Java-merketyper

I denne veiledningen vil vi lære om forskjellige typer Java-merknader ved hjelp av eksempler.

Java-merknader er metadata (data om data) for programkildekoden vår. Det er flere forhåndsdefinerte merknader gitt av Java SE. Videre kan vi også lage tilpassede merknader i henhold til våre behov.

Hvis du ikke vet hva merknader er, kan du gå til veiledningen for Java-merknader.

Disse kommentarene kan kategoriseres som:

1. Forhåndsdefinerte kommentarer

  • @Deprecated
  • @Override
  • @SuppressWarnings
  • @SafeVarargs
  • @FunctionalInterface

2. Egendefinerte merknader

3. Meta-merknader

  • @Retention
  • @Documented
  • @Target
  • @Inherited
  • @Repeatable

Forhåndsdefinerte merketyper

1. @Foreldet

Den @Deprecatedannotering er en markør som indikerer annotering elementet (klasse, metode, felt, etc) avvikles og er blitt erstattet av en nyere element.

Dens syntaks er:

 @Deprecated accessModifier returnType deprecatedMethodName() (… )

Når et program bruker elementet som er erklært utdatert, genererer kompilatoren en advarsel.

Vi bruker Javadoc- @deprecatedkoden for å dokumentere det utdaterte elementet.

 /** * @deprecated * why it was deprecated */ @Deprecated accessModifier returnType deprecatedMethodName() (… )

Eksempel 1: @Foreldet annoteringseksempel

 class Main ( /** * @deprecated * This method is deprecated and has been replaced by newMethod() */ @Deprecated public static void deprecatedMethod() ( System.out.println("Deprecated method"); ) public static void main(String args()) ( deprecatedMethod(); ) )

Produksjon

 Utfaset metode

2. @Override

De @Overridekommentar angir at en fremgangsmåte for en underklasse styrer fremgangsmåten i superklassen med den samme metode navn, returtype, og parameterlisten.

Det er ikke obligatorisk å bruke @Overridenår du overstyrer en metode. Imidlertid, hvis vi bruker det, gir kompilatoren en feil hvis noe er galt (for eksempel feil parametertype) mens metoden overstyres.

Eksempel 2: @Override kommentareksempel

 class Animal ( // overridden method public void display()( System.out.println("I am an animal"); ) ) class Dog extends Animal ( // overriding method @Override public void display()( System.out.println("I am a dog"); ) public void printMessage()( display(); ) ) class Main ( public static void main(String() args) ( Dog dog1 = new Dog(); dog1.printMessage(); ) )

Produksjon 

 jeg er en hund

I dette eksemplet, ved å lage et objekt hund1 av hundeklassen, kan vi kalle metoden sin printMessage () som deretter utfører display()uttalelsen.

Siden display()er definert i begge klassene, overstyrer metoden for underklasse Dog metoden til superklassen Animal. Derfor display()kalles underklassen.

3. @SuppressWarnings

Som navnet antyder, @SuppressWarningsinstruerer kommentaren kompilatoren til å undertrykke advarsler som genereres mens programmet kjøres.

Vi kan spesifisere hvilken type advarsler som skal undertrykkes. Advarslene som kan undertrykkes er kompilatorspesifikke, men det er to kategorier advarsler: avvik og ukontrollert .

For å undertrykke en bestemt advarselskategori bruker vi:

 @SuppressWarnings("warningCategory")

For eksempel,

 @SuppressWarnings("deprecated")

For å undertrykke flere advarselskategorier bruker vi:

 @SuppressWarnings(("warningCategory1", "warningCategory2"))

For eksempel,

 @SuppressWarnings(("deprecated", "unchecked"))

Kategori deprecatedinstruerer kompