refactor: StackArray

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

@@ -0,0 +1,51 @@
+package com.thealgorithms.datastructures.stacks;
+
+/**
+ * A generic interface for Stack data structures.
+ *
+ * @param <T> the type of elements in this stack
+ */
+public interface Stack<T> {
+
+    /**
+     * Adds an element to the top of the stack.
+     *
+     * @param value The element to add.
+     */
+    void push(T value);
+
+    /**
+     * Removes the element at the top of this stack and returns it.
+     *
+     * @return The element popped from the stack.
+     * @throws IllegalStateException if the stack is empty.
+     */
+    T pop();
+
+    /**
+     * Returns the element at the top of this stack without removing it.
+     *
+     * @return The element at the top of this stack.
+     * @throws IllegalStateException if the stack is empty.
+     */
+    T peek();
+
+    /**
+     * Tests if this stack is empty.
+     *
+     * @return {@code true} if this stack is empty; {@code false} otherwise.
+     */
+    boolean isEmpty();
+
+    /**
+     * Returns the size of this stack.
+     *
+     * @return The number of elements in this stack.
+     */
+    int size();
+
+    /**
+     * Removes all elements from this stack.
+     */
+    void makeEmpty();
+}

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

@@ -3,170 +3,80 @@
 /**
  * This class implements a Stack using a regular array.
  *
- * <p>
- * A stack is exactly what it sounds like. An element gets added to the top of
- * the stack and only the element on the top may be removed. This is an example
- * of an array implementation of a Stack. So an element can only be
- * added/removed from the end of the array. In theory stack have no fixed size,
- * but with an array implementation it does.
+ * @param <T> the type of elements in this stack
  */
-public class StackArray {
+public class StackArray<T> implements Stack<T> {
 
-    /**
-     * Driver Code
-     */
-    public static void main(String[] args) {
-        // Declare a stack of maximum size 4
-        StackArray myStackArray = new StackArray(4);
-
-        assert myStackArray.isEmpty();
-        assert !myStackArray.isFull();
-
-        // Populate the stack
-        myStackArray.push(5);
-        myStackArray.push(8);
-        myStackArray.push(2);
-        myStackArray.push(9);
-
-        assert !myStackArray.isEmpty();
-        assert myStackArray.isFull();
-        assert myStackArray.peek() == 9;
-        assert myStackArray.pop() == 9;
-        assert myStackArray.peek() == 2;
-        assert myStackArray.size() == 3;
-    }
-
-    /**
-     * Default initial capacity.
-     */
     private static final int DEFAULT_CAPACITY = 10;
 
-    /**
-     * The max size of the Stack
-     */
     private int maxSize;
-
-    /**
-     * The array representation of the Stack
-     */
-    private int[] stackArray;
-
-    /**
-     * The top of the stack
-     */
+    private T[] stackArray;
     private int top;
 
-    /**
-     * init Stack with DEFAULT_CAPACITY
-     */
+    @SuppressWarnings("unchecked")
     public StackArray() {
         this(DEFAULT_CAPACITY);
     }
 
-    /**
-     * Constructor
-     *
-     * @param size Size of the Stack
-     */
+    @SuppressWarnings("unchecked")
     public StackArray(int size) {
-        maxSize = size;
-        stackArray = new int[maxSize];
-        top = -1;
+        if (size <= 0) {
+            throw new IllegalArgumentException("Stack size must be greater than 0");
+        }
+        this.maxSize = size;
+        this.stackArray = (T[]) new Object[size];
+        this.top = -1;
     }
 
-    /**
-     * Adds an element to the top of the stack
-     *
-     * @param value The element added
-     */
-    public void push(int value) {
-        if (!isFull()) { // Checks for a full stack
-            top++;
-            stackArray[top] = value;
-        } else {
+    @Override
+    public void push(T value) {
+        if (isFull()) {
             resize(maxSize * 2);
-            push(value); // don't forget push after resizing
         }
+        stackArray[++top] = value;
     }
 
-    /**
-     * Removes the top element of the stack and returns the value you've removed
-     *
-     * @return value popped off the Stack
-     */
-    public int pop() {
-        if (!isEmpty()) { // Checks for an empty stack
-            return stackArray[top--];
+    @Override
+    public T pop() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Stack is empty, cannot pop element");
         }
-
-        if (top < maxSize / 4) {
+        T value = stackArray[top--];
+        if (top + 1 < maxSize / 4 && maxSize > DEFAULT_CAPACITY) {
             resize(maxSize / 2);
-            return pop(); // don't forget pop after resizing
-        } else {
-            System.out.println("The stack is already empty");
-            return -1;
         }
+        return value;
     }
 
