A hashing data structure, such as HashMap in Java, uses a hash function to map keys to their corresponding values in an underlying array.

When a key-value pair is added to the HashMap, the key is first passed through the hash function, which generates a hash code for the key. The hash code is then used to determine the index in the array where the value associated with the key should be stored.

To handle collisions, where multiple keys generate the same hash code, the HashMap stores a linked list at each array index. When a new key-value pair is added to the HashMap, it is added to the linked list at the appropriate index.

When retrieving a value from the HashMap, the key is first passed through the hash function to generate the corresponding index in the array. If the linked list at the index contains the desired key, the corresponding value is returned. If the linked list does not contain the desired key, the search continues by iterating through the linked list until the key is found or the end of the list is reached.

When the number of elements in the HashMap exceeds a certain threshold, the HashMap is resized to reduce the likelihood of collisions and improve performance. The resize operation involves creating a new array with a larger capacity, rehashing all the existing key-value pairs into the new array, and discarding the old array.

Overall, the HashMap data structure provides fast insertion, deletion, and retrieval of key-value pairs, as long as the hash function distributes keys evenly across the underlying array and collisions are minimized.

**What is the time complexity in terms of the big o notation of pushing and retrieving an element from a HashMap?**

In a HashMap data structure, the time complexity for pushing (adding) an element and retrieving an element depends on the number of elements in the HashMap and the efficiency of the hash function.

In the average case, assuming a good hash function and uniform distribution of keys, the time complexity for adding an element and retrieving an element from a HashMap is O(1), or constant time. This is because the hash function allows for quick calculation of the index where the element should be stored or retrieved from in the underlying array.

However, in the worst-case scenario, when there are many collisions due to poor hash function design or uneven distribution of keys, the time complexity for adding an element and retrieving an element from a HashMap can be O(n), where n is the number of elements in the HashMap. In this case, each element would need to be checked sequentially in the linked list stored at the index of the array generated by the hash function.

Therefore, it is important to choose an appropriate hash function and monitor the number of collisions to ensure the average case time complexity of O(1) is achieved.

**What is a collision in HashMap?**

Collision in a HashMap occurs when two or more keys have the same hash code, resulting in them being stored at the same index in the underlying array. When a collision occurs, HashMap uses a linked list data structure to store the key-value pairs at the corresponding index.

When a new key-value pair is added to the HashMap and a collision occurs, the HashMap appends the new pair to the end of the linked list at the corresponding index. When retrieving a value associated with a key that has collided, the HashMap must iterate through the linked list at the corresponding index until it finds the key.

Collisions can occur for a variety of reasons, including poor hash function design or a high load factor, which is the ratio of the number of elements to the size of the underlying array. To reduce collisions and improve performance, it is important to choose a good hash function and to monitor and adjust the load factor as needed. Additionally, when the number of elements in the HashMap becomes too large, it is typically resized to reduce the likelihood of collisions.

**Which is the best technique to handle collision in a HashMap? **

The best technique to handle collisions in a HashMap is to use separate chaining, which involves storing each colliding key-value pair in a linked list at the corresponding index of the underlying array.

Separate chaining is a simple and effective way to handle collisions because it allows for an arbitrary number of keys with the same hash code to be stored in the same index of the array. Additionally, separate chaining has a relatively small memory overhead and is easy to implement.

Other collision resolution techniques include open addressing, which involves finding the next available index in the array when a collision occurs and probing the array until an empty slot is found. However, open addressing can be more complex to implement and may require more memory than separate chaining.

In summary, separate chaining is the recommended technique for handling collisions in a HashMap due to its simplicity, effectiveness, and low memory overhead.

**What is the default size of the load factor in the hashing-based collection?**

The default load factor for a hashing-based collection, such as a HashMap in Java, is 0.75.

The load factor is a measure of how full the underlying array is allowed to become before the collection is resized to maintain efficiency. The load factor is defined as the ratio of the number of elements in the collection to the size of the underlying array.

A load factor of 0.75 means that the underlying array is resized when the collection is 75% full. The default load factor of 0.75 is a good balance between space utilization and performance, but it can be adjusted as needed depending on the specific use case.

A higher load factor can result in more collisions and longer search times, but it can also reduce memory usage by allowing the underlying array to become more densely populated. Conversely, a lower load factor can reduce collisions and search times, but it can also increase memory usage by requiring a larger underlying array.