The annotation type definition looks similar to an interface definition where the keyword interface is preceded by the at sign (@) (@ = AT, as in annotation type). Annotation types are a form of interface, which will be covered in a later lesson. For the moment, you do not need to understand interfaces.
You could use a marker interface, which is more or less the predecessor to annotations when it comes to classes. If you define it to extend the interface you were mentioning, you can be sure that any class marked with it also implements your interface.
Java @interface||Java Annotation ||Java
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The hash code of an annotation is the sum of the hash codes of its members (including those with default values), as defined below: The hash code of an annotation member is (127 times the hash code of the member-name as computed by String.hashCode()) XOR the hash code of the member-value, as defined below: The hash code of a member-value depends on its type: The hash code of a primitive value v is equal to WrapperType.valueOf(v).hashCode(), where WrapperType is the wrapper type corresponding to the primitive type of v (Byte, Character, Double, Float, Integer, Long, Short, or Boolean). The hash code of a string, enum, class, or annotation member-value I v is computed as by calling v.hashCode(). (In the case of annotation member values, this is a recursive definition.) The hash code of an array member-value is computed by calling the appropriate overloading of Arrays.hashCode on the value. (There is one overloading for each primitive type, and one for object reference types.) Overrides:hashCode in class ObjectReturns:the hash code of this annotationSee Also:Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)
toStringString toString()Returns a string representation of this annotation. The details of the representation are implementation-dependent, but the following may be regarded as typical: @com.acme.util.Name(first=Alfred, middle=E., last=Neuman) Overrides:toString in class ObjectReturns:a string representation of this annotationannotationTypeClass
A method is marked with the @Override annotationwhenever it is legal. This includes a class method overriding a superclass method, a class methodimplementing an interface method, and an interface method respecifying a superinterfacemethod.
The only purpose is to mark a declaration. These annotations contain no members and do not consist of any data. Thus, its presence as an annotation is sufficient. Since the marker interface contains no members, simply determining whether it is present or absent is sufficient. @Override is an example of Marker Annotation.
Category 5: Repeating Annotations These are the annotations that can be applied to a single item more than once. For an annotation to be repeatable it must be annotated with the @Repeatable annotation, which is defined in the java.lang.annotation package. Its value field specifies the container type for the repeatable annotation. The container is specified as an annotation whose value field is an array of the repeatable annotation type. Hence, to create a repeatable annotation, firstly the container annotation is created, and then the annotation type is specified as an argument to the @Repeatable annotation.
@FunctionalInterface @FunctionalInterface annotation, introduced in Java SE 8, indicates that the type declaration is intended to be a functional interface, as defined by the Java Language Specification.
Java defines a set of annotations that are built into the language. Of the seven standard annotations, three are part of java.lang, and the remaining four are imported from java.lang.annotation.[5][6]
In addition to processing an annotation using an annotation processor, a Java programmer can write his own code that uses reflections to process the annotation. Java SE 5 supports a new interface that is defined in the java.lang.reflect package. This package contains the interface called AnnotatedElement that is implemented by the Java reflection classes including Class, Constructor, Field, Method, and Package. The implementations of this interface are used to represent an annotated element of the program currently running in the Java Virtual Machine. This interface allows annotations to be read reflectively.
The AnnotatedElement interface provides access to annotations having RUNTIME retention. This access is provided by the getAnnotation, getAnnotations, and isAnnotationPresent methods. Because annotation types are compiled and stored in byte code files just like classes, the annotations returned by these methods can be queried just like any regular Java object. A complete example of processing an annotation is provided below:
This annotation can be used on an interface to declare a set of activity types. Each method in an interface annotated with this annotation represents an activity type. An interface can't have both @Workflow and @Activities annotations-
Specifies the registration options of an activity type. This annotation can be used on an interface annotated with @Activities or the methods within. If specified in both places, then the annotation used on the method takes effect.
When used on a method in an interface annotated with the @Workflow annotation, identifies that the method is used to retrieve the latest workflow execution state. There can be at most one method with this annotation in an interface with the @Workflow annotation. Methods with this annotation must not take any parameters and must have a return type other than void.
When used on a method in an interface annotated with the @Workflow annotation, identifies a signal that can be received by executions of the workflow type declared by the interface. Use of this annotation is required to define a signal method.
When used on an interface annotated with the @Workflow annotation, indicates that the workflow type should not be registered with Amazon SWF. One of @WorkflowRegistrationOptions and @SkipRegistrationOptions annotations must be used on an interface annotated with @Workflow, but not both.
This annotation is used on an interface to declare a workflow type. An interface decorated with this annotation should contain exactly one method that is decorated with the @Execute annotation to declare an entry point for your workflow.
Java Annotation is a tag that represents the metadata i.e. attached with class, interface, methods or fields to indicate some additional information which can be used by java compiler and JVM.
The java.lang.annotation.ElementType enum declares many constants to specify the type of element where annotation is to be applied such as TYPE, METHOD, FIELD etc. Let's see the constants of ElementType enum:
In real scenario, java programmer only need to apply annotation. He/She doesn't need to create and access annotation. Creating and Accessing annotation is performed by the implementation provider. On behalf of the annotation, java compiler or JVM performs some additional operations.
In similar way, we can create generic interfaces in java. We can also have multiple type parameters as in Map interface. Again we can provide parameterized value to a parameterized type also, for example new HashMap(); is valid.
Java Generic Type Naming convention helps us understanding code easily and having a naming convention is one of the best practices of Java programming language. So generics also comes with its own naming conventions. Usually, type parameter names are single, uppercase letters to make it easily distinguishable from java variables. The most commonly used type parameter names are:
Notice the isEqual method signature showing syntax to use generics type in methods. Also, notice how to use these methods in our java program. We can specify type while calling these methods or we can invoke them like a normal method. Java compiler is smart enough to determine the type of variable to be used, this facility is called type inference.
Suppose we want to add Integers to a list of integers in a method, we can keep the argument type as List but it will be tied up with Integers whereas List and List can also hold integers, so we can use a lower bound wildcard to achieve this. We use generics wildcard (?) with super keyword and lower bound class to achieve this. We can pass lower bound or any supertype of lower bound as an argument, in this case, java compiler allows to add lower bound object types to the list.
Generics in Java was added to provide type-checking at compile time and it has no use at run time, so java compiler uses type erasure feature to remove all the generics type checking code in byte code and insert type-casting if necessary. Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead. For example, if we have a generic class like below;
The @WorkflowMethod annotation has a name parameter, for example: @WorkflowMethod(name = "YourWorkflowType").It can be used to denote the Workflow type. If not set, the Workflow type defaults to the short name of the Workflow interface,in the example above being FileProcessingWorkflow.Methods annotated with @WorkflowMethod can have any number of parameters.We recommend passing a single parameter that contains all the input fields.This allows adding fields in a backward compatible manner.
Note that this approach does not apply to @WorkflowMethod annotations, meaning that when using a base interface, it shouldnot include any @WorkflowMethod methods.To illustrate this, lets' say that we define the following invalid code:
An Abstract value type is a manually-written non-final (usually abstract) class orinterface (or even annotation type) that defines the value type and is annotated with theorg.immutables.value.Value.Immutable annotation.It may contain attributes and other metadata, as well as regular Java methods (and fields, if necessary).It is strongly recommended that abstract value types not introduce visible mutable state.Abstract value types are used as the source model for generated code. Get started!. 2ff7e9595c
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