Hash Table Concept and Hash Functions in DSA Java - Build from Scratch

Hash Table Concept and Hash Functions

📖 Scenario: Imagine you are organizing a small library. You want to quickly find books by their titles. To do this, you decide to use a hash table, which helps you store and find books fast by turning their titles into numbers called hash codes.

🎯 Goal: You will create a simple hash table using an array and a hash function. You will store book titles in the hash table, then retrieve the book title using the book title.

📋 What You'll Learn

Create a hash table array of size 10

Write a hash function that converts a book title to an index

Store book titles in the hash table

Retrieve and print the stored book title for a given book title

💡 Why This Matters

🌍 Real World

Hash tables are used in many applications like databases, caches, and dictionaries to quickly find data using keys.

💼 Career

Understanding hash tables and hash functions is essential for software developers to optimize data storage and retrieval.

1

Create the hash table array

Create a String array called hashTable with size 10 to store book titles.

DSA Java

// Create a String array called hashTable with size 10
// Your code here

Hint

Use new String[10] to create an array of size 10.

2

Write the hash function

Create a method called hashFunction that takes a String called key and returns an int. The method should calculate the sum of the ASCII values of all characters in key and return the sum modulo 10.

DSA Java

String[] hashTable = new String[10];

// Create hashFunction method below
// Your code here

Hint

Loop through each character in the string and add its ASCII value to sum. Use modulo 10 to get the index.

3

Store book titles in the hash table

Use the hashFunction to find the index for the book title "JavaBasics" and store it in the hashTable array at that index.

DSA Java

String[] hashTable = new String[10];

public static int hashFunction(String key) {
    int sum = 0;
    for (int i = 0; i < key.length(); i++) {
        sum += key.charAt(i);
    }
    return sum % 10;
}

// Store "JavaBasics" in hashTable using hashFunction
// Your code here

Hint

Call hashFunction with "JavaBasics" and save the result in index. Then assign "JavaBasics" to hashTable[index].

4

Retrieve and print the stored book title

Use the hashFunction to find the index of "JavaBasics" and print the value stored in hashTable at that index.

DSA Java

String[] hashTable = new String[10];

public static int hashFunction(String key) {
    int sum = 0;
    for (int i = 0; i < key.length(); i++) {
        sum += key.charAt(i);
    }
    return sum % 10;
}

int index = hashFunction("JavaBasics");
hashTable[index] = "JavaBasics";

// Print the book title stored at the index for "JavaBasics"
// Your code here

Hint

Use System.out.println(hashTable[index]) to print the stored book title.

Practice

1. What is the main purpose of a hash function in a hash table?

easy

A. To encrypt the data stored in the table

B. To sort the keys in ascending order

C. To convert a key into an index for storing values

D. To count the number of elements in the table

Solution

1. Step 1: Understand the role of a hash function

   A hash function takes a key and transforms it into an index number that points to a location in the hash table.

2. Step 2: Compare options with the hash function role

   Only To convert a key into an index for storing values describes converting a key into an index, which is the main purpose of a hash function.

3. Final Answer:

   To convert a key into an index for storing values -> Option C

4. Quick Check:

   Hash function = key to index [OK]

Hint: Hash function maps keys to indexes in the table [OK]

Common Mistakes:

• Thinking hash functions sort keys
• Confusing hashing with encryption
• Believing hash functions count elements

2. Which of the following is the correct way to declare a HashMap in Java using built-in classes?

easy

A. Hashtable map = new Hashtable();

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

C. HashTable map = new HashTable<>();

D. HashMap map = new HashMap();

Solution

1. Step 1: Identify correct class names and syntax

   Java's standard hash table class is HashMap (case-sensitive). Hashtable is also valid but legacy and requires type parameters.

2. Step 2: Check each option's syntax

   HashMap map = new HashMap<>(); uses correct generic syntax with HashMap. Hashtable map = new Hashtable(); misses generics. HashTable map = new HashTable<>(); uses wrong class name 'HashTable' (capital T). HashMap map = new HashMap(); reverses generic placement incorrectly.

3. Final Answer:

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

4. Quick Check:

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

Hint: Use HashMap with generics and diamond operator <> [OK]

Common Mistakes:

• Using wrong class name 'HashTable'
• Omitting generics for type safety
• Incorrect generic syntax placement

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

import java.util.HashMap;
public class Main {
  public static void main(String[] args) {
    HashMap map = new HashMap<>();
    map.put("apple", 10);
    map.put("banana", 20);
    map.put("apple", 30);
    System.out.println(map.get("apple"));
  }
}

medium

A. 30

B. 20

C. 10

D. null

Solution

1. Step 1: Understand put() behavior in HashMap

   When a key already exists, put() replaces the old value with the new one.

2. Step 2: Trace the put() calls for key "apple"

   First put("apple", 10) adds key with value 10. Second put("apple", 30) replaces value 10 with 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last value for "apple" = 30 [OK]

Hint: Last put for a key overwrites previous value [OK]

Common Mistakes:

• Assuming values accumulate instead of overwrite
• Confusing keys and values
• Expecting null for existing keys

4. Identify the error in the following Java code that uses a hash table:

import java.util.HashMap;
public class Main {
  public static void main(String[] args) {
    HashMap map = new HashMap<>();
    map.put(1, "One");
    map.put(2, "Two");
    System.out.println(map.get("1"));
  }
}

medium

A. HashMap cannot store Integer keys

B. Missing import statement for HashMap

C. Incorrect syntax for put() method

D. Using wrong key type in get() method

Solution

1. Step 1: Check key types used in HashMap and get()

   The HashMap is declared with Integer keys. The get() method is called with a String "1" instead of Integer 1.

2. Step 2: Understand type mismatch effect

   Since keys are Integer, get("1") returns null because "1" (String) is not equal to 1 (Integer).

3. Final Answer:

   Using wrong key type in get() method -> Option D

4. Quick Check:

   Key type mismatch causes null result [OK]

Hint: Match key types exactly when using get() [OK]

Common Mistakes:

• Passing String key instead of Integer
• Ignoring type safety warnings
• Assuming automatic type conversion

5. You want to design a hash function for a hash table that stores student IDs (integers) as keys. Which of the following hash functions is best to minimize collisions for IDs mostly ranging from 1000 to 9999?

hard

A. hash(key) = key % 100

B. hash(key) = key % 10000

C. hash(key) = key / 1000

D. hash(key) = key % 10

Solution

1. Step 1: Analyze the key range and hash function outputs

   Keys range from 1000 to 9999. Using modulo 10 (hash(key) = key % 10) gives only 10 buckets, causing many collisions. Modulo 10000 (hash(key) = key % 10000) returns the key itself, which is too large for typical table size. Division by 1000 (hash(key) = key / 1000) gives values 1 to 9, only 9 buckets, too small.

2. Step 2: Evaluate modulo 100 (hash(key) = key % 100)

   Modulo 100 gives values from 0 to 99, creating 100 buckets. This spreads keys better and reduces collisions compared to options A and C.

3. Final Answer:

   hash(key) = key % 100 -> Option A

4. Quick Check:

   More buckets (100) reduce collisions best [OK]

Hint: Choose modulo near expected bucket count for spread [OK]

Common Mistakes:

• Choosing too few buckets causing collisions
• Using modulo equal to key range (too large)
• Using division that reduces bucket count too much