Declaring custom annotations

Declaring an annotation type

Annotations are declared with @interface (not interface). Elements look like zero-argument methods with optional default values:

public @interface Review {
    String author();           // required — no default
    String comment() default "";
    int priority() default 1;  // defaults to 1 if omitted
}

Using the annotation:

@Review(author = "alice", comment = "needs tests", priority = 2)
public void processOrder() { /* ... */ }

Element type rules

Annotation element types are restricted to: - Primitives (int, boolean, etc.) - String - Class (e.g. Class<?>) - An enum type - Another annotation type - A 1D array of any of the above

Complex types like List<String> are not allowed.

The value() shorthand

If an annotation has a single element named value (and all other elements have defaults), callers can omit the element name:

public @interface Tag {
    String value();
}

@Tag("important")   // shorthand for @Tag(value = "important")
public void foo() {}

Marker annotations

An annotation with no elements is a marker annotation — its presence alone conveys information:

public @interface ThreadSafe {}   // marker — no elements

@ThreadSafe
public class SafeCounter { /* ... */ }