(0);
         map.remove(rv);
+        downHeapify(0);
         return rv;
     }
 
+    /**
+     * Restores the heap property by moving the item at the given index downwards.
+     *
+     * @param pi the index of the current item
+     */
     private void downHeapify(int pi) {
         int lci = 2 * pi + 1;
         int rci = 2 * pi + 2;
@@ -62,15 +103,35 @@ private void downHeapify(int pi) {
         }
     }
 
+    /**
+     * Retrieves the maximum item from the heap without removing it.
+     *
+     * @return the maximum item
+     */
     public T get() {
-        return this.data.get(0);
+        if (isEmpty()) {
+            throw new IllegalStateException("Heap is empty");
+        }
+        return this.data.getFirst();
     }
 
-    // t has higher property then return +ve
+    /**
+     * Compares two items to determine their order.
+     *
+     * @param t the first item
+     * @param o the second item
+     * @return a positive integer if t is greater than o, negative if t is less, and zero if they are equal
+     */
     private int isLarger(T t, T o) {
         return t.compareTo(o);
     }
 
+    /**
+     * Swaps two items in the heap and updates their indices in the map.
+     *
+     * @param i index of the first item
+     * @param j index of the second item
+     */
     private void swap(int i, int j) {
         T ith = this.data.get(i);
         T jth = this.data.get(j);
@@ -80,9 +141,16 @@ private void swap(int i, int j) {
         map.put(jth, i);
     }
 
+    /**
+     * Updates the priority of the specified item by restoring the heap property.
+     *
+     * @param item the item whose priority is to be updated
+     */
     public void updatePriority(T item) {
+        if (!map.containsKey(item)) {
+            throw new IllegalArgumentException("Item not found in the heap");
+        }
         int index = map.get(item);
-        // because we enter lesser value then old vale
         upHeapify(index);
     }
 }

src/test/java/com/thealgorithms/datastructures/heaps/GenericHeapTest.java

@@ -13,114 +13,81 @@ public class GenericHeapTest {
     private GenericHeap<Integer> heap;
 
     @BeforeEach
-    public void setUp() {
+    void setUp() {
         heap = new GenericHeap<>();
     }
 
     @Test
-    public void testGenericHeapAddAndGet() {
-        heap.add(19);
-        heap.add(36);
-        heap.add(100);
-        heap.add(-17);
-        heap.add(3);
-
-        // Check that the largest element (100) is at the top of the heap
-        assertEquals(100, heap.get());
-    }
-
-    @Test
-    public void testGenericHeapRemove() {
-        heap.add(19);
-        heap.add(36);
-        heap.add(100);
-        heap.add(-17);
-        heap.add(3);
-
-        // Verify that the largest element is removed correctly
-        assertEquals(100, heap.remove());
-
-        // The new element at the top should be 36
-        assertEquals(36, heap.get());
+    void testAddAndGet() {
+        heap.add(10);
+        heap.add(20);
+        heap.add(5);
 
-        // Check that the size is correct after removal
-        assertEquals(4, heap.size());
+        assertEquals(20, heap.get());
     }
 
     @Test
-    public void testGenericHeapSize() {
-        assertTrue(heap.isEmpty());
-
+    void testRemove() {
         heap.add(10);
         heap.add(20);
+        heap.add(5);
 
-        // Check that the size is correct
-        assertEquals(2, heap.size());
-
-        heap.remove();
-
-        // After removal, the size should be 1
-        assertEquals(1, heap.size());
+        assertEquals(20, heap.remove());
+        assertEquals(10, heap.get());
     }
 
     @Test
-    public void testGenericHeapIsEmpty() {
-        // Verify that the heap is initially empty
+    void testIsEmpty() {
         assertTrue(heap.isEmpty());
-
-        heap.add(15);
-
-        // Now the heap should not be empty
+        heap.add(1);
         assertFalse(heap.isEmpty());
-
-        heap.remove();
-
-        // After removing the one element, it should be empty again
-        assertTrue(heap.isEmpty());
     }
 
     @Test
-    public void testGenericHeapUpdatePriority() {
-        heap.add(19);
-        heap.add(36);
-        heap.add(100);
-        heap.add(-17);
-        heap.add(3);
-
-        // Verify that the largest element initially is 100
-        assertEquals(100, heap.get());
+    void testSize() {
+        assertEquals(0, heap.size());
+        heap.add(1);
+        heap.add(2);
+        assertEquals(2, heap.size());
+    }
 
-        heap.remove();
+    @Test
+    void testUpdatePriority() {
+        heap.add(10);
+        heap.add(20);
+        heap.add(5);
 
-        // Simulates a change in priority by increasing the value of 100 to 44
-        heap.add(44);
+        heap.updatePriority(10);
+        assertEquals(20, heap.get());
 
-        // Now, the new high should be 25
-        assertEquals(44, heap.get());
+        heap.add(30);
+        heap.updatePriority(20); // 20 will be moved up
+        assertEquals(30, heap.get());
     }
 
     @Test
-    public void testGenericHeapRemoveUntilEmpty() {
-        heap.add(5);
-        heap.add(3);
-        heap.add(4);
-        heap.add(1);
-        heap.add(2);
+    void testRemoveFromEmptyHeap() {
+        Exception exception = assertThrows(IllegalStateException.class, () -> heap.remove());
+        assertEquals("Heap is empty", exception.getMessage());
+    }
 
-        // Remove all items and check that they are removed in descending order
-        assertEquals(5, heap.remove());
-        assertEquals(4, heap.remove());
-        assertEquals(3, heap.remove());
-        assertEquals(2, heap.remove());
-        assertEquals(1, heap.remove());
+    @Test
+    void testGetFromEmptyHeap() {
+        Exception exception = assertThrows(IllegalStateException.class, () -> heap.get());
+        assertEquals("Heap is empty", exception.getMessage());
+    }
 
-        // Empty heap
-        assertTrue(heap.isEmpty());
+    @Test
+    void testUpdatePriorityForNonExistentItem() {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> heap.updatePriority(100));
+        assertEquals("Item not found in the heap", exception.getMessage());
     }
 
     @Test
-    public void testGenericHeapAddNullItem() {
-        // Check null item
-        assertThrows(IllegalArgumentException.class, () -> { heap.add(null); });
+    void testDisplay() {
+        heap.add(10);
+        heap.add(20);
+        heap.add(5);
+        heap.display(); // Just checking no exceptions are thrown.
     }
 }