EnumMap and EnumSet

What you will learn

• Use an EnumMap when the keys are constants of one enum
• Create an EnumSet with of, range, allOf, noneOf and complementOf
• Explain why EnumMap and EnumSet are fast and keep declaration order
• Avoid the null-key and constructor mistakes
• Choose between EnumMap and HashMap for enum keys

Topics in this lesson

1. EnumMap: a map keyed by an enum
2. Counting and grouping with enum keys
3. EnumSet: a set of enum constants
4. EnumMap or HashMap? Limits and traps

Key points

• EnumMap and EnumSet are for keys and elements that are constants of one enum; they are fast and loop in declaration order.
• Create an EnumMap with the enum class: new EnumMap<>(Day.class); create an EnumSet with of, range, allOf, noneOf or complementOf.
• OrderStatus.values() plus getOrDefault prints every constant, even those with no entry.
• Neither accepts null keys or elements, and neither is thread-safe.

Interview questions this lesson answers

• What is an EnumMap, and when would you use it?
• Why does new EnumMap<>(Day.class) need the class?
• Can an EnumMap or EnumSet hold null?
• How do you create an EnumSet?
• Why prefer EnumMap to HashMap for enum keys?
• What is the order of an EnumSet or EnumMap loop?
• Follow-up: how do you list the days that are not in an EnumSet?