In Java, Stack is a class that extends Vector and implements a standard Last In First Out (LIFO) stack data structure. It inherits the synchronization properties of Vector, making it thread-safe but with similar performance drawbacks due to synchronization overhead.

Advantages #

1. LIFO Behavior: The stack operates with a Last In First Out order, which is useful in many algorithms and processes, such as parsing expressions, backtracking, or implementing recursive algorithms iteratively.
2. Thread-Safe: Since Stack extends Vector, it is synchronized, making it thread-safe by default. This can be useful when multiple threads need to share the same stack.
3. Easy-to-Use API: Stack provides simple methods like push(), pop(), peek(), which directly support typical stack operations.

Disadvantages #

1. Performance Overhead: Since Stack inherits from Vector, it inherits its synchronized nature, which can lead to performance issues when synchronization is not required. This makes it slower compared to modern, non-synchronized alternatives like ArrayDeque.
2. Legacy Class: Stack is considered a legacy class, and modern alternatives such as ArrayDeque or LinkedList are preferred for stack operations in non-threaded applications due to better performance and flexibility.
3. Unnecessary Synchronization: If the stack is only used by one thread, the inherent synchronization is unnecessary and can degrade performance.

Comparison with Deque (ArrayDeque or LinkedList) #

In modern Java development, Deque (like ArrayDeque or LinkedList) is preferred for stack implementations. Here’s why:

• Non-Synchronized: Deque is not synchronized by default, leading to better performance in non-concurrent scenarios.
• More Flexibility: Deque can be used as both a stack and a queue, providing greater flexibility.
• Better Performance: ArrayDeque in particular offers O(1) performance for push, pop, and peek operations without synchronization overhead.

Time and Space Complexity #

Examples #

1. Initialization and Pushing Elements:

   • Time Complexity: O(1) for each push

   // Initialize a Stack
   Stack<Integer> stack = new Stack<>();
   
   // Push elements onto the stack
   stack.push(1);   // [1]
   stack.push(2);   // [1, 2]
   stack.push(3);   // [1, 2, 3]
   

2. Accessing the Top Element (peek):

   • Time Complexity: O(1)

   int topElement = stack.peek();  // Returns 3, the element at the top
   

3. Popping an Element:

   • Time Complexity: O(1)

   int poppedElement = stack.pop();  // Removes and returns 3, stack becomes [1, 2]
   

4. Searching for an Element:

   • Time Complexity: O(n)

   int position = stack.search(2);  // Returns the 1-based position of the element (2nd from top)
   

5. Checking if the Stack is Empty:

   • Time Complexity: O(1)

   boolean isEmpty = stack.isEmpty();  // Returns false if the stack has elements
   

6. Iterating Over the Stack:

   • Time Complexity: O(n)

   // Iterating through the stack (from bottom to top)
   for (Integer item : stack) {
       System.out.println(item);
   }
   

7. Thread-Safe Access:

   • Time Complexity: O(1) for push and pop operations

   // Stack is thread-safe for concurrent use
   synchronized (stack) {
       stack.push(5);
       int value = stack.pop();
   }
   

8. Reversing a String Using Stack:

   public class StringReverser {
       public static String reverse(String str) {
           Stack<Character> stack = new Stack<>();
   
           // Push all characters onto the stack
           for (char c : str.toCharArray()) {
               stack.push(c);
           }
   
           // Pop all characters from the stack and append to result
           StringBuilder reversed = new StringBuilder();
           while (!stack.isEmpty()) {
               reversed.append(stack.pop());
           }
           return reversed.toString();
       }
   
       public static void main(String[] args) {
           String result = reverse("hello");
           System.out.println(result);  // Output: "olleh"
       }
   }
   

9. Balanced Parentheses Checker Using Stack:

   public class ParenthesesChecker {
       public static boolean isBalanced(String expression) {
           Stack<Character> stack = new Stack<>();
   
           for (char c : expression.toCharArray()) {
               if (c == '(' || c == '{' || c == '[') {
                   stack.push(c);
               } else if (c == ')' || c == '}' || c == ']') {
                   if (stack.isEmpty()) {
                       return false;
                   }
                   char top = stack.pop();
                   if ((c == ')' && top != '(') ||
                       (c == '}' && top != '{') ||
                       (c == ']' && top != '[')) {
                       return false;
                   }
               }
           }
           return stack.isEmpty();
       }
   
       public static void main(String[] args) {
           String expression = "{[()]}";
           System.out.println(isBalanced(expression));  // Output: true
       }
   }
   

Conclusion: #

• Stack in Java provides a thread-safe, synchronized implementation of a LIFO (Last In, First Out) data structure. It is simple to use and provides convenient methods for typical stack operations such as push, pop, and peek.
• Advantages: It is thread-safe, easy to use, and provides fast access for stack-related operations.
• Disadvantages: Stack is a legacy class, and the synchronized methods introduce unnecessary overhead in single-threaded contexts. Modern alternatives such as ArrayDeque or LinkedList (when used as a stack) are usually preferred due to better performance in non-threaded environments.