Peek Front Element of Queue in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the front element of the queue without removing it.

DSA Java

int front = queue.[1]();

Drag options to blanks, or click blank then click option'

Apop

Bpoll

Cremove

Dpeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which is not a Queue method.

Using poll() which removes the element.

The peek() method returns the front element of the queue without removing it.

2fill in blank

medium

Complete the code to check if the queue is empty before peeking the front element.

DSA Java

if (!queue.[1]()) {
    int front = queue.peek();
}

Drag options to blanks, or click blank then click option'

Aempty

BisFull

CisEmpty

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using isFull() which is not a standard Queue method.

Using size() directly without comparison.

The isEmpty() method checks if the queue has no elements.

3fill in blank

hard

Fix the error in the code to correctly peek the front element of the queue.

DSA Java

int front = queue.[1]();

Drag options to blanks, or click blank then click option'

Atop

Bpeek

Cfront

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which is for stacks.

Using top() which is not a Queue method.

The correct method to peek the front element in a Java Queue is peek().

4fill in blank

hard

Fill both blanks to create a safe peek operation that returns -1 if the queue is empty.

DSA Java

int front = queue.[1]() != null ? queue.[2]() : -1;

Drag options to blanks, or click blank then click option'

Apeek

Bpoll

Cremove

Delement

Attempts:

3 left

💡 Hint

Common Mistakes

Using element() which throws exception if empty.

Using poll() which removes the element.

Using peek() twice safely checks and returns the front element or -1 if empty.

5fill in blank

hard

Fill both blanks to peek the front element safely and print it or print 'Empty' if queue is empty.

DSA Java

Integer front = queue.[1]();
if (front == null) {
    System.out.println([2]);
} else {
    System.out.println(front);
}

Drag options to blanks, or click blank then click option'

Apeek

B"Empty"

D"No elements"

Attempts:

3 left

💡 Hint

Common Mistakes

Using element() which throws exception if empty.

Printing null instead of a message.

peek() returns null if empty, so we print "Empty" or the front element.

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