Hash Map vs Array vs Linked List for Lookup in DSA Java - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA Java

int value = numbers[1]2;

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

A(

B[

C{

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for array access.

Using curly braces or angle brackets which are invalid for arrays.

In Java, arrays use square brackets [] to access elements by index.

2fill in blank

medium

Complete the code to check if the linked list contains the value 5.

DSA Java

boolean found = list[1](5);

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

Acontains

Bsearch

Cfind

Dhas

Attempts:

3 left

💡 Hint

Common Mistakes

Using search() which is not a LinkedList method.

Using find() or has() which do not exist for LinkedList.

The LinkedList class in Java has a contains() method to check if an element exists.

3fill in blank

hard

Fix the error in the code to get the value for key "apple" from the hash map.

DSA Java

String value = map[1]("apple");

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

Aget

Bfetch

CgetValue

Dlookup

Attempts:

3 left

💡 Hint

Common Mistakes

Using getValue() which does not exist in HashMap.

Using fetch() or lookup() which are not Java HashMap methods.

The HashMap class uses get() to retrieve a value by key.

4fill in blank

hard

Fill both blanks to create a hash map and add a key-value pair.

DSA Java

HashMap<String, Integer> map = new HashMap<>();
map.[1]("banana", [2]);

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

Aput

B5

Cadd

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using add() which is not a HashMap method.

Using a string instead of an integer for the value.

Use put() to add a key-value pair to a HashMap. The value 5 is an integer.

5fill in blank

hard

Fill all three blanks to iterate over an array and print each element.

DSA Java

for (int [1] = 0; [2] < arr.length; [3]++) {
    System.out.println(arr[i]);
}

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

Ai

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the loop parts.

Using 'j' which is not used in the loop body.

The loop variable is 'i' used consistently for initialization, condition, and increment.

Practice

1. Which data structure provides the fastest average lookup time for a key-value pair?

easy

A. Hash Map

B. Array

C. Linked List

D. Stack

Solution

1. Step 1: Understand lookup times for each data structure

   Hash Map uses hashing to find keys quickly, usually in constant time O(1). Arrays require searching by index or linear search for values, O(1) for index but O(n) for value lookup. Linked Lists require linear search O(n).

2. Step 2: Compare average lookup times

   Hash Map average lookup is O(1), Array lookup by index is O(1) but by value is O(n), Linked List lookup is O(n).

3. Final Answer:

   Hash Map -> Option A

4. Quick Check:

   Fastest lookup = Hash Map [OK]

Hint: Hash Map uses keys for O(1) lookup, arrays and lists do not [OK]

Common Mistakes:

• Confusing array index lookup with value lookup
• Assuming linked list lookup is fast
• Thinking stack is for key-value lookup

2. Which of the following is the correct way to declare a HashMap in Java for storing Integer keys and String values?

easy

A. HashMap map = new HashMap();

B. HashMap map = new HashMap<>();

C. HashMap map = new HashMap();

D. HashMap map = new HashMap<>();

Solution

1. Step 1: Check generic types in HashMap declaration

   Java uses wrapper classes like Integer, not primitive int, for generics. So HashMap is invalid.

2. Step 2: Check constructor syntax

   Using diamond operator <> is correct for type inference. HashMap map = new HashMap<>(); is valid and preferred.

3. Final Answer:

   HashMap map = new HashMap<>(); -> Option D

4. Quick Check:

   Correct generic syntax = HashMap map = new HashMap<>(); [OK]

Hint: Use wrapper classes and diamond operator for HashMap declaration [OK]

Common Mistakes:

• Using primitive types like int in generics
• Omitting diamond operator causing raw type warning
• Swapping key and value types in constructor

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

import java.util.*;
public class Test {
  public static void main(String[] args) {
    List<Integer> list = Arrays.asList(10, 20, 30);
    Map map = new HashMap<>();
    for (int i = 0; i < list.size(); i++) {
      map.put(list.get(i), "Value" + i);
    }
    System.out.println(map.get(20));
  }
}

medium

A. Value0

B. 20

C. Value1

D. null

Solution

1. Step 1: Understand the loop and map population

   The loop runs i from 0 to 2. For each i, map puts key list.get(i) and value "Value" + i. So keys 10, 20, 30 map to "Value0", "Value1", "Value2" respectively.

2. Step 2: Check the output of map.get(20)

   Key 20 maps to "Value1" so printing map.get(20) outputs "Value1".

3. Final Answer:

   Value1 -> Option C

4. Quick Check:

   map.get(20) = Value1 [OK]

Hint: Map keys match list values; index i forms value string [OK]

Common Mistakes:

• Confusing key with value in map.get()
• Assuming map stores values in insertion order
• Expecting numeric output instead of string

4. Identify the error in the following Java code snippet that tries to find if a value exists in a LinkedList:

LinkedList<String> list = new LinkedList<>();
list.add("apple");
list.add("banana");
if (list.contains("orange")) {
  System.out.println("Found orange");
} else {
  System.out.println("Orange not found");
}

medium

A. No error; code works correctly

B. LinkedList does not have contains() method

C. Must use list.search("orange") instead of contains()

D. Contains method only works for integers

Solution

1. Step 1: Check LinkedList methods

   Java's LinkedList class inherits from Collection and has a contains() method to check if an element exists.

2. Step 2: Verify code logic

   The code adds "apple" and "banana" to list, then checks if "orange" is contained. Since "orange" is not in list, it prints "Orange not found". This is correct behavior.

3. Final Answer:

   No error; code works correctly -> Option A

4. Quick Check:

   LinkedList.contains() works as expected [OK]

Hint: LinkedList inherits contains() from Collection interface [OK]

Common Mistakes:

• Thinking contains() is not available for LinkedList
• Using non-existent search() method
• Assuming contains() only works for integers

5. You need to store a large dataset of user IDs (integers) and their names (strings) for fast lookup by user ID. Which data structure is best and why? Choose the best option considering lookup speed, memory, and insertion time.

hard

A. Array, because it provides O(1) lookup by index

B. Hash Map, because it provides average O(1) lookup by key

C. Linked List, because it allows easy insertion and deletion

D. Binary Search Tree, because it keeps data sorted

Solution

1. Step 1: Analyze lookup requirements

   We need fast lookup by user ID (key). Array lookup by index is O(1), but user IDs are not guaranteed to be continuous indices.

2. Step 2: Compare data structures for key-based lookup

   Linked List lookup is O(n), too slow. Binary Search Tree lookup is O(log n), slower than Hash Map average O(1). Hash Map stores key-value pairs with fast average lookup.

3. Step 3: Consider insertion and memory

   Hash Map has good insertion time and reasonable memory overhead. Array wastes memory if IDs are sparse. BST keeps data sorted but slower lookup.

4. Final Answer:

   Hash Map, because it provides average O(1) lookup by key -> Option B

5. Quick Check:

   Best for fast key lookup = Hash Map [OK]

Hint: Use Hash Map for fast key-based lookup, not arrays or lists [OK]

Common Mistakes:

• Choosing array assuming user IDs are continuous
• Ignoring lookup time differences
• Preferring BST for speed over Hash Map