Java

Annotations do not change action of a compiled program
Annotations are not pure comments as they can change the way a program is treated by compiler
- Annotations can be used by the compiler to detect errors or suppress warnings
- Software tools can process annotation information to generate code, XML files, and so forth
- Some annotations are available to be examined at runtime
Annotations help to associate metadata (information) to the program elements i.e. instance variables, constructors, methods, classes, etc.

Marker Annotations, contain no members and do not consist any data, such as @Override
Single value Annotations, contains only one member and allow a shorthand form of specifying the value of the member, the name of the member must be value
Full Annotations, consist of multiple data members/ name, value, pairs

java.lang.annotation
- @Retention
  - RetentionPolicy.RUNTIME, annotation should be available via reflection at runtime
  - RetentionPolicy.CLASS, annotation is stored in the .class file, but not available at runtime
  - RetentionPolicy.SOURCE, annotation is only available in the source code, and not in the .class files and not a runtime.
- @Documented, when defining an annotation, to ensure that classes using your annotation show this in their generated JavaDoc
- @Target, specify which Java elements your custom annotation can be used to annotate
  - ElementType.ANNOTATION_TYPE
  - ElementType.CONSTRUCTOR
  - ElementType.FIELD
  - ElementType.LOCAL_VARIABLE
  - ElementType.METHOD
  - ElementType.PACKAGE
  - ElementType.PARAMETER
  - ElementType.TYPE
- @Inherited, causes the annotation for a superclass to be inherited by a subclass
java.lang
- @Deprecated, an element has been deprecated
- @Override, override a method from a superclass, if it doesn’t, a compile-time error will result
- @SuppressWarnings, suppress specified compiler warnings
  - deprecation, deprecated element
  - unchecked, unchecked warning is generated when a legacy code interfaces with a code that use generics

//Info.java
import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Documented
@Target({ElementType.METHOD, ElementType. TYPE})
@Inherited 
@interface Info
{
	String Name() default "None";
	String ExpiryDate() default "10.01.1980";
}

@Info(Name="Car", ExpiryDate="10.14.2017")
public class Car
{
	@Info(Name="f1", ExpiryDate="10.01.2017")
	public void f1()
	{
		System.out.println("f1 in Car ...");
	}

	@Info(Name="f2", ExpiryDate="10.14.2017")
	public void f2()
	{
		System.out.println("f2 in Car ...");
	}
}

import java.lang.annotation.*;

public class Buick extends Car
{
	@Override
	public void f2()
	{
		System.out.println("f2 in Buick ...");
	}

	@Deprecated
	public void f3()
	{
		System.out.println("f3 in Buick ...");
	}
}