| Criteria | EnumSet | HashSet |
|---|---|---|
| Element Type | Only stores elements of a single enum type. | Can store any objects. |
| Performance | Typically faster than HashSet due to optimized bit vector implementation. | Good general-purpose performance, based on hash table. |
| Ordering | Maintains elements in their natural order (i.e., the order in which enum constants are declared). | Does not guarantee any specific order. |
| Null Elements | Does not support null elements. | Allows one null element. |
| Thread Safety | No | No |
| Memory Footprint | Typically less memory-consuming than HashSet due to internal bit vector optimizations. | Uses more memory due to underlying HashMap structure. |
| Use-case | When you need a set implementation specifically for enum types, offering high performance and minimal memory footprint. | General-purpose set implementation for any object type. |
| Internal Implementation | Bit vector. If the universe of enum constants is small enough, it uses a single long for storage. | Backed by a HashMap where the set’s elements are the keys. |
Example: Difference Between EnumSet and HashSet in Java
import java.util.EnumSet;
import java.util.HashSet;
import java.util.Set;
public class EnumSetVsHashSetDemo {
// Define an example enum for demonstration
public enum Days {
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}
public static void main(String[] args) {
// Using an EnumSet
Set<Days> daysEnumSet = EnumSet.of(Days.MONDAY, Days.WEDNESDAY, Days.FRIDAY);
// Using a HashSet
Set<Days> daysHashSet = new HashSet<>();
daysHashSet.add(Days.MONDAY);
daysHashSet.add(Days.WEDNESDAY);
daysHashSet.add(Days.FRIDAY);
System.out.println("EnumSet contents: " + daysEnumSet);
System.out.println("HashSet contents: " + daysHashSet);
}
}
Output:
EnumSet contents: [MONDAY, WEDNESDAY, FRIDAY] HashSet contents: [MONDAY, WEDNESDAY, FRIDAY]
Explanation:
1. EnumSet: It is a specialized set implementation for use with enum types. EnumSet is represented internally as a vector of bits. It is extremely compact and efficient. All basic operations execute in constant time.
2. HashSet: It is a general-purpose Set implementation backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order; it can change over time as elements are added and removed.
3. In the provided example:
– For EnumSet: We used the of method to create an EnumSet containing certain days. The printed order is the natural order of the enum constants.
– For HashSet: We added days one by one to the set. The printed order might appear the same as the insertion order in this small example, but HashSet doesn't guarantee any specific order.
4. When to use:
– Use EnumSet when working exclusively with enum types and need an efficient set implementation.
– Use HashSet for a general-purpose set implementation.
The main difference is in the internal implementation and efficiency. For operations on enum types, EnumSet is typically faster and more memory-efficient than HashSet.