Priority Queue Introduction and Concept in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a PriorityQueue of integers in Java.

DSA Java

PriorityQueue<Integer> pq = new PriorityQueue[1]();

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

A<>

B()

C{}

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of angle brackets for generics.

Using curly braces or square brackets which are invalid here.

In Java, to create a generic PriorityQueue, you use angle brackets <> after the class name.

2fill in blank

medium

Complete the code to add an element 10 to the priority queue pq.

DSA Java

pq.[1](10);

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Ainsert

Benqueue

Cpush

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert or push which are not methods of PriorityQueue.

Using enqueue which is not a Java PriorityQueue method.

In Java's PriorityQueue, the method to add elements is add().

3fill in blank

hard

Fix the error in the code to remove the highest priority element from pq.

DSA Java

int highest = pq.[1]();

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Aremove

Bpoll

Cpop

Ddequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which is not a PriorityQueue method.

Using remove which throws exception if empty.

Using dequeue which is not a PriorityQueue method.

poll() removes and returns the head of the PriorityQueue or returns null if empty.

4fill in blank

hard

Fill both blanks to check if pq is empty and get its size.

DSA Java

boolean empty = pq.[1]();
int size = pq.[2]();

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AisEmpty

Bsize

Clength

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using length or count which are not PriorityQueue methods.

Confusing isEmpty() with size().

isEmpty() checks if the queue is empty, size() returns the number of elements.

5fill in blank

hard

Fill all three blanks to create a max-heap PriorityQueue using a comparator.

DSA Java

PriorityQueue<Integer> maxPQ = new PriorityQueue<>([1], [2].reverseOrder());
maxPQ.[3](5);

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A(10)

BCollections

Cadd

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong constructor parameters.

Using wrong method to add elements.

Confusing initial capacity with element values.

The constructor takes initial capacity (e.g., 10) and a comparator like Collections.reverseOrder(). Then add() inserts elements.

Practice

1. What is the main characteristic of a Priority Queue compared to a regular queue?

easy

A. Elements are dequeued based on their priority, not just order of insertion

B. Elements are always dequeued in the order they were inserted

C. It only stores elements of the same value

D. It does not allow duplicate elements

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they were added (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on priority, not insertion order.

3. Final Answer:

   Elements are dequeued based on their priority, not just order of insertion -> Option A

4. Quick Check:

   Priority Queue = Priority-based removal [OK]

Hint: Priority queue removes highest priority first, not oldest [OK]

Common Mistakes:

• Confusing priority queue with FIFO queue
• Thinking priority queue sorts elements permanently
• Assuming duplicates are not allowed

2. Which of the following is the correct way to declare a Priority Queue of integers in Java?

easy

A. PriorityQueue<int> pq = new PriorityQueue<>();

B. PriorityQueue<Integer> pq = new PriorityQueue<>();

C. PriorityQueue pq = new PriorityQueue<int>();

D. PriorityQueue<Integer> pq = new PriorityQueue<int>();

Solution

1. Step 1: Use wrapper class for generics

   Java generics require wrapper classes, so use Integer, not int.

2. Step 2: Correct generic syntax

   Use diamond operator <> on the right side without type inside.

3. Final Answer:

   PriorityQueue<Integer> pq = new PriorityQueue<>(); -> Option B

4. Quick Check:

   Use wrapper Integer and diamond operator [OK]

Hint: Use wrapper classes (Integer) with generics, not primitives [OK]

Common Mistakes:

• Using primitive types (int) with generics
• Specifying type twice on right side
• Not using diamond operator

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

import java.util.*;
public class TestPQ {
  public static void main(String[] args) {
    PriorityQueue<Integer> pq = new PriorityQueue<>();
    pq.add(30);
    pq.add(10);
    pq.add(20);
    System.out.print(pq.poll() + ", " + pq.poll() + ", " + pq.poll());
  }
}

medium

A. 10, 20, 30

B. 30, 20, 10

C. 20, 10, 30

D. 10, 30, 20

Solution

1. Step 1: Understand PriorityQueue default order

   Java's PriorityQueue by default is a min-heap, so smallest element is removed first.

2. Step 2: Trace poll() calls

   Elements added: 30, 10, 20. Poll removes 10, then 20, then 30.

3. Final Answer:

   10, 20, 30 -> Option A

4. Quick Check:

   Min-heap poll order = 10, 20, 30 [OK]

Hint: PriorityQueue.poll() returns smallest element first by default [OK]

Common Mistakes:

• Assuming insertion order is preserved
• Thinking PriorityQueue is max-heap by default
• Confusing poll() with peek()

4. Identify the error in the following code snippet that uses a Priority Queue in Java:

import java.util.PriorityQueue;
public class Demo {
  public static void main(String[] args) {
    PriorityQueue pq = new PriorityQueue();
    pq.add("apple");
    pq.add(10);
    System.out.println(pq.poll());
  }
}

medium

A. PriorityQueue requires initial capacity in constructor

B. PriorityQueue must be imported from java.util.concurrent package

C. PriorityQueue cannot mix different data types like String and Integer

D. PriorityQueue does not support add() method

Solution

1. Step 1: Check PriorityQueue type safety

   Without generics, PriorityQueue stores Objects but cannot compare different types.

2. Step 2: Mixing String and Integer causes ClassCastException

   Adding both String and Integer causes runtime error when ordering elements.

3. Final Answer:

   PriorityQueue cannot mix different data types like String and Integer -> Option C

4. Quick Check:

   Mixed types cause runtime error in PriorityQueue [OK]

Hint: PriorityQueue elements must be mutually comparable [OK]

Common Mistakes:

• Ignoring type safety and mixing types
• Wrong import statement
• Assuming add() method is unsupported

5. You want to create a Priority Queue in Java that always returns the largest element first. Which of the following is the best way to achieve this?

hard

A. Use PriorityQueue<Integer> pq = new PriorityQueue<>(); and add elements normally

B. Use PriorityQueue<Integer> pq = new PriorityQueue<>(null);

C. Use PriorityQueue<Integer> pq = new PriorityQueue<>(Comparator.naturalOrder());

D. Use PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());

Solution

1. Step 1: Understand default PriorityQueue order

   By default, PriorityQueue is a min-heap, so smallest element is returned first.

2. Step 2: Use reverse order comparator for max-heap behavior

   Passing Collections.reverseOrder() makes it return largest element first.

3. Final Answer:

   PriorityQueue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder()); -> Option D

4. Quick Check:

   Reverse order comparator = max-heap PriorityQueue [OK]

Hint: Use Collections.reverseOrder() to get max element first [OK]

Common Mistakes:

• Assuming default PriorityQueue is max-heap
• Passing null comparator causes NullPointerException
• Using naturalOrder() returns min element first