Introduction
Java's HashMap and HashSet classes are frequently used together since both the map and the Set rely on the hashCode() method to perform their essential functions. So what exactly is the dissimilarity between these two data structures? This article highlights their crucial characteristics and clarifies their purposes in Java programming so you can utilize them more in your applications.
HashMap vs. HashSet
If you're new to Java, you might wonder what the difference is between HashMap and HashSet. Both are used to store data, but they work in different ways. A HashMap uses a key-value pair that maps one object to another, whereas a HashSet store's unique values. If you want your Collection of objects to include duplicates, then use a HashSet. A key with two steals in a HashMap can reference two things; however, it can only have one value for each key in a HashSet.
Difference Between HashMap and HashSet in Tabular Form
|
Parameters of
Comparison | HashMap | HashSet |
| Define | HashMap is a data structure that holds key-value pairs | HashSet is a data structure that reserves data in an unordered fashion but does not allow duplicate entries |
| Speed | Faster | Slower |
| Insertion | Use put | Use Add |
| Implement | Map interface | Set interface |
What is HashMap?
HashMap is a data structure that holds key-value pairs. It is equivalent to a dictionary or a lookup table. The keys in a HashMap are unique and used to access the values. The values can be anything, including another data structure. For example, you could have a HashMap of integers and strings. You would put all the integers together as a key and all the strings together as a value. In this case, there would be two separate sets: one with the ints as keys and one with the strings as keys. However, if you had an integer object instead of an integer (because you're using JavaScript), then it would just be stored in both sets because Javascript does not support any structuring for objects (like structs or classes).
What does HashMap Implement?
HashMap is a key-value data structure that stores data in a hash table. The keys are used to access the values, which are stored in buckets. When you add a new key-value pair to a HashMap, the key is hashed and used to determine which bucket the value will be held in. If there is already a value in that bucket, the new deal will replace it. Keys cannot be duplicated. Values can have any type but must all have the same type.
Dummy Values in HashMap
HashMap is a key-value data structure that maps keys to values. A key can be any object, but a deal can only be an integer. If you try to insert a non-integer value into a HashMap, it will throw an exception. HashSet is a key-only data structure that stores keys. Discounts are not allowed in HashSet. If you push to insert a value into a HashSet, it will throw an exception. HashSet has no order and doesn't allow duplicate keys; each key maps to a single value. HashMap has a charge - if you put two objects with the same key in one of these, they will map to two different positions in the table, meaning one will take precedence over the other if looked up by that key. Duplicate keys are possible in HashMap; when inserted into a set like this, they map to both locations, and one takes precedence over the other when looked up by that key.
Objects Required During an Add Operation in HashMap
If you're adding an element to a HashMap, you'll need the following:
- The key for the element you're adding
- The value for the element you're adding
- A reference to the HashMap object
- The size of the HashMap (this is used to determine where to add your new element)
- The load factor of the HashMap (this is used to determine when to resize the HashMap)
- A reference to the Entry object for the key you're adding (this is used to find a current entry with the exact key)
- A reference to the Entry object for the new element you're adding
Adding and Storing Mechanism in HashMap
HashMap performs by reserving key-value pairs in a hash table. When you add a new element to a HashMap, it will calculate the key's hash code and use that to determine where to store the value in the table. When you want to retrieve an element from a HashMap, it will again calculate the key's hash code and use that to find the value in the table.
Speed of HashMap
HashMap is generally faster than HashSet because it uses a key-value pair. In a key-value couple, each key is associated with one value. A HashSet, on the other hand, can store multiple values. So, if you're looking for speed, HashMap is the way to go. However, there are some disadvantages.
Insertion Method in HashMap
A HashMap store's key-value pairs. When you add a new element to a HashMap, it uses the key's hashcode to determine where to store the value. The key is then mapped to the matter, and both are held in the HashMap. If you try to add a legend already in the HashMap, the new value will replace the old one.
What is HashSet?
HashSet is a data structure that reserves data in an unordered fashion but does not allow duplicate entries. It is similar to an array but with some key differences. One of the main differences is that HashSet uses a hashing algorithm to determine where to store data, whereas an array stores data in sequential memory locations. This makes HashSet much faster than an array when searching for data. Another difference is that HashSet can only store one instance of each piece of data, whereas an array can store multiple instances.
What does HashSet Implement?
