Understanding Java EnumMap: Concept, Usage, and Implementation

In Java, the enum type is a useful data type for defining a fixed set of constants, and EnumMap is a high‑performance Map implementation based on enum types.

EnumMap stores key‑value pairs in an internal array, ensuring all enum values are covered and providing fast lookup, insertion, and removal.

What is EnumMap

EnumMap is a generic class extending AbstractMap and implementing Serializable and Cloneable . Its key type must be an enum, while the value type can be any object.

public class EnumMap
, V> extends AbstractMap
implements java.io.Serializable, Cloneable

How to use EnumMap

Define an enum, then create an EnumMap instance with the enum class, and put entries using put() , retrieve with get() , and remove with remove() .

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class EnumMapTest {
    enum Color { RED, GREEN, BLUE }
    @Test
    public void testColorEnumMap() {
        EnumMap
colorMap = new EnumMap<>(Color.class);
        colorMap.put(Color.RED, "#FF0000");
        colorMap.put(Color.GREEN, "#00FF00");
        colorMap.put(Color.BLUE, "#0000FF");
        System.out.println(colorMap);
    }
}

The output shows the mapping of each enum constant to its corresponding string.

Implementation principle

EnumMap uses an internal Object array whose length equals the number of enum constants; each position stores the value for the corresponding enum key, enabling O(1) access.

Advanced example – Command pattern

interface Command { void execute(); }
class StartCommand implements Command { @Override public void execute() { System.out.println("Start command executed"); } }
class StopCommand implements Command { @Override public void execute() { System.out.println("Stop command executed"); } }
enum CommandType { START, STOP }
public class CommandExample {
    private static final EnumMap
commandMap = new EnumMap<>(CommandType.class);
    static {
        commandMap.put(CommandType.START, new StartCommand());
        commandMap.put(CommandType.STOP, new StopCommand());
    }
    public static void main(String[] args) {
        commandMap.get(CommandType.START).execute();
        commandMap.get(CommandType.STOP).execute();
    }
}

This demonstrates using EnumMap to map enum command types to concrete command implementations.

Test case

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class EnumMapTest {
    enum Weekday { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
    @Test
    public void testWeekDayEnumMap() {
        EnumMap
map = new EnumMap<>(Weekday.class);
        map.put(Weekday.MONDAY, "星期一");
        map.put(Weekday.TUESDAY, "星期二");
        map.put(Weekday.WEDNESDAY, "星期三");
        System.out.println(" map = " + map);
        assertTrue(map.containsKey(Weekday.MONDAY));
        assertFalse(map.containsKey(Weekday.SATURDAY));
        System.out.println(" map.size() = " + map.size());
        assertEquals(3, map.size());
        System.out.println("map.get(Weekday.MONDAY) = " + map.get(Weekday.MONDAY));
        assertEquals("星期一", map.get(Weekday.MONDAY));
        map.remove(Weekday.MONDAY);
        assertEquals(2, map.size());
    }
}

The test verifies basic EnumMap operations such as insertion, lookup, size checking, and removal.

Conclusion

EnumMap provides a convenient and highly efficient way to map enum keys to values, making it especially suitable for scenarios where enums are used as map keys in backend development.