Difference Between HashSet vs HashMap
In this article HashSet vs HashMap, two of the key elements of collection framework, HashSet and HashMap, is generally thought of as a collective item and is referred interchangeably. But there is a whole lot of differences between these two. Let us start by discussing what collection framework is and then we would look into them one by one, then the similarities between them and at the end discussion about the differences and some common errors pointing to their interchangeable usage tendency. These two elements are widely used in multi-threading concepts as well.
Terminologies of HashSet and HashMap
Below are the terminologies of HashSet and HashMap:
1. Collection Framework: This type of framework enables the storage and manipulation of a group of objects. This is a collective architecture of interfaces, classes, and algorithms. In simple words, the framework will allow to build a building with different elements like brick, cement, rods, etc. which are interfaces, classes, and algorithms.
This architecture was devised keeping in mind:
- This framework should be high-performance.
- Allow different types of collection to work similarly.
- Easily scalable and adaptable.
2. Collection: These are standard groups of classes/interfaces each performing specified tasks. Some of the groups are fully implemented and some of them provide skeletal support.
3. Hash: Hashing is a function that is used for mapping arbitrary sized data to fixed-size values.
4. HashSet: As the name suggests, this type represents the implementation of set A set interface has the only element hashed to it. This type of interface doesn’t allow duplicate elements.
5. HashMap: This has an implementation of the map interface (associative map) where there is a key-value pair representation. This type of interface doesn’t allow duplicate keys.
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Similarities between HashSet and HashMap
- Both of these concepts are unsynchronized. This poses danger for usage in the threading option. In case we want to use them in thread-safe operation, we would need to explicitly synchronize them.
- There is no guarantee on the order of element remaining constant.
- Digging deeper we see that HashSet source code is backed by HashMap.
- Time performance for basic operation like adding/insertion, deleting/removal is constant.
- Both of them use the same function to continue maintaining the distinctive elements in the data, hashCode() and equals() are the methods used.
Head to Head Comparison between HashSet vs HashMap (Infographics)
Below is the top 8 comparison between HashSet vs HashMap:
Key Differences between HashSet vs HashMap
- The key difference between HashSet and HashMap is that the hashing function used for HashSet works only on one element whereas, for HashMap, the function works on two elements.
- While insertion of new value in a HashMap with the key already existing, the new value will be overwritten on the previous value. Whereas in HashSet during insertion of a new value which already exists, insertion won’t be allowed.
- In HashSet, the objects are stored. For example, HashSet of string objects will be depicted as {‘You’, ‘have’, ‘a’, ‘good’, ‘day’}. In HashMap the similar sentence is represented with a key-value pair. For example, {1à‘You’, 2à ‘have’, 3à ‘a’, 4à ‘good’, 5à ‘day’}. The key is integer type and value is a string.
- In terms of usage, if the task to perform a check for the presence of an element, we use Set implementation. The code is cleaner and more understandable. If the task is storing data for elements or requires faster search operations based on keys, we use Map implementation.
Comparison Table of HashSet vs HashMap
The table below summarizes the comparisons between HashSet vs HashMap:
| Genre | How is it implemented/used in HashSet? | How is it implemented/used in HashMap? |
| Hierarchy | HashSet is implemented using a set interface by extending the collection interface. | HashMap has its hierarchy and is completely different from the collection interface. |
| Data Storage | The data is stored as objects in HashSet | The data in HashMap is stored as key-value pair. In layman terms, the data has a key, which needs to be distinct, and value attached to the key. |
| Internal Structure | Internally HashMap data structure is used for the storage of data elements in HashSet. In layman terms, if data is requested to get stored using HashSet, internally HashMap will be used for storage. | Internally HashMap uses an array of Entry<k,v> objects for storage of data. Here the ‘k’ is the key and ‘v’ is the value. And both together form the Entry for a key-value pair. |
| Duplicate Values | Duplicate elements are not allowed. During insertion, if a duplicate element is found HashSet will not change as the insertion would not take place. | Elements can be duplicated in the data. But the key should be unique. |
| Insertion Operation | One object i.e. value is used for the insertion process in HashSet. add() function is used for insertion. | Two objects are required for the insertion process to take place. One has to be key and other as value. put() method is used for insertion. |
| Performance/ Complexity | The values in HashSet is used for calculating hashcode value. The hashcode value is used for accessing the object. This value can be the same for 2 values, thus affecting performance. The complexity of HashSet is O(n). | The values in HashMap are associated with unique keys. This key is used for accessing the object. Hence the operations in HashMap is faster. The complexity of HashMap is O(1). To achieve the order of complexity O(1), and an efficient hashing algorithm is needed. |
| Usage | When the uniqueness of data is required HashSet is used. For example, storing a week of days. | HashMap is widely used until maintenance of the uniqueness of data is inevitable. |
| Null Values | Only one null value can be stored in HashSet. ‘null’ value is considered as a single element and since duplicate elements are not allowed, hence only one ‘null’ value is allowed. | There can be multiple null values HashMap can hold as it doesn’t put any restriction on duplicate values. But, only one null key is allowed as duplicate keys are not allowed in HashMap. |
Conclusion
Though there are noticeable differences between HashSet and HashMap, sometimes they are interchangeably used which might lead to faulty implementations. Though HashMap is internally used for HashSet, it might be common thinking that they can be interchangeably used without much resistance, but one must be careful about the usage. The key differences in terms of usage highlighted above might give a good platform for choosing which type needs to be used when.
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