Stack in Java

A Stack follows LIFO — Last In, First Out. The last element you put in is the first one you take out. Think of a stack of plates: you add to the top and remove from the top.

Java has a Stack class, but it is a legacy class from Java 1.0. For new code, use ArrayDeque as a stack. This lesson covers both.

The Stack Class — push, pop, peek

Run this example to see its output.

Line by line:

• stack.push("Third") — Third goes on top. Stack is now [First, Second, Third] (top = Third).
• stack.peek() — returns "Third" without removing it.
• stack.pop() — removes and returns "Third".
• stack.search("First") — returns 2 (Second is at position 1 from top, First is at position 2).

Real Use Case: Checking Balanced Brackets

A stack is perfect for this — open brackets get pushed, close brackets check against the top.

Run this example to see its output.

Preferred Alternative: ArrayDeque as Stack

The Stack class is synchronized on every method — slow in single-threaded code. ArrayDeque is faster and the official recommendation.

Run this example to see its output.

ArrayDeque.push() = addFirst(). ArrayDeque.pop() = removeFirst(). Behaviour identical to Stack, but faster.

Common Mistake: Calling pop() on Empty Stack

Always check empty() or isEmpty() before popping when the stack might be empty.

Practice Task

Task: Use a Stack<Integer> (or ArrayDeque<Integer>) to reverse an array. Push all elements onto the stack, then pop them into a new array. The popped order is the reverse.

Hint: Push elements in order 0 to n-1. Pop n times into positions 0 to n-1 of the result array.

Solution:

Run this example to see its output.

What to observe: Pushing 1,2,3,4,5 puts 5 on top. Popping gives 5,4,3,2,1. The stack's LIFO property does the reversal automatically — no index math needed.