Tutorial by Examples

The Standard Edition of Java comes with some annotations predefined. You do not need to define them by yourself and you can use them immediately. They allow the compiler to enable some fundamental checking of methods, classes and code. @Override This annotation applies to a method and says that th...

Java's Reflection API allows the programmer to perform various checks and operations on class fields, methods and annotations during runtime. However, in order for an annotation to be at all visible at runtime, the RetentionPolicy must be changed to RUNTIME, as demonstrated in the example below: @i...

Annotation types are defined with @interface. Parameters are defined similar to methods of a regular interface. @interface MyAnnotation { String param1(); boolean param2(); int[] param3(); // array parameter } Default values @interface MyAnnotation { String param1() defau...

You can fetch the current properties of the Annotation by using Reflection to fetch the Method or Field or Class which has an Annotation applied to it, and then fetching the desired properties. @Retention(RetentionPolicy.RUNTIME) @interface MyAnnotation { String key() default "foo";...

Until Java 8, two instances of the same annotation could not be applied to a single element. The standard workaround was to use a container annotation holding an array of some other annotation: // Author.java @Retention(RetentionPolicy.RUNTIME) public @interface Author { String value(); } ...

By default class annotations do not apply to types extending them. This can be changed by adding the @Inherited annotation to the annotation definition Example Consider the following 2 Annotations: @Inherited @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface Inher...

This example demonstrates how to do compile time checking of an annotated element. The annotation The @Setter annotation is a marker can be applied to methods. The annotation will be discarded during compilation not be available afterwards. package annotation; import java.lang.annotation.Eleme...

The Java Language Specification describes Annotations as follows: An annotation is a marker which associates information with a program construct, but has no effect at run time. Annotations may appear before types or declarations. It is possible for them to appear in a place where they could a...

When Java annotations were first introduced there was no provision for annotating the target of an instance method or the hidden constructor parameter for an inner classes constructor. This was remedied in Java 8 with addition of receiver parameter declarations; see JLS 8.4.1. The receiver param...

An Annotation parameter can accept multiple values if it is defined as an array. For example the standard annotation @SuppressWarnings is defined like this: public @interface SuppressWarnings { String[] value(); } The value parameter is an array of Strings. You can set multiple values by u...