EnumSet & EnumMap in Java

Both EnumSet and EnumMap are special Collection classes designed exclusively for enum types.

They are extremely fast, lightweight, memory-efficient, and perform better than traditional Set/Map implementations when working with enums.

Both belong to the java.util package.

What is EnumSet?

EnumSet is a specialized Set implementation created only for enum types. It stores enum constants using a bit-vector representation, making it extremely fast and memory-efficient.

• Stores only enum constants
• Backed internally by a bit vector (bitwise operations → very fast)
• All elements must be from the same enum
• Maintains natural order (enum declaration order)

Key Features of EnumSet

• Works only with Enums --> Cannot store any other type.
• Backed by Bit Vector --> Highly optimized for speed and memory.
• Natural Ordering --> Order follows the enum declaration.
• Null Not Allowed --> Adding null → NullPointerException.
• Very Fast (O(1) operations) --> Faster than HashSet for add, remove, and contains.

Common EnumSet Methods

EnumSet Examples

1. Creating an EnumSet

enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

EnumSet<Day> weekdays = EnumSet.of(Day.MON, Day.TUE, Day.WED, Day.THU, Day.FRI);

2. Selecting All Values

EnumSet<Day> allDays = EnumSet.allOf(Day.class);

3. Range Example

EnumSet<Day> midWeek = EnumSet.range(Day.TUE, Day.THU);

4. Complement Example

EnumSet<Day> weekend = EnumSet.complementOf(weekdays);

When to Use EnumSet?

Use EnumSet when:

• You need to store multiple enum values
• You want fast and memory-efficient operations
• You need range or complement functionality
• The order must follow enum declaration

Common Use Cases:

• File permissions (READ, WRITE, EXECUTE)
• App features / flags
• UI access permissions
• Days, workflow states, categories

Advantages of EnumSet

• Extremely fast (bitwise operations)
• Very memory-efficient
• Natural ordering maintained
• Rich helper methods (allOf, range, complementOf)
• Safer and cleaner than bit masking

Limitations of EnumSet

• Works only with enum types
• Null not allowed
• Not thread-safe (use Collections.synchronizedSet() if needed)

What is EnumMap?

EnumMap is a specialized Map implementation where keys must be enum constants.

It is backed internally by a simple array, making it faster and lighter than HashMap whenever the keys are enums.

• Keys must be enum type
• Values can be any type
• Internally backed by an array, making it extremely fast
• Maintains natural ordering of keys (enum declaration order)
• Null keys not allowed, null values allowed

Key Features of EnumMap

• Keys must be Enum  --> Using non-enum keys is not allowed.
• Very Fast and Memory Efficient --> Faster than HashMap when keys are enum constants.
• Maintains Natural Order --> Order is exactly the same as enum declaration.
• Null Handling

1. Null keys → throws NullPointerException
2. Null values → allowed

• Backed by Array --> Predictable, compact, and efficient internal structure.

EnumMap Methods

EnumMap Example

Enum Declaration

enum Status { NEW, IN_PROGRESS, COMPLETED }

EnumMap Usage

EnumMap<Status, String> map = new EnumMap<>(Status.class);

map.put(Status.NEW, "Task Created");
map.put(Status.IN_PROGRESS, "Task Running");
map.put(Status.COMPLETED, "Task Done");

System.out.println(map);

Output

{NEW=Task Created, IN_PROGRESS=Task Running, COMPLETED=Task Done}

When to Use EnumMap?

Use EnumMap when:

• Keys are enum values
• You want fast lookup and update
• You want predictable ordering
• You want memory-efficient storage

Common Use Cases:

• Finite-state machines
• Task/workflow states
• Configuration mapping
• Caching enum-based data

Advantages of EnumMap

• Faster than HashMap for enum keys
• Very memory-efficient
• Maintains key order
• Type-safe and clean design

Limitations of EnumMap

• Only enum keys allowed
• Null keys not allowed
• Not thread-safe (use Collections.synchronizedMap() if required)

EnumSet vs EnumMap