Peek Front Element of Queue in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the front element in this queue?

Consider the following Java code that uses a queue. What will be printed after peeking the front element?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<Integer> q = new LinkedList<>();
        q.add(10);
        q.add(20);
        q.add(30);
        System.out.println(q.peek());
    }
}

A 10

B 20

C 30

D null

💡 Hint

Peek returns the front element without removing it.

The peek() method returns the first element in the queue without removing it. Since 10 was added first, it is at the front.

❓ Predict Output

intermediate

What happens when peeking an empty queue?

What will be the output of the following Java code?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<String> q = new LinkedList<>();
        System.out.println(q.peek());
    }
}

A 0

B Throws NoSuchElementException

C Empty string ""

D null

💡 Hint

Peek returns null if the queue is empty.

The peek() method returns null if the queue is empty instead of throwing an exception.

❓ Predict Output

advanced

What is the output after multiple peek and poll operations?

Given the following Java code, what will be printed?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<Integer> q = new LinkedList<>();
        q.add(5);
        q.add(15);
        q.add(25);
        System.out.println(q.peek());
        q.poll();
        System.out.println(q.peek());
        q.poll();
        System.out.println(q.peek());
    }
}

A 5\n15\n25

B 5\n5\n5

C 15\n25\nnull

D 5\n15\nnull

💡 Hint

Peek shows front element; poll removes it.

Initially, peek returns 5. After poll removes 5, peek returns 15. After poll removes 15, peek returns 25.

🔧 Debug

advanced

Identify the error when peeking a queue declared as List

What error will this Java code produce?

DSA Java

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> q = new ArrayList<>();
        q.add(1);
        q.add(2);
        System.out.println(q.peek());
    }
}

A Prints 1

B Runtime NullPointerException

C Compilation error: cannot find symbol method peek()

D Compilation error: incompatible types

💡 Hint

List interface does not have peek() method.

The List interface does not define a peek() method, so the compiler will report cannot find symbol error.

🧠 Conceptual

expert

Which data structure guarantees O(1) peek for front element?

Among these data structures, which one guarantees constant time O(1) peek operation for the front element?

A Stack

B Linked List based Queue

C Binary Search Tree

D Array List

💡 Hint

Peek means accessing front without removal.

A linked list based queue keeps a reference to the front node, so peek is O(1). ArrayList provides O(1) access by index but does not guarantee front element peek in a queue context. BST and Stack do not provide front element peek in O(1).

Practice

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

easy

A. Returns the front element without removing it

B. Removes and returns the front element

C. Adds an element to the end of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the purpose of peek()

   The peek() method is designed to look at the front element of the queue without changing the queue.

2. Step 2: Compare with other queue methods

   Unlike poll() which removes the front element, peek() only returns it without removal.

3. Final Answer:

   Returns the front element without removing it -> Option A

4. Quick Check:

   peek() returns front element without removal [OK]

Hint: peek() shows front without removing [OK]

Common Mistakes:

• Confusing peek() with poll() which removes element
• Thinking peek() adds elements
• Assuming peek() checks if queue is empty

2. Which of the following is the correct syntax to peek the front element of a Java Queue named q?

easy

A. q.getFirst();

B. q.front();

C. q.peek();

D. q.pop();

Solution

1. Step 1: Recall Java Queue methods

   The standard method to look at the front element without removing it is peek().

2. Step 2: Check other options

   front() and pop() are not methods of Java Queue interface; getFirst() belongs to Deque, not Queue.

3. Final Answer:

   q.peek(); -> Option C

4. Quick Check:

   Use peek() to view front element [OK]

Hint: Use q.peek() to see front element [OK]

Common Mistakes:

• Using pop() which is not in Queue interface
• Using front() which doesn't exist in Java Queue
• Confusing getFirst() with peek()

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

import java.util.*;
public class Main {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.add(10);
    q.add(20);
    q.add(30);
    System.out.println(q.peek());
    q.poll();
    System.out.println(q.peek());
  }
}

medium

A. 10\n10

B. 30\n20

C. 20\n30

D. 10\n20

Solution

1. Step 1: Add elements to queue

   Elements 10, 20, 30 are added in order. Queue is now [10, 20, 30].

2. Step 2: Peek and poll operations

   First peek() returns 10 (front). Then poll() removes 10. Next peek() returns 20 (new front).

3. Final Answer:

   10\n20 -> Option D

4. Quick Check:

   peek() before poll = 10, after poll = 20 [OK]

Hint: peek() shows front; poll() removes front [OK]

Common Mistakes:

• Assuming peek() removes element
• Confusing poll() with peek()
• Expecting peek() to return last element

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

import java.util.*;
public class Main {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.add(5);
    q.add(15);
    int front = q.peek();
    System.out.println(front);
  }
}

medium

A. peek() returns Integer, cannot assign to int without unboxing

B. No error, code runs fine and prints 5

C. Queue interface does not have peek() method

D. Must cast peek() result to int explicitly

Solution

1. Step 1: Check peek() return type

   peek() returns an Integer object, which Java auto-unboxes to int automatically.

2. Step 2: Verify code correctness

   Code compiles and runs fine, printing 5 as expected.

3. Final Answer:

   No error, code runs fine and prints 5 -> Option B

4. Quick Check:

   Auto-unboxing allows Integer to int assignment [OK]

Hint: Java auto-unboxes Integer to int [OK]

Common Mistakes:

• Thinking explicit cast needed from Integer to int
• Assuming peek() returns primitive int
• Believing Queue lacks peek() method

5. Given a queue q containing integers, write a Java method peekOrDefault that returns the front element if the queue is not empty, or returns -1 if it is empty. Which of the following implementations is correct?

hard

A. public int peekOrDefault(Queue<Integer> q) { return q.isEmpty() ? -1 : q.peek(); }

B. public int peekOrDefault(Queue<Integer> q) { return q.peek() == null ? -1 : q.poll(); }

C. public int peekOrDefault(Queue<Integer> q) { return q.poll() == null ? -1 : q.peek(); }

D. public int peekOrDefault(Queue<Integer> q) { if(q.peek() == null) return -1; else return q.poll(); }

Solution

1. Step 1: Check for empty queue safely

   Use q.isEmpty() to check if queue has elements before peeking.

2. Step 2: Return front or default

   If empty, return -1; else return q.peek() which does not remove element.

3. Step 3: Analyze other options

   public int peekOrDefault(Queue<Integer> q) { return q.peek() == null ? -1 : q.poll(); }: public int peekOrDefault(Queue<Integer> q) { return q.peek() == null ? -1 : q.poll(); } peeks to check but polls to return, removing the front element—wrong. public int peekOrDefault(Queue<Integer> q) { return q.poll() == null ? -1 : q.peek(); }: public int peekOrDefault(Queue<Integer> q) { return q.poll() == null ? -1 : q.peek(); } polls first; if not empty, removes front then peeks new front—wrong. public int peekOrDefault(Queue<Integer> q) { if(q.peek() == null) return -1; else return q.poll(); }: public int peekOrDefault(Queue<Integer> q) { if(q.peek() == null) return -1; else return q.poll(); } returns poll() in else, removing element—wrong.

4. Final Answer:

   public int peekOrDefault(Queue<Integer> q) { return q.isEmpty() ? -1 : q.peek(); } -> Option A

5. Quick Check:

   Check empty first, then peek without removal [OK]

Hint: Check isEmpty() before peek() to avoid null [OK]

Common Mistakes:

• Using poll() which removes element
• Peeking to check null but then polling which removes
• Not handling empty queue before peeking