Enhance docs, remove main, add tests in StackArray (#6019)

Author: Hardvan

src/main/java/com/thealgorithms/datastructures/stacks/StackArray.java

@@ -1,7 +1,13 @@
 package com.thealgorithms.datastructures.stacks;
 
 /**
- * This class implements a Stack using a regular array.
+ * Implements a generic stack using an array.
+ *
+ * <p>This stack automatically resizes when necessary, growing to accommodate additional elements and
+ * shrinking to conserve memory when its size significantly decreases.
+ *
+ * <p>Elements are pushed and popped in LIFO (last-in, first-out) order, where the last element added
+ * is the first to be removed.
  *
  * @param <T> the type of elements in this stack
  */
@@ -13,11 +19,20 @@ public class StackArray<T> implements Stack<T> {
     private T[] stackArray;
     private int top;
 
+    /**
+     * Creates a stack with a default capacity.
+     */
     @SuppressWarnings("unchecked")
     public StackArray() {
         this(DEFAULT_CAPACITY);
     }
 
+    /**
+     * Creates a stack with a specified initial capacity.
+     *
+     * @param size the initial capacity of the stack, must be greater than 0
+     * @throws IllegalArgumentException if size is less than or equal to 0
+     */
     @SuppressWarnings("unchecked")
     public StackArray(int size) {
         if (size <= 0) {
@@ -28,6 +43,11 @@ public StackArray(int size) {
         this.top = -1;
     }
 
+    /**
+     * Pushes an element onto the top of the stack. Resizes the stack if it is full.
+     *
+     * @param value the element to push
+     */
     @Override
     public void push(T value) {
         if (isFull()) {
@@ -36,6 +56,13 @@ public void push(T value) {
         stackArray[++top] = value;
     }
 
+    /**
+     * Removes and returns the element from the top of the stack. Shrinks the stack if
+     * its size is below a quarter of its capacity, but not below the default capacity.
+     *
+     * @return the element removed from the top of the stack
+     * @throws IllegalStateException if the stack is empty
+     */
     @Override
     public T pop() {
         if (isEmpty()) {
@@ -48,6 +75,12 @@ public T pop() {
         return value;
     }
 
+    /**
+     * Returns the element at the top of the stack without removing it.
+     *
+     * @return the top element of the stack
+     * @throws IllegalStateException if the stack is empty
+     */
     @Override
     public T peek() {
         if (isEmpty()) {
@@ -56,28 +89,72 @@ public T peek() {
         return stackArray[top];
     }
 
+    /**
+     * Resizes the internal array to a new capacity.
+     *
+     * @param newSize the new size of the stack array
+     */
     private void resize(int newSize) {
         @SuppressWarnings("unchecked") T[] newArray = (T[]) new Object[newSize];
         System.arraycopy(stackArray, 0, newArray, 0, top + 1);
         stackArray = newArray;
         maxSize = newSize;
     }
 
+    /**
+     * Checks if the stack is full.
+     *
+     * @return true if the stack is full, false otherwise
+     */
     public boolean isFull() {
         return top + 1 == maxSize;
     }
 
+    /**
+     * Checks if the stack is empty.
+     *
+     * @return true if the stack is empty, false otherwise
+     */
     @Override
     public boolean isEmpty() {
         return top == -1;
     }
 
-    @Override public void makeEmpty() { // Doesn't delete elements in the array but if you call
-        top = -1; // push method after calling makeEmpty it will overwrite previous values
+    /**
+     * Empties the stack, marking it as empty without deleting elements. Elements are
+     * overwritten on subsequent pushes.
+     */
+    @Override
+    public void makeEmpty() {
+        top = -1;
     }
 
+    /**
+     * Returns the number of elements currently in the stack.
+     *
+     * @return the size of the stack
+     */
     @Override
     public int size() {
         return top + 1;
     }
+
+    /**
+     * Returns a string representation of the stack.
+     *
+     * @return a string representation of the stack
+     */
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("StackArray [");
+        for (int i = 0; i <= top; i++) {
+            sb.append(stackArray[i]);
+            if (i < top) {
+                sb.append(", ");
+            }
+        }
+        sb.append("]");
+        return sb.toString();
+    }
 }

src/test/java/com/thealgorithms/datastructures/stacks/StackArrayTest.java

@@ -21,11 +21,11 @@ void testPushAndPop() {
         stack.push(4);
         stack.push(5);
 
-        Assertions.assertEquals(5, stack.pop()); // Stack follows LIFO, so 5 should be popped first
-        Assertions.assertEquals(4, stack.pop()); // Next, 4 should be popped
-        Assertions.assertEquals(3, stack.pop()); // Followed by 3
-        Assertions.assertEquals(2, stack.pop()); // Then 2
-        Assertions.assertEquals(1, stack.pop()); // Finally 1
+        Assertions.assertEquals(5, stack.pop());
+        Assertions.assertEquals(4, stack.pop());
+        Assertions.assertEquals(3, stack.pop());
+        Assertions.assertEquals(2, stack.pop());
+        Assertions.assertEquals(1, stack.pop());
     }
 
