How do Interfaces define an operation?

1. Operation name: required
2. Any number of arguments is allowed
3. Return data type: required for a return value, but return values themselves are optional
4. Visibility: required before code generation
5. Class operation: optional
6. Argument name: required for each argument, but arguments themselves are optional
7. Argument data type: required for each argument, but arguments themselves are optional
8. Constraints: optional