EnumSet / Bit Vector

📋 Quick Reference

Property	Value
Type	Bit vector for enum constants
Add/Remove/Contains	O(1) - single bit operation
Iteration	O(n) where n = enum size, not set size
Memory	One bit per enum constant (extremely compact)
Null	Not allowed
Best For	Sets of enum values, flags, permissions

One-liner: Ultra-efficient bit vector set for enum types - O(1) operations with minimal memory.

🎮 Interactive Visualizer

Watch how EnumSet uses bits to represent set membership:

EnumSet Bit Operations
O(1) all opsMemory efficient
1 / 12
Initialize EnumSet with empty bitmask (0). Each enum constant maps to a bit position based on ordinal().
DayOfWeek Enum → Bitmask Mapping
bit 00
MON
bit 10
TUE
bit 20
WED
bit 30
THU
bit 40
FRI
bit 50
SAT
bit 60
SUN
elements=0b0000000(0 in decimal)
Set Contents:
Empty set
💡 Why EnumSet? Memory Efficiency!
EnumSet
8 bytes
1 long for up to 64 enum values
✓ Single CPU instruction per operation
HashSet
~280 bytes
For 7 enum values (~40 bytes each)
Hash computation + object overhead
35x smaller memory footprint!
Pseudocode
1class EnumSet<E extends Enum<E>> {
2  private long elements = 0L;
3 
4  void add(E e) {
5    int ordinal = e.ordinal();
6    elements |= (1L << ordinal);
7  }
8 
9  void remove(E e) {
10    int ordinal = e.ordinal();
11    elements &= ~(1L << ordinal);
12  }
13 
14  boolean contains(E e) {
15    int ordinal = e.ordinal();
16    return (elements & (1L<<ordinal)) != 0;
17  }
18}
Variables
elements
=0b0000000
Keyboard Shortcuts
P Play / Pause
[ Step back
] Step forward
R Reset
Speed
Bit set (1)
Bit set (1)
Bit clear (0)
Bit clear (0)
Current operation
Current operation
Changed bit
Changed bit
P · [ ] · R
Loading visualizer...

Try these operations:

Add elements - see bits flip to 1
Remove elements - see bits flip to 0
Union/intersection - observe bitwise operations
Notice: each enum constant is just one bit!

🔧 Key Operations

Creating

JAVA(17 lines)
Code
Loading syntax highlighter...

Adding/Removing

JAVA(8 lines)
Code
Loading syntax highlighter...

Querying

JAVA(4 lines)
Code
Loading syntax highlighter...

Iteration

JAVA(9 lines)
Code
Loading syntax highlighter...

📊 Complexity Analysis

Operation	Time	Notes
`add(e)`	O(1)	Single bit set
`remove(e)`	O(1)	Single bit clear
`contains(e)`	O(1)	Single bit check
`addAll/removeAll`	O(1)	Bitwise OR/AND
`size()`	O(1)	Popcount (bit counting)
`iteration`	O(n)	n = total enum constants
`equals`	O(1)	Compare bit vectors

Bit Vector Representation

enum Permission { READ, WRITE, EXECUTE, DELETE }

Bit positions:  DELETE  EXECUTE  WRITE  READ
                  3        2       1      0

EnumSet.of(READ, WRITE) = 0011 (binary) = 3
EnumSet.of(READ, EXECUTE) = 0101 (binary) = 5
EnumSet.allOf() = 1111 (binary) = 15
EnumSet.noneOf() = 0000 (binary) = 0

Operations:
add(EXECUTE):    0011 | 0100 = 0111
remove(READ):    0111 & 1110 = 0110
contains(WRITE): 0110 & 0010 = 0010 (truthy)

Internal Implementation

JAVA(5 lines)
Code
Loading syntax highlighter...

✅ When to Use EnumSet

Good Use Cases

Permission/role flags - user permissions, feature flags
State combinations - multiple states at once
Day/month subsets - working days, quarter months
Configuration options - enabled features
Replacing bit flags - type-safe alternative to int flags

Avoid When

Non-enum types - use HashSet instead
Need insertion order - EnumSet uses declaration order
Need null elements - EnumSet prohibits null
Dynamic values - enums are compile-time fixed

🧩 Common Patterns

Permissions System

JAVA(27 lines)
Code
Loading syntax highlighter...

Feature Flags

JAVA(7 lines)
Code
Loading syntax highlighter...

Replacing Bit Flags

JAVA(9 lines)
Code
Loading syntax highlighter...

⚠️ Common Pitfalls

1. Using HashSet Instead of EnumSet

JAVA(5 lines)
Code
Loading syntax highlighter...

2. Forgetting Declaration Order

JAVA(6 lines)
Code
Loading syntax highlighter...

3. Creating from Empty Collection

JAVA(5 lines)
Code
Loading syntax highlighter...

4. Expecting Null Support

JAVA(6 lines)
Code
Loading syntax highlighter...

🎤 Interview Tips

Q: Why is EnumSet so fast?

"

It represents the set as a bit vector (typically a single long). Each enum constant maps to a bit position. add/remove/contains are single bit operations - literally O(1) with tiny constant factors.

Q: How much memory does EnumSet use?

"

For enums with ≤64 constants: 8 bytes (one long) plus object overhead. For larger enums: ceil(n/64) longs. Compare to HashSet which uses ~32 bytes per element.

Q: EnumSet vs EnumMap?

"

EnumSet is a Set of enum values (no associated values). EnumMap is a Map with enum keys and any values. Both use array indexing by ordinal for efficiency.

Q: Can EnumSet work with any enum?

"

Yes, any enum type. The type parameter ensures type safety. Implementation automatically chooses RegularEnumSet (≤64 constants) or JumboEnumSet (>64 constants).

Previous: Part 7: LinkedHashMap

Next: Part 9: Binary Search

Visualizer: EnumSet from @tomaszjarosz/react-visualizers