There are a few critical differences between HashMap and HashSet. For one, HashSet implements the Set interface, which means it can only store unique elements. This is because Set doesn't allow duplicate elements and will only add a part if it's not already present in the Set. In contrast, HashMap stores both keys and values so that you can have multiple objects with the same value for their key (known as collisions). You also have to ensure you're not storing duplicate values for your keys when using a Map. If two or more values are mapped to the same key in your map, those items will be stored with each other. However, because of this uniqueness constraint on hash sets, they require much less memory than maps do when they're used to store identical data types: The size of hash sets scales linearly with the number of items added while hashing maps scale exponentially with more data.
Dummy Values in HashSet
A HashSet is a data structure that keeps data in an unordered fashion. A vital characteristic of a HashSet is that it does not allow duplicate values. It will be ignored if you try to add a value to a HashSet that already exists. Using the Collection, you can find out how many times each element appears in the Set. Size () method or iterating over its features with a for-each loop. In addition, Iterator allows adding new elements to an existing collection at any position within the iteration - which is impossible with HashMaps because they only allow insertion at one end of their list. The size() method on Iterator returns the number of elements currently stored in the hash set.
Objects Required During an Add Operation in HashSet
To add an object to a HashSet, you need to:
- Call the add() method, passing in the object you want to add.
- The add() method calls the hashCode() method on the object to be added.
- The hashCode() method returns an int value, which is used as the index to store the object in the backing array.
- If there isn't already an object stored at that index, the thing is added to the array.
- If there is already an object stored at that index, though, things get a little more complicated.
Adding and Storing Mechanism in HashSet
HashSet is a Set that store the data in key, value format. To add an element in HashSet, we need to specify both key and value. While storing data, HashSet internally uses hashing technique to calculate the hashcode of both key and value. The calculated hashcode is used as an index to store objects in the bucket. A key is considered a duplicate if it generates the same hashcode as that of another critical present in Set. In such a case, only one key is allowed to store in Set, and other keys are rejected.
Speed of HashSet
HashSet is much faster than HashMap because it uses a different data structure internally. HashSet uses a hash table, a data structure that stores items in an array using a hashing function. This function determines where each item is stored in the array. This benefit is that it can be accessed quickly because the computer can calculate the index of where an item is stored using the hashing function.
Insertion Method in HashSet
A HashSet is a Set that uses a hash table for storage. A Set is a data structure that holds unique elements. The main difference between HashMap and HashSet is that HashMap stores key-value pairs while HashSet stores values.
You can assume a HashSet as an Index that doesn't allow duplicates. So, when you add an element to a HashSet, it first checks to see if that element is already in the Set. If it is, it does nothing. If it isn't, it adds the element to the Set.
The insertion method in HashSet checks if the added element is already in the Set. If it isn't, it calls the add method, which will add the element to the Set.
Main Differences Between HashMap and HashSet in Points
- HashMap is a map while HashSet is a set.
- HashMap accepts key-value pairs while HashSet only accepts single values.
- The keys in a HashMap must be unique, while the values in a HashSet can be duplicated.
- The key in a HashMap is used to access the corresponding value, while there is no such concept in a HashSet.
- The order of elements in a HashMap is not guaranteed, while the order of elements in a HashSet is secured by default.
- A null key and multiple null values are allowed in a HashMap, but only a single null value is permitted in a HashSet.
- The elements in a HashMap do not have to implement the equals() or hashCode() methods, unlike in a HashSet where they are required.
- In a HashMap, if an element with its key already exists, then it will replace that element with its value instead of creating another one which does not happen with a HashSet since it can only hold unique values.
Conclusion
A HashMap is a data structure that stows key-value teams, while a HashSet is a data structure that only stores values. Both have pros and cons, so it relies on your distinct necessities as to which one you should use. For example, if you want to store unique items in a collection, use HashSet. On the other hand, if you wish to fast retrieve things based on their keys, then use HashMap.
Frequently Asked Questions
What is the difference between HashMap and HashSet?
HashMap and HashSet are two critical data structures in Java. They store data in key-value pairs but differ in how they work.
What is the difference between HashSet and HashMap, and Hashtable?
There are three main differences between HashSet, HashMap, and Hashtable:
- HashSet doesn't allow null values, while both HashMap and Hashtable do.
- HashMap is not synchronized, while both HashSet and Hashtable are.
- Finally, the Iterator of HashMap is fail-fast, while for both HashSet and Hashtable, it is not.
Why does HashSet use HashMap?
HashSet is a collection that stores unique elements, meaning no duplicates are allowed. To do this, it uses a HashMap internally. Every time a part is added to the HashSet, it's also added to the HashMap. The key of the map entry is the element being added, and the value is a dummy object.
What is the difference between HashMap and set in Java?
HashMap is a data structure that maps keys to values. It is implemented using a hash table. A key is an object you use to retrieve a value from a HashMap. A value is an object that can be retrieved by using a key.
