Working of HashMap in Java

In Java, HashMap is a class that implements the Map interface and is used to store key-value pairs. HashMap uses a hash table to store the data, which provides constant-time performance for basic operations like get() and put().

Here’s a brief overview of how HashMap works:

When a key-value pair is added to the HashMap using the put() method, a hash code is generated for the key. The hash code is an integer value that represents the key.
The hash code is then used to calculate the index of the entry in the hash table where the key-value pair will be stored. The index is calculated using a hash function.
If the index is already occupied by an existing entry, a collision occurs. In this case, the HashMap uses a linked list to store all the entries that have the same hash code.
When the get() method is called with a key, the hash code for the key is calculated, and the index in the hash table is found. If there is a collision at that index, the linked list is traversed to find the entry with the matching key.
When the remove() method is called with a key, the hash code for the key is calculated, and the index in the hash table is found. If there is a collision at that index, the linked list is traversed to find the entry with the matching key, which is then removed.

HashMap provides constant-time performance for basic operations like get() and put() because the hash code for the key is used to directly calculate the index in the hash table where the key-value pair will be stored or retrieved. However, if there are many collisions in the hash table, the performance of HashMap may degrade. To avoid collisions, it’s important to choose good hash functions and avoid having too many elements in the map.

What is Hashing:

Hashing is a technique used in computer science to transform data of arbitrary size into a fixed size value, usually a number or a string of characters. The fixed size value is called a hash, and the process of creating a hash from data is called hashing.

Hashing is used in a variety of applications, including data storage, encryption, and authentication. In data storage, hashing is used to efficiently search for and retrieve data from large datasets, like in the case of HashMaps in Java.

Hashing works by taking input data of arbitrary size and using a hash function to map the input to a fixed-size output. The hash function takes the input data and applies a series of mathematical operations to it, producing a hash value that is unique to that input data. Ideally, two different input values should not produce the same hash value, but this is not always possible due to the limited size of the output.

Hashing is commonly used in conjunction with other algorithms, like encryption and compression, to provide additional security and efficiency. In authentication systems, hashing is used to verify that a user’s password matches the hash of their original password. When a user creates a password, it is hashed and stored in a database. When the user logs in, the password they enter is hashed and compared to the stored hash. If the hashes match, the user is granted access. This way, even if the database is compromised, an attacker will not be able to obtain the user’s password, since they only have access to the hashed value.

What is HashMap:

In Java, HashMap is a class that implements the Map interface and provides a way to store key-value pairs. A HashMap is essentially a collection of key-value pairs, where each key is unique and maps to a single value.

A HashMap uses a hash table to store the key-value pairs, which allows for constant-time performance for basic operations like get() and put(). The hash table is an array of buckets, each of which contains a linked list of entries. Each entry represents a key-value pair.

When a key-value pair is added to the HashMap, a hash code is generated for the key, which is used to calculate the index of the entry in the hash table where the key-value pair will be stored. If the index is already occupied by an existing entry, a collision occurs, and the entry is added to the linked list of entries for that bucket.

When the get() method is called with a key, the hash code for the key is calculated, and the index in the hash table is found. If there is a collision at that index, the linked list is traversed to find the entry with the matching key.

Similarly, when the remove() method is called with a key, the hash code for the key is calculated, and the index in the hash table is found. If there is a collision at that index, the linked list is traversed to find the entry with the matching key, which is then removed.

Insert Key, Value pair in HashMap:

To insert a key-value pair in a HashMap in Java, you can use the put() method. Here’s an example:

import java.util.HashMap;

public class Example {
    public static void main(String[] args) {
        // Create a new HashMap
        HashMap<String, Integer> map = new HashMap<>();

        // Add a new key-value pair
        map.put("apple", 1);

        // Add another key-value pair
        map.put("banana", 2);

        // Print the contents of the HashMap
        System.out.println(map);
    }
}

In this example, we create a new HashMap with keys of type String and values of type Integer. We then use the put() method to add two key-value pairs to the map: "apple" with a value of 1, and "banana" with a value of 2.

Finally, we print the contents of the HashMap using the println() method. The output should be: {apple=1, banana=2}.

Note that if the HashMap already contains the key, calling put() with the same key will update the existing value.

Calculating Index:

In Java’s HashMap, the index of an entry in the hash table is calculated based on the hash code of the key using the following formula:

index = (hash & (tableSize - 1))

where hash is the hash code of the key, and tableSize is the size of the hash table. The & operator is a bitwise AND operator.

The reason for using tableSize - 1 is because the size of the hash table is typically a power of 2. When you subtract 1 from a power of 2, you get a binary number that consists of all 1’s. For example, if the tableSize is 16, tableSize - 1 is 15, which is 0b1111 in binary.

Taking the bitwise AND of a hash code with a binary number that consists of all 1’s effectively masks the hash code to the lower n bits, where n is the number of bits needed to represent the tableSize. This ensures that the index falls within the range of the hash table.

For example, if the hash code is 0b10011011 and the tableSize is 16, the index would be:

index = (0b10011011 & (16 - 1))
      = (0b10011011 & 0b1111)
      = 0b1011
      = 11

So the entry with this key would be stored at index 11 in the hash table.

Note that if the HashMap is resized, the index for a given key may change, as the hash code is rehashed using the new table size.

Hash Collision:

A hash collision occurs in a hash table when two different keys generate the same hash code and therefore map to the same index in the hash table. This can happen because the number of possible hash codes is typically much smaller than the number of possible keys.

When a hash collision occurs, the HashMap uses a linked list to store multiple entries at the same index in the hash table. Each entry in the linked list represents a key-value pair that maps to the same index.

When you insert a key-value pair into a HashMap and a hash collision occurs, the new entry is added to the linked list at the corresponding index. When you retrieve a value from a HashMap, the hash code for the key is used to locate the index in the hash table, and the linked list at that index is searched for the entry with the matching key.

In general, hash collisions can reduce the performance of a HashMap because they require additional work to search through the linked list. However, if the hash table is implemented correctly and has a good hash function, the number of hash collisions should be small, and the performance of the HashMap should remain high.

To reduce the likelihood of hash collisions, it’s important to choose a good hash function that produces a uniform distribution of hash codes across the key space, and to avoid having too many elements in the map. If the number of elements in the map grows too large, the likelihood of hash collisions increases.

get() method in HashMap:

In Java’s HashMap, the get() method is used to retrieve the value associated with a given key in the hash table. The method signature is as follows:

public V get(Object key)

where key is the key whose associated value you want to retrieve, and V is the type of the values stored in the HashMap.

To retrieve the value associated with a key using the get() method, the hash code of the key is first calculated. The hash code is then used to find the index of the entry in the hash table. If the entry at the corresponding index has the same key as the one you are looking for, the associated value is returned. If there is a linked list of entries at the corresponding index, the linked list is searched for an entry with the same key, and the associated value is returned if found. If no matching entry is found, null is returned.

Here’s an example of how to use the get() method:

import java.util.HashMap;

public class Example {
    public static void main(String[] args) {
        // Create a new HashMap
        HashMap<String, Integer> map = new HashMap<>();

        // Add some key-value pairs
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("orange", 3);

        // Retrieve the value associated with the key "banana"
        int value = map.get("banana");

        // Print the value
        System.out.println(value);
    }
}

In this example, we create a HashMap and add three key-value pairs to it. We then retrieve the value associated with the key "banana" using the get() method and store it in an int variable named value. Finally, we print the value, which should be 2.

Note that if the key you pass to the get() method is not present in the HashMap, null will be returned. It’s important to handle this case in your code to avoid NullPointerExceptions.