Elements

Annotation interfaces can contain any number of elements.

@interface Todo {
    String description();
}

These are declared in the same way as methods in an interface, save for some important restrictions.

The method should take no arguments.

@interface Todo {
    // Cannot take arguments
    String description(int x);
}

And the return value needs to be one of a few built-in types like Class, String, int, boolean, etc.¹, an enum, or an array of one of those.

enum Priority {
    HIGH,
    LOW
}

class PersonInCharge {}

@interface Todo {
    // String ✅
    String description();

    // int ✅
    int someNumber();

    // boolean ✅
    boolean isImportant();

    // Class ✅
    Class<?> someRelatedClass();

    // Arrays ✅
    String[] notes();

    // Enums ✅
    Priority priority();

    // Other Classes ❌
    PersonInCharge personInCharge();
}