Why Peek Top Element of Stack in DSA Java?

The Big Idea

Peek lets you see the top without touching--saving your stack from chaos!

The Scenario

Imagine you have a stack of books on your desk. You want to see the title of the book on the very top without removing it. If you try to pick up the top book every time just to check, you might mess up the order or drop some books.

The Problem

Manually lifting the top book each time is slow and risky. You might accidentally remove it or disturb the stack. Similarly, without a simple way to peek, checking the top element in a stack can cause errors or extra work.

The Solution

The peek operation lets you look at the top element of the stack without removing it. It's like glancing at the top book's title without touching the stack. This keeps the order safe and saves time.

Before vs After

✗ Before

if (!stack.isEmpty()) {
  int top = stack.pop();
  System.out.println(top);
  stack.push(top);
}

✓ After

if (!stack.isEmpty()) {
  int top = stack.peek();
  System.out.println(top);
}

What It Enables

Peek makes it easy to check the current top item instantly without changing the stack's order.

Real Life Example

When undoing actions in a text editor, peek helps to see the last action without removing it, so you can decide whether to undo or not.

Key Takeaways

Manual checking risks changing the stack order.

Peek safely shows the top element without removal.

It saves time and prevents errors in stack operations.

Practice

1. What does the peek() method do in a stack?

easy

A. Checks if the stack is empty

B. Removes and returns the top element

C. Adds a new element to the top

D. Returns the top element without removing it

Solution

1. Step 1: Understand the peek method purpose

   The peek method is designed to look at the top element of the stack without changing the stack.

2. Step 2: Compare with other stack methods

   Unlike pop, peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the top element without removing it -> Option D

4. Quick Check:

   peek() = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing peek with pop
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the top element of a Java Stack named stack?

easy

A. stack.peek();

B. stack.top();

C. stack.getTop();

D. stack.pop();

Solution

1. Step 1: Recall Java Stack method names

   The Java Stack class uses peek() to view the top element without removing it.

2. Step 2: Check other options for correctness

   top() and getTop() are not valid Stack methods; pop() removes the top element.

3. Final Answer:

   stack.peek(); -> Option A

4. Quick Check:

   Correct syntax for peek is stack.peek() [OK]

Hint: Use stack.peek() to view top element [OK]

Common Mistakes:

• Using pop() instead of peek()
• Using non-existent methods like top()
• Missing parentheses in method call

3. What will be the output of the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
stack.push(30);
System.out.println(stack.peek());

medium

A. 10

B. 30

C. 20

D. StackEmptyException

Solution

1. Step 1: Trace the push operations

   Elements pushed in order: 10 (bottom), 20, then 30 (top).

2. Step 2: Understand peek output

   peek() returns the top element without removing it, which is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Top element after pushes = 30 [OK]

Hint: Peek shows last pushed element [OK]

Common Mistakes:

• Confusing peek with pop output
• Assuming peek returns bottom element
• Expecting exception on non-empty stack

4. Identify the error in the following code snippet that tries to peek the top element of a stack:

Stack<String> stack = new Stack<>();
System.out.println(stack.peek());

medium

A. Throws EmptyStackException at runtime

B. Prints empty string

C. Compilation error: peek() not found

D. No error, prints null

Solution

1. Step 1: Check stack initialization and usage

   The stack is created but no elements are pushed before peek is called.

2. Step 2: Understand peek behavior on empty stack

   Calling peek() on an empty stack throws EmptyStackException at runtime.

3. Final Answer:

   Throws EmptyStackException at runtime -> Option A

4. Quick Check:

   peek() on empty stack throws exception [OK]

Hint: Peek on empty stack throws exception [OK]

Common Mistakes:

• Assuming peek returns null if empty
• Expecting compile-time error
• Thinking it prints empty string

5. Given a stack with elements [5, 10, 15] (5 at bottom, 15 at top), which sequence of operations will peek the top element without removing it and then add 20 on top?

1. stack.peek();
2. stack.push(20);
3. stack.pop();
4. stack.peek();

Select the correct order of operations to achieve this.

hard

A. 3, 1, 2

B. 2, 1

C. 1, 2

D. 1, 3, 2

Solution

1. Step 1: Peek top element without removing

   Operation 1 (stack.peek()) returns 15 without removing it.

2. Step 2: Add 20 on top

   Operation 2 (stack.push(20)) adds 20 on top, so stack becomes [5, 10, 15, 20].

3. Final Answer:

   1, 2 -> Option C

4. Quick Check:

   Peek then push = 1, 2 [OK]

Hint: Peek first, then push new element [OK]

Common Mistakes:

• Popping before peek removes top
• Pushing before peek changes top
• Incorrect operation order