     @Test
@@ -34,34 +34,34 @@ void testPeek() {
         stack.push(20);
         stack.push(30);
 
-        Assertions.assertEquals(30, stack.peek()); // Peek should return 30, the top of the stack
-        Assertions.assertEquals(3, stack.size()); // Size should remain 3 after peek
+        Assertions.assertEquals(30, stack.peek());
+        Assertions.assertEquals(3, stack.size());
 
         stack.pop();
-        Assertions.assertEquals(20, stack.peek()); // After popping, peek should return 20
+        Assertions.assertEquals(20, stack.peek());
     }
 
     @Test
     void testIsEmpty() {
-        Assertions.assertTrue(stack.isEmpty()); // Initially, the stack should be empty
+        Assertions.assertTrue(stack.isEmpty());
         stack.push(42);
-        Assertions.assertFalse(stack.isEmpty()); // After pushing an element, the stack should not be empty
+        Assertions.assertFalse(stack.isEmpty());
         stack.pop();
-        Assertions.assertTrue(stack.isEmpty()); // After popping the only element, the stack should be empty again
+        Assertions.assertTrue(stack.isEmpty());
     }
 
     @Test
     void testResizeOnPush() {
-        StackArray<Integer> smallStack = new StackArray<>(2); // Start with a small stack size
+        StackArray<Integer> smallStack = new StackArray<>(2);
         smallStack.push(1);
         smallStack.push(2);
-        Assertions.assertTrue(smallStack.isFull()); // Initially, the stack should be full
+        Assertions.assertTrue(smallStack.isFull());
 
-        smallStack.push(3); // This push should trigger a resize
-        Assertions.assertFalse(smallStack.isFull()); // The stack should no longer be full after resize
-        Assertions.assertEquals(3, smallStack.size()); // Size should be 3 after pushing 3 elements
+        smallStack.push(3);
+        Assertions.assertFalse(smallStack.isFull());
+        Assertions.assertEquals(3, smallStack.size());
 
-        Assertions.assertEquals(3, smallStack.pop()); // LIFO behavior check
+        Assertions.assertEquals(3, smallStack.pop());
         Assertions.assertEquals(2, smallStack.pop());
         Assertions.assertEquals(1, smallStack.pop());
     }
@@ -74,13 +74,13 @@ void testResizeOnPop() {
         stack.push(3);
         stack.push(4);
 
-        stack.pop(); // Removing elements should trigger a resize when less than 1/4 of the stack is used
         stack.pop();
         stack.pop();
-        Assertions.assertEquals(1, stack.size()); // After popping, only one element should remain
+        stack.pop();
+        Assertions.assertEquals(1, stack.size());
 
         stack.pop();
-        Assertions.assertTrue(stack.isEmpty()); // The stack should be empty now
+        Assertions.assertTrue(stack.isEmpty());
     }
 
     @Test
@@ -90,32 +90,40 @@ void testMakeEmpty() {
         stack.push(3);
         stack.makeEmpty();
 
-        Assertions.assertTrue(stack.isEmpty()); // The stack should be empty after calling makeEmpty
-        Assertions.assertThrows(IllegalStateException.class, stack::pop); // Popping from empty stack should throw exception
+        Assertions.assertTrue(stack.isEmpty());
+        Assertions.assertThrows(IllegalStateException.class, stack::pop);
     }
 
     @Test
     void testPopEmptyStackThrowsException() {
-        Assertions.assertThrows(IllegalStateException.class, stack::pop); // Popping from an empty stack should throw an exception
+        Assertions.assertThrows(IllegalStateException.class, stack::pop);
     }
 
     @Test
     void testPeekEmptyStackThrowsException() {
-        Assertions.assertThrows(IllegalStateException.class, stack::peek); // Peeking into an empty stack should throw an exception
+        Assertions.assertThrows(IllegalStateException.class, stack::peek);
     }
 
     @Test
     void testConstructorWithInvalidSizeThrowsException() {
-        Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(0)); // Size 0 is invalid
-        Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(-5)); // Negative size is invalid
+        Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(0));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(-5));
     }
 
     @Test
     void testDefaultConstructor() {
-        StackArray<Integer> defaultStack = new StackArray<>(); // Using default constructor
-        Assertions.assertEquals(0, defaultStack.size()); // Initially, size should be 0
+        StackArray<Integer> defaultStack = new StackArray<>();
+        Assertions.assertEquals(0, defaultStack.size());
 
         defaultStack.push(1);
-        Assertions.assertEquals(1, defaultStack.size()); // After pushing, size should be 1
+        Assertions.assertEquals(1, defaultStack.size());
+    }
+
+    @Test
+    void testToString() {
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        Assertions.assertEquals("StackArray [1, 2, 3]", stack.toString());
     }
 }