HashSet Vs HashMap in Java

HashSet and HashMap are both classes that provide collection frameworks for storing and manipulating data. While they have similar names, they serve different purposes and have distinct characteristics.

HashSet

  1. HashSet is an implementation of the Set interface.
  2. It does not allow duplicate elements. If you attempt to add an element that already exists in the HashSet, it will not be added.
  3. Elements in a HashSet are not ordered. There is no guarantee on the order in which elements will be stored or retrieved.
  4. It uses hashing to store elements, which enables fast lookup and retrieval.
  5. HashSet is primarily used when you want to store a collection of unique elements and you do not require any specific ordering of the elements.

HashSet Implementation

import java.util.*; class TestClass { public static void main (String[] args) { //create a HashSet Object HashSet days=new HashSet(); // add elements to the HashSet days.add("Sunday"); days.add("Monday"); days.add("Tuesday"); days.add("Wednesday"); days.add("Thursday"); days.add("Friday"); days.add("Saturday"); //Iterate through HashSet Iterator itr=days.iterator(); while(itr.hasNext()){ System.out.println(itr.next()); } } }

Examples of use cases for HashSet include removing duplicates from a list, checking membership, or finding the intersection or difference between two sets.

Output
Monday Thursday Friday Sunday Wednesday Tuesday Saturday

HashMap

  1. HashMap is an implementation of the Map interface.
  2. It stores data in key-value pairs, where each key is unique.
  3. Elements in a HashMap are not ordered. The order of key-value pairs is not guaranteed.
  4. HashMap allows null values for both keys and values.
  5. It also uses hashing to store elements, allowing for efficient key-based retrieval.
  6. HashMap is commonly used when you need to associate values with unique keys and perform efficient lookups or updates based on those keys.

HashMap Implementation

import java.util.*; class TestClass { public static void main (String[] args) { //Create HashMap HashMap < Integer,String > days = new HashMap < Integer,String >(); //Add Key/Value pairs days.put(1,"Sunday"); days.put(2,"Monday"); days.put(3,"Tuesday"); days.put(4,"Wednesday"); Set < Map.Entry < Integer,String > > set = days.entrySet(); for (Map.Entry < Integer,String > sg : set) { System.out.println("Key :"+sg.getKey() + " Value :"+days.get(sg.getKey())); } } }

Examples of use cases for HashMap include caching values, indexing data, or implementing a dictionary.

Output
Key :1 Value :Sunday Key :2 Value :Monday Key :3 Value :Tuesday Key :4 Value :Wednesday

Conclusion

HashSet is used when you need a collection of unique elements without any specific ordering, while HashMap is used when you need to store key-value pairs with efficient lookup capabilities.






Related Topics
  1. Java Interview Questions-Core Faq - 1
  2. Java Interview Questions-Core Faq - 2
  3. Java Interview Questions-Core Faq - 3
  4. Features of Java Programming Language (2024)
  1. Difference between Java and JavaScript?
  2. What is the difference between JDK and JRE?
  3. What gives Java its 'write once and run anywhere' nature?
  4. What is JVM and is it platform independent?
  5. What is Just-In-Time (JIT) compiler?
  6. What is the garbage collector in Java?
  7. What is NullPointerException in Java
  8. Difference between Stack and Heap memory in Java
  9. How to set the maximum memory usage for JVM?
  10. What is numeric promotion?
  11. Generics in Java
  12. Static keyword in Java
  13. What are final variables in Java?
  14. How Do Annotations Work in Java?
  15. How do I use the ternary operator in Java?
  16. What is instanceof keyword in Java?
  17. How ClassLoader Works in Java?
  18. What are fail-safe and fail-fast Iterators in Java
  19. What are method references in Java?
  20. "Cannot Find Symbol" compile error
  21. Difference between system.gc() and runtime.gc()
  22. How to convert TimeStamp to Date in Java?
  23. Does garbage collection guarantee that a program will not run out of memory?
  24. How setting an Object to null help Garbage Collection?
  25. How do objects become eligible for garbage collection?
  26. How to calculate date difference in Java
  27. Difference between Path and Classpath in Java
  28. Is Java "pass-by-reference" or "pass-by-value"?
  29. Difference between static and nonstatic methods java
  30. Why Java does not support pointers?
  31. What is a package in Java?
  32. What are wrapper classes in Java?
  33. What is singleton class in Java?
  34. Difference between Java Local Variable, Instance Variable and a Class Variable?
  35. Can a top level class be private or protected in Java
  36. Are Polymorphism , Overloading and Overriding similar concepts?
  37. Locking Mechanism in Java
  38. Why Multiple Inheritance is Not Supported in Java
  39. Why Java is not a pure Object Oriented language?
  40. Static class in Java
  41. Difference between Abstract class and Interface in Java
  42. Why do I need to override the equals and hashCode methods in Java?
  43. Why does Java not support operator overloading?
  44. Anonymous Classes in Java
  45. Static Vs Dynamic class loading in Java
  46. Why am I getting a NoClassDefFoundError in Java?
  47. How to Generate Random Number in Java
  48. What's the meaning of System.out.println in Java?
  49. What is the purpose of Runtime and System class in Java?
  50. The finally Block in Java
  51. Difference between final, finally and finalize
  52. What is try-with-resources in java?
  53. What is a stacktrace?
  54. Why String is immutable in Java ?
  55. What are different ways to create a string object in Java?
  56. Difference between String and StringBuffer/StringBuilder in Java
  57. Difference between creating String as new() and literal | Java
  58. How do I convert String to Date object in Java?
  59. How do I create a Java string from the contents of a file?
  60. What actually causes a StackOverflow error in Java?
  61. Why is char[] preferred over String for storage of password in Java
  62. What is I/O Filter and how do I use it in Java?
  63. Serialization and Deserialization in Java
  64. Understanding transient variables in Java
  65. What is Externalizable in Java?
  66. What is the purpose of serialization/deserialization in Java?
  67. What is the Difference between byte stream and Character streams
  68. How to append text to an existing file in Java
  69. How to convert InputStream object to a String in Java
  70. What is the difference between Reader and InputStream in Java
  71. Introduction to Java threads
  72. Synchronization in Java
  73. Static synchronization Vs non static synchronization in Java
  74. Deadlock in Java with Examples
  75. What is Daemon thread in Java
  76. Implement Runnable vs Extend Thread in Java
  77. What is the volatile keyword in Java
  78. What are the basic interfaces of Java Collections Framework
  79. Difference between ArrayList and Vector | Java
  80. What is the difference between ArrayList and LinkedList?
  81. What is the difference between List and Set in Java
  82. Difference between HashMap and Hashtable in Java?
  83. How does the hashCode() method of java works?
  84. Difference between capacity() and size() of Vector in Java
  85. What is a Java ClassNotFoundException?
  86. How to fix java.lang.UnsupportedClassVersionError