Use "interface" as Annotation

Java Tutorials - Herong's Tutorial Examples

∟Use "interface" as Annotation

This section provides a tutorial example on how to use 'interface' type to do the same job, roughly, as 'annotation' type.

Based on the Java Language Specifications, an annotation type is actually an interface type.

So I think the Java compiler is actually:

Converting "@interface" (annotation type) declaration statements into "interface" declaration statements.
Converting annotation type element declarations into interface method declarations.
Converting annotation invocation statements into "class" declaration statements to implement the declared interface type.
Converting annotated information (annotation element-value pairs) into methods with return statements overriding those in the interface type.

Let's look at the following sample program that uses the annotation type:

/* HeaderAnnotation.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class HeaderAnnotation {
   // Annotation declaration
   @interface Header {
      String usage();    // Usage of the targeted construct
      int version();     // Version number of targeted construct
   }

   // Annotation invocation
   @Header(
      usage="The execution entry point.",
      version=10
   )
   public static void main(String[] arg) {
      System.out.println("Hello world!");
   }
}

We can rewrite the above program using interface type:

/* HeaderInterface.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class HeaderInterface {
   // Annotation declaration
   interface Header {
      public String usage();    // Usage of the targeted construct
      public int version();     // Version number of targeted construct
   }

   // Annotation invocation
   class Header1 implements Header {
      public String usage() { return "The execution entry point."; }
      public int version() { return 10; }
   }
   public static void main(String[] arg) {
      System.out.println("Hello world!");
   }
}

If you compile and run both programs, you will get:

herong> java HeaderAnnotation.java
Hello world!

herong> java HeaderInterface.java
Hello world!

As you can see, we managed to do the same job, roughly, using interface type instead of annotation type.

Table of Contents

About This Book

JDK - Java Development Kit

Execution Process, Entry Point, Input and Output

Primitive Data Types and Literals

Control Flow Statements

Bits, Bytes, Bitwise and Shift Operations

Managing Bit Strings in Byte Arrays

Reference Data Types and Variables

Enum Types and Enum Constants

StringBuffer - The String Buffer Class

System Properties and Runtime Object Methods

Generic Classes and Parameterized Types

Generic Methods and Type Inference

Lambda Expressions and Method References

Java Modules - Java Package Aggregation

Execution Threads and Multi-Threading Java Programs

ThreadGroup Class and "system" ThreadGroup Tree

Synchronization Technique and Synchronized Code Blocks

Deadlock Condition Example Programs

Garbage Collection and the gc() Method

Assert Statements and -ea" Option

►Annotation Statements and Declarations

What Is Annotation

►Use "interface" as Annotation

Default Values for Annotation Elements

Single-Element Annotation Invocation

No-Element (Marker) Annotation Invocation

getAnnotations() Method - Annotation APIs

Predefined Annotation Types

Java Related Terminologies

Archived Tutorials

References

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