-    /**
-     * Returns the element at the top of the stack
-     *
-     * @return element at the top of the stack
-     */
-    public int peek() {
-        if (!isEmpty()) { // Checks for an empty stack
-            return stackArray[top];
-        } else {
-            System.out.println("The stack is empty, cant peek");
-            return -1;
+    @Override
+    public T peek() {
+        if (isEmpty()) {
+            throw new IllegalStateException("Stack is empty, cannot peek element");
         }
+        return stackArray[top];
     }
 
     private void resize(int newSize) {
-        int[] transferArray = new int[newSize];
-
-        for (int i = 0; i < stackArray.length; i++) {
-            transferArray[i] = stackArray[i];
-        }
-        // This reference change might be nice in here
-        stackArray = transferArray;
+        @SuppressWarnings("unchecked") T[] newArray = (T[]) new Object[newSize];
+        System.arraycopy(stackArray, 0, newArray, 0, top + 1);
+        stackArray = newArray;
         maxSize = newSize;
     }
 
-    /**
-     * Returns true if the stack is empty
-     *
-     * @return true if the stack is empty
-     */
-    public boolean isEmpty() {
-        return (top == -1);
+    public boolean isFull() {
+        return top + 1 == maxSize;
     }
 
-    /**
-     * Returns true if the stack is full
-     *
-     * @return true if the stack is full
-     */
-    public boolean isFull() {
-        return (top + 1 == maxSize);
+    @Override
+    public boolean isEmpty() {
+        return top == -1;
     }
 
-    /**
-     * Deletes everything in the Stack
-     *
-     * <p>
-     * Doesn't delete elements in the array but if you call push method after
-     * calling makeEmpty it will overwrite previous values
-     */
-    public void makeEmpty() { // Doesn't delete elements in the array but if you call
+    @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
     }
 
-    /**
-     * Return size of stack
-     *
-     * @return size of stack
-     */
+    @Override
     public int size() {
         return top + 1;
     }

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

@@ -0,0 +1,121 @@
+package com.thealgorithms.datastructures.stacks;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class StackArrayTest {
+
+    private Stack<Integer> stack;
+
+    @BeforeEach
+    void setUp() {
+        stack = new StackArray<>(5); // Initialize a stack with capacity of 5
+    }
+
+    @Test
+    void testPushAndPop() {
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        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
+    }
+
+    @Test
+    void testPeek() {
+        stack.push(10);
+        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
+
+        stack.pop();
+        Assertions.assertEquals(20, stack.peek()); // After popping, peek should return 20
+    }
+
+    @Test
+    void testIsEmpty() {
+        Assertions.assertTrue(stack.isEmpty()); // Initially, the stack should be empty
+        stack.push(42);
+        Assertions.assertFalse(stack.isEmpty()); // After pushing an element, the stack should not be empty
+        stack.pop();
+        Assertions.assertTrue(stack.isEmpty()); // After popping the only element, the stack should be empty again
+    }
+
+    @Test
+    void testResizeOnPush() {
+        StackArray<Integer> smallStack = new StackArray<>(2); // Start with a small stack size
+        smallStack.push(1);
+        smallStack.push(2);
+        Assertions.assertTrue(smallStack.isFull()); // Initially, the stack should be full
+
+        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
+
+        Assertions.assertEquals(3, smallStack.pop()); // LIFO behavior check
+        Assertions.assertEquals(2, smallStack.pop());
+        Assertions.assertEquals(1, smallStack.pop());
+    }
+
+    @Test
+    void testResizeOnPop() {
+        StackArray<Integer> stack = new StackArray<>(4);
+        stack.push(1);
+        stack.push(2);
+        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.assertTrue(stack.isEmpty()); // The stack should be empty now
+    }
+
+    @Test
+    void testMakeEmpty() {
+        stack.push(1);
+        stack.push(2);
+        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
+    }
+
+    @Test
+    void testPopEmptyStackThrowsException() {
+        Assertions.assertThrows(IllegalStateException.class, stack::pop); // Popping from an empty stack should throw an exception
+    }
+
+    @Test
+    void testPeekEmptyStackThrowsException() {
+        Assertions.assertThrows(IllegalStateException.class, stack::peek); // Peeking into an empty stack should throw an exception
+    }
+
+    @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
+    }
+
+    @Test
+    void testDefaultConstructor() {
+        StackArray<Integer> defaultStack = new StackArray<>(); // Using default constructor
+        Assertions.assertEquals(0, defaultStack.size()); // Initially, size should be 0
+
+        defaultStack.push(1);
+        Assertions.assertEquals(1, defaultStack.size()); // After pushing, size should be 1
+    }
+}