Peek Front Element of Queue in DSA Java - Execution Trace

Concept Flow - Peek Front Element of Queue

Start with Queue

↓

Check if Queue is empty?

Yes→Return null or error

No↓

Access front element without removing

↓

Return front element

↓

Done

The flow checks if the queue is empty, then accesses and returns the front element without removing it.

Execution Sample

DSA Java

Queue<Integer> q = new LinkedList<>();
q.add(10);
q.add(20);
int front = q.peek();
System.out.println(front);

This code adds two elements to the queue and then peeks at the front element without removing it.

Execution Table

Step	Operation	Nodes in Queue	Pointer Changes	Visual State
1	Initialize empty queue	[]	head=null, tail=null	null
2	Add 10	[10]	head=10, tail=10	10 -> null
3	Add 20	[10, 20]	head=10, tail=20	10 -> 20 -> null
4	Peek front element	[10, 20]	head=10, tail=20 (no change)	10 -> 20 -> null
5	Return front element	[10, 20]	head=10, tail=20 (no change)	Output: 10

💡 Peek operation ends after returning the front element without modifying the queue.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
head	null	Node(10)	Node(10)	Node(10)	Node(10)
tail	null	Node(10)	Node(20)	Node(20)	Node(20)
queue size	0	1	2	2	2
peeked value	N/A	N/A	N/A	10	10

Key Moments - 3 Insights

Why does peek not remove the front element from the queue?

As shown in execution_table rows 4 and 5, peek only accesses the front node without changing head or tail pointers, so the queue remains unchanged.

What happens if we peek when the queue is empty?

In the concept_flow, if the queue is empty, peek returns null or an error, preventing access to a non-existent front element.

Why do head and tail pointers not change during peek?

Because peek only reads the front element without removing it, the pointers remain the same as shown in execution_table rows 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the visual state of the queue?

A 10 -> 20 -> null

B 20 -> 10 -> null

C 10 -> null

D null

💡 Hint

Refer to the 'Visual State' column in execution_table row 3.

At which step does the peeked value become available?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'peeked value' variable in variable_tracker after each step.

If the queue was empty at step 4, what would be the expected output?

A 10

B null or error

C 20

D Queue size increases

💡 Hint

Look at concept_flow decision for empty queue before peek.

Concept Snapshot

Queue peek operation:
- Checks if queue is empty
- Returns front element without removing
- head and tail pointers stay unchanged
- Does not modify queue size
- Returns null if queue empty

Full Transcript

This visualization shows how the peek operation on a queue works step-by-step. We start with an empty queue, add two elements 10 and 20, then peek at the front element. The peek operation accesses the front element 10 without removing it or changing the queue structure. The head and tail pointers remain the same, and the queue size does not change. If the queue were empty, peek would return null or an error. This helps understand that peek is a safe way to see the front without modifying the queue.

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