Peek Front Element of Queue in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a queue?

It shows the front element of the queue without removing it, so you can see who is next without changing the queue.

beginner

In a queue, which end is the 'front'?

The front is the end where elements are removed (dequeued). It's like the front of a line where the first person is served.

intermediate

What happens if you peek an empty queue?

You get a special value (like null or exception) because there is no front element to show.

beginner

Why is peek useful compared to dequeue?

Peek lets you check the next item without removing it, so the queue stays the same. Dequeue removes the item.

intermediate

In Java's Queue interface, which method is used to peek the front element safely?

The method 'peek()' returns the front element or null if the queue is empty, without removing it.

What does the peek() method do in a queue?

A Removes and returns the front element

B Adds an element to the back

C Returns the front element without removing it

D Checks if the queue is empty

peek() shows the front element without removing it.

If a queue is empty, what does peek() return in Java?

A Returns null

B Returns zero

C Throws an exception

D Returns the last element

peek() returns null if the queue is empty.

Which end of the queue does peek() look at?

A Random

B Middle

C Back

D Front

peek() shows the front element of the queue.

What is the difference between peek() and poll() in a queue?

A peek() removes element, poll() does not

B peek() shows front without removing, poll() removes front

C Both do the same

D peek() adds element, poll() removes element

peek() shows front without removing; poll() removes and returns front.

Why might you use peek() instead of dequeue?

A To check the next element without changing the queue

B To add an element

C To clear the queue

D To reverse the queue

peek() lets you see the next element without removing it.

Explain how the peek operation works in a queue and why it is useful.

Think about looking at the first person in line without asking them to leave.

Describe what happens when you peek an empty queue in Java.

What does peek() return if no one is in line?

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