Pop Operation on Stack in DSA Java - Build from Scratch

Pop Operation on Stack

📖 Scenario: You are managing a stack of books. You want to remove the top book from the stack to read it.

🎯 Goal: Build a simple stack using an array and perform the pop operation to remove the top item.

📋 What You'll Learn

Create an integer array called stack with these exact values: 10, 20, 30, 40, 50

Create an integer variable called top and set it to 4 (index of the last element)

Write code to perform the pop operation by removing the top element and updating top

Print the stack elements from bottom to top after the pop operation

💡 Why This Matters

🌍 Real World

Stacks are used in undo features, browser history, and expression evaluation.

💼 Career

Understanding stack operations is essential for software development and technical interviews.

1

Create the stack array

Create an integer array called stack with these exact values: 10, 20, 30, 40, 50

DSA Java

// Your code here

Hint

Use int[] stack = {10, 20, 30, 40, 50}; to create the array.

2

Set the top index

Create an integer variable called top and set it to 4 (index of the last element in stack)

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] stack = {10, 20, 30, 40, 50};
        // Your code here
    }
}

Hint

Use int top = 4; to set the top index.

3

Perform the pop operation

Write code to perform the pop operation by removing the top element and updating top to top - 1

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] stack = {10, 20, 30, 40, 50};
        int top = 4;
        // Remove the top element and update top
        // Your code here
    }
}

Hint

Update top by subtracting 1 to remove the top element.

4

Print the stack after pop

Use a for loop with variable i from 0 to top inclusive to print the stack elements separated by spaces

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] stack = {10, 20, 30, 40, 50};
        int top = 4;
        top = top - 1;
        // Print stack elements from bottom to top
        // Your code here
    }
}

Hint

Use a for loop from 0 to top and print each element with a space.

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