Enum Types - Beginning Java 8 Fundamentals - page 714

You can also nest an enum type inside another enum type or an interface. The following are valid enum type
declarations:
public enum OuterEnum {
C1, C2, C3;
public enum NestedEnum {
C4, C5, C6;
}
}
public interface MyInterface {
int operation1();
int operation2();
public enum AnotherNestedEnum {
CC1, CC2, CC3;
}
}
Implementing an Interface to an Enum Type
An enum type may implement interfaces. The rules for an enum type implementing an interface are the same as the
rules for a class implementing an interface. An enum type is never inherited by another enum type. Therefore, you
cannot declare an enum type as abstract. This also implies that if an enum type implements an interface, it must also
provide implementation for all abstract methods in that interface.
The program in Listing 18-8 declares a Command interface. The program in Listing 18-9 declares an enum type
called CommandList that implements the Command interface. Each enum constant implements the execute() method
of the Command interface. Alternatively, you can implement the execute() method in the enum type body and omit
the implementations from some or all enum constants. Listing 18-10 demonstrates using the enum constants in the
CommandList enum type as Command type.
Listing 18-8. A Command Interface
// Command.java
package com.jdojo.enums;
public interface Command {
void execute();
}
Listing 18-9. A CommandList Enum Type Implementing the Command Interface
// CommandList.java
package com.jdojo.enums;
public enum CommandList implements Command {
RUN {