Pop Operation on Stack in DSA Java - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty?

No↓

Remove top element

↓

Update top pointer

↓

Return removed element

Yes↓

Stack Underflow Error or Null Return

The pop operation first checks if the stack is empty. If not, it removes the top element, updates the top pointer, and returns the removed element. If empty, it signals underflow.

Execution Sample

DSA Java

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
int popped = stack.pop();
System.out.println(stack);

This code pushes 10 and 20 onto the stack, then pops the top element (20), and prints the remaining stack.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Push 10	10	top -> 10	10 -> null
2	Push 20	20 -> 10	top -> 20	20 -> 10 -> null
3	Pop	20 -> 10	top -> 20 (before pop)	20 -> 10 -> null
4	Remove top element (20)	10	top updated to 10	10 -> null
5	Return popped element	10	top -> 10	10 -> null
6	Print stack	10	top -> 10	10 -> null
7	Pop on empty stack (if tried)	empty	top -> null	null

💡 Execution stops after pop returns the top element or signals underflow if stack is empty.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
stack	empty	10	20 -> 10	10	10
top pointer	null	points to 10	points to 20	points to 10	points to 10
popped element	undefined	undefined	undefined	20	20

Key Moments - 3 Insights

Why do we check if the stack is empty before popping?

Because popping from an empty stack causes an error or underflow. The execution_table row 7 shows the empty stack case where pop cannot proceed.

What happens to the top pointer after popping?

The top pointer moves down to the next element below the popped one. See execution_table rows 3 and 4 where top moves from 20 to 10.

Does the popped element remain in the stack after pop?

No, the popped element is removed from the stack. Execution_table row 4 shows the stack state after removal.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the top pointer pointing to after pop?

A 20

B null

C 10

D 30

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 4.

At which step does the stack become empty if we pop all elements?

A Step 5

B Step 7

C Step 3

D Step 1

💡 Hint

Look at the 'Nodes in Stack' column and find when it shows 'empty'.

If we push 30 after step 5, what will be the new top pointer?

A points to 30

B points to 20

C points to 10

D null

💡 Hint

Pushing adds a new top element; check how top pointer changes in earlier push steps.

Concept Snapshot

Pop Operation on Stack:
- Checks if stack is empty before popping
- Removes and returns the top element
- Updates top pointer to next element
- Causes underflow if stack is empty
- Stack visual: top -> element -> ... -> null

Full Transcript

The pop operation on a stack first checks if the stack is empty to avoid errors. If not empty, it removes the top element, updates the top pointer to the next element, and returns the removed element. If the stack is empty, it signals underflow or returns null. The visual execution shows pushing 10 and 20, then popping 20, leaving 10 as the new top. The top pointer moves accordingly, and the stack state updates after each operation.

Practice

1. What does the pop() operation do in a stack?

easy

A. Removes and returns the top element of the stack

B. Adds an element to the bottom of the stack

C. Returns the bottom element without removing it

D. Checks if the stack is empty

Solution

1. Step 1: Understand the stack structure

   A stack follows Last In First Out (LIFO) principle, so operations affect the top element.

2. Step 2: Define pop operation

   The pop() removes and returns the top element from the stack.

3. Final Answer:

   Removes and returns the top element of the stack -> Option A

4. Quick Check:

   pop() removes top element [OK]

Hint: Pop always removes the last added item from stack [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack named stack in Java?

easy

A. stack.pop();

B. pop(stack);

C. stack.remove();

D. pop();

Solution

1. Step 1: Recall Java Stack method syntax

   In Java, pop() is a method called on the stack object.

2. Step 2: Identify correct method call

   The correct syntax is calling pop() on the stack instance: stack.pop();

3. Final Answer:

   stack.pop(); -> Option A

4. Quick Check:

   Method call on object = stack.pop() [OK]

Hint: Pop is called as a method on the stack object [OK]

Common Mistakes:

• Using pop() without object
• Using remove() instead of pop()
• Passing stack as argument to pop

3. What will be the output after executing the following Java code?

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

medium

A. 30
[20, 10]

B. 10
[20, 30]

C. 20
[10, 30]

D. 30
[10, 20]

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Apply pop operation

   Pop removes top element 30, so popped = 30. Remaining stack is [10, 20].

3. Final Answer:

   30 [10, 20] -> Option D

4. Quick Check:

   pop() returns last pushed = 30 [OK]

Hint: Pop returns last pushed element, stack shrinks from top [OK]

Common Mistakes:

• Confusing order of elements after pop
• Printing popped element incorrectly
• Assuming pop removes bottom element

4. Consider this Java code snippet:

Stack<String> stack = new Stack<>();
stack.push("A");
stack.push("B");
stack.pop();
stack.pop();
stack.pop();

What error will occur when running this code?

medium

A. No error, code runs fine

B. EmptyStackException at third pop

C. NullPointerException at second pop

D. IndexOutOfBoundsException at first pop

Solution

1. Step 1: Count push and pop calls

   Two elements pushed: "A", "B". Then three pops called.

2. Step 2: Identify error on extra pop

   First two pops remove "B" and "A". Third pop tries to remove from empty stack causing EmptyStackException.

3. Final Answer:

   EmptyStackException at third pop -> Option B

4. Quick Check:

   Pop on empty stack throws EmptyStackException [OK]

Hint: Popping empty stack causes EmptyStackException [OK]

Common Mistakes:

• Assuming pop returns null on empty stack
• Confusing exception types
• Ignoring extra pop call

5. You have a stack with elements [5, 10, 15, 20] (5 at bottom, 20 at top). You perform two pop operations, then push 25, then pop once. What is the final stack content?

hard

A. [5, 10, 25]

B. [5, 10, 15]

C. [5, 10]

D. [5, 10, 15, 25]

Solution

1. Step 1: Perform two pop operations

   Initial stack: bottom 5,10,15, top 20. First pop removes 20, second pop removes 15. Stack now: [5, 10].

2. Step 2: Push 25 and pop once

   Push 25: stack becomes [5, 10, 25]. Pop removes 25, stack returns to [5, 10].

3. Step 3: Verify final stack

   Final stack is [5, 10].

4. Final Answer:

   [5, 10] -> Option C

5. Quick Check:

   Pop removes top, push adds top [OK]

Hint: Track pushes and pops stepwise to find final stack [OK]

Common Mistakes:

• Forgetting last pop removes pushed element
• Mixing order of elements
• Assuming push adds at bottom