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What is a HashMap vs HashSet?

HashMap Stores elements in form of key-value pair i.e each element has its corresponding key which is required for its retrieval during iteration. HashSet stores only objects no such key value pairs maintained. Put method of hash map is used to add element in hashmap.

tutorialspoint.com - Difference between HashMap and HashSet in Java. - Tutorialspoint

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HashMap and HashSet both are one of the most important classes of Java Collection framework.

Following are the important differences between HashMap and HashSet.

Sr. No. Key HashMap HashSet 1 Implementation Hashmap is the implementation of Map interface. Hashset on other hand is the implementation of set interface. 2 Internal implementation Hashmap internally do not implements hashset or any set for its implementation. Hashset internally uses Hashmap for its implementation. 3 Storage of elements HashMap Stores elements in form of key-value pair i.e each element has its corresponding key which is required for its retrieval during iteration. HashSet stores only objects no such key value pairs maintained. 4 Method to add element Put method of hash map is used to add element in hashmap. On other hand add method of hashset is used to add element in hashset. 5 Index performance Hashmap due to its unique key is faster in retrieval of element during its iteration. HashSet is completely based on object so compared to hashmap is slower. 6 Null Allowed Single null key and any number of null value can be inserted in hashmap without any restriction. On other hand Hashset allows only one null value in its collection,after which no null value is allowed to be added.

Example of Hashmap vs Hashset

JavaTester.java

Live Demo

import java.util.HashSet; public class JavaTester { public static void main(String[] args){ HashSet hs = new HashSet(); hs.add("John"); hs.add("Smith"); hs.add("Peter"); System.out.println("Before adding duplicate values

" + hs); hs.add("John"); hs.add("Smith"); System.out.println("

After adding duplicate values

" + hs); hs.add(null); hs.add(null); System.out.println("

After adding null values

" + hs); } }

Output

Before adding duplicate values [John, Smith, Peter] After adding duplicate values [John, Smith, Peter] After adding null values [null, John, Smith, Peter]

Example

JavaTester.java

Live Demo

import java.util.HashMap; public class JavaTester { public static void main(String[] args){ HashMap hm = new HashMap(); hm.put(12, "John"); hm.put(2, "Smith"); hm.put(7, "Peter"); System.out.println("

HashMap object output :

" + hm); hm.put(12, "Smith"); System.out.println("

After inserting duplicate key :

" + hm); } }

Output

HashMap object output : {2=Smith, 7=Peter, 12=John} After inserting duplicate key : {2=Smith, 7=Peter, 12=John}

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Is map faster than ArrayList?

stackoverflow.com - Using ArrayList or HashMap for better speed - Stack Overflow

The ArrayList has O(n) performance for every search, so for n searches its performance is O(n^2). The HashMap has O(1) performance for every search (on average), so for n searches its performance will be O(n). While the HashMap will be slower at first and take more memory, it will be faster for large values of n. The reason the ArrayList has O(n) performance is that every item must be checked for every insertion to make sure it is not already in the list. We will do n insertions, so it is O(n^2) for the whole operation. The reason the HashMap has O(1) performance is that the hashing algorithm takes the same time for every key and then the lookup to find the key also takes constant time. There can be occasions when the hash table exceeds its load factor and needs to be reallocated, and that it why it is constant on avarage.

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