Pop Operation on Stack in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after popping from the stack?

Consider the following Java code that uses a stack. What will be printed after the pop operation?

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        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);
    }
}

A

10
[20, 30]

B

20
[10, 30]

C

30
[10, 20]

D

30
[20, 10]

💡 Hint

Remember that pop removes the top element from the stack.

The stack initially has 10 at bottom, then 20, then 30 at top. Popping removes 30 and returns it. The stack then contains [10, 20].

❓ Predict Output

intermediate

What happens when popping from an empty stack?

What error will this Java code produce when trying to pop from an empty stack?

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();
        stack.pop();
    }
}

A No error, returns null

B NullPointerException

C IndexOutOfBoundsException

D java.util.EmptyStackException

💡 Hint

Think about what happens if you remove an element from an empty container.

The Stack class throws EmptyStackException when pop() is called on an empty stack.

🔧 Debug

advanced

Why does this pop operation cause a runtime error?

Identify the cause of the runtime error in the following Java code involving stack pop operation.

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack<String> stack = new Stack<>();
        stack.push("apple");
        stack.push(null);
        String item = stack.pop();
        System.out.println(item.length());
    }
}

A NullPointerException because popped element is null

B EmptyStackException because stack is empty

C ClassCastException due to wrong type

D No error, prints length of "apple"

💡 Hint

Check what happens when you call a method on a null object.

The popped element is null, so calling length() on it causes NullPointerException.

🧠 Conceptual

advanced

What is the state of the stack after these operations?

Given an empty stack, the following operations are performed: push(5), push(10), pop(), push(15), pop(). What is the content of the stack now?

A [5]

B [10]

C [15]

D []

💡 Hint

Track each operation step by step.

Push 5: stack=[5]; push 10: stack=[5,10]; pop(): removes 10, stack=[5]; push 15: stack=[5,15]; pop(): removes 15, stack=[5].

🚀 Application

expert

What is the output of this code with multiple pops?

What will be printed after executing this Java code that pops multiple times from a stack?

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack<Character> stack = new Stack<>();
        stack.push('a');
        stack.push('b');
        stack.push('c');
        System.out.print(stack.pop());
        System.out.print(stack.pop());
        System.out.print(stack.pop());
    }
}

A cab

B cba

C bac

D abc

💡 Hint

Remember stack is last-in, first-out.

Stack has 'a' bottom, then 'b', then 'c' top. Popping prints 'c', then 'b', then 'a'.

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