Peek Top Element of Stack in DSA Java - Build from Scratch

Peek Top Element of Stack

📖 Scenario: Imagine you are managing a stack of books. You want to see which book is currently on the top without removing it.

🎯 Goal: Build a simple Java program that creates a stack of integers, then peeks at the top element to see it without removing it.

📋 What You'll Learn

Create a stack of integers with exact values

Add a variable to hold the top element

Use the peek method to get the top element

Print the top element

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and managing function calls.

💼 Career

Understanding stack operations like peek is essential for software development roles that involve data structure manipulation and algorithm design.

1

Create a stack with integers

Create a Stack<Integer> called books and add these integers in order: 10, 20, 30.

DSA Java

// Create the stack and add integers 10, 20, 30
// Your code here

Hint

Use Stack<Integer> books = new Stack<>(); to create the stack. Use books.push(value); to add each integer.

2

Add a variable to hold the top element

Create an int variable called topBook to hold the top element of the stack.

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack books = new Stack();
        books.push(10);
        books.push(20);
        books.push(30);

        // Create int variable topBook
        // Your code here
    }
}

Hint

Declare int topBook; inside the main method after adding the books.

3

Use peek to get the top element

Assign the top element of books to topBook using the peek() method.

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack books = new Stack();
        books.push(10);
        books.push(20);
        books.push(30);

        int topBook;
        // Assign topBook using peek
        // Your code here
    }
}

Hint

Use topBook = books.peek(); to get the top element without removing it.

4

Print the top element

Print the value of topBook using System.out.println(topBook);.

DSA Java

import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        Stack books = new Stack();
        books.push(10);
        books.push(20);
        books.push(30);

        int topBook;
        topBook = books.peek();
        // Print topBook
        // Your code here
    }
}

Hint

Use System.out.println(topBook); to display the top element.

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