Java Annotation Interface

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Syntax and Declaration

public @interface StandardAnnotation {
    // Element declarations
}

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Annotation Elements

1. No Parameters: The method declaration must not accept any arguments.
2. No Exceptions: The method must not have a throws clause.
3. No Type Parameters: Generic methods are not permitted.
4. No Cyclic Dependencies: An annotation interface cannot contain an element of its own type, either directly or indirectly.

public @interface ConfigMetadata {
    String author();
    int revision();
}

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Permitted Return Types

• Primitive types (int, boolean, double, etc.)
• java.lang.String
• java.lang.Class (including parameterized forms like Class<?> or Class<? extends Number>)
• enum types
• Other annotation interfaces
• Single-dimensional arrays of any of the types listed above

import java.lang.annotation.ElementType;

public @interface ComplexAnnotation {
    boolean isEnabled();                  // Primitive
    String[] tags();                      // Array of String
    Class<?> targetClass();               // Class type
    ElementType targetEnum();             // Enum type
    StandardAnnotation nestedConfig();    // Nested annotation
}

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Default Values

public @interface Timeout {
    int duration() default 1000;
    String unit() default "MILLISECONDS";
    String[] fallbackMethods() default {}; // Empty array instead of null
}

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The value Element

public @interface SingleValueAnnotation {
    String[] value();
}

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Meta-Annotations

• @Target: Specifies the Java elements (e.g., TYPE, METHOD, FIELD) to which the annotation interface can be applied.
• @Retention: Dictates the lifecycle of the annotation (SOURCE, CLASS, or RUNTIME).
• @Inherited: Indicates that the annotation should be automatically inherited by subclasses of the annotated class.
• @Documented: Instructs Javadoc to include the annotation in the generated documentation.
• @Repeatable: Allows the same annotation interface to be applied multiple times to a single declaration.

import java.lang.annotation.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface ScopedAnnotation {
    String value() default "singleton";
}