Java Object Oriented Design - Java Restrictions on Annotation Types

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Restrictions on Annotation Types

An annotation type cannot inherit from another annotation type.

Every annotation type implicitly inherits the java.lang.annotation.Annotation interface, which is declared as follows:

package  java.lang.annotation;

public interface  Annotation  { 
   boolean equals(Object  obj); 
   int  hashCode();
   String toString();
   Class<? extends  Annotation> annotationType();
}

Method declarations in an annotation type cannot specify any parameters.

Method declarations in an annotation type cannot have a throws clause.

The return type of a method declared in an annotation type must be one of the following types:

• Any primitive type: byte, short, int, long, float, double, boolean, and char
• java.lang.String
• java.lang.Class
• An enum type
• An annotation type
• An array of any of the above mentioned type, for example, String[], int[], etc.

The return type cannot be a nested array. For example, you cannot have a return type of String[][] or int[][].

You can declare the annotation method as shown:

public  @interface MyAnnotation {
    Class element1();  // Any Class type
    Class<Test> element2();  // Only  Test   class  type
    Class<? extends  Test>  element3(); // Test   or  its  subclass type
}

An annotation type cannot be generic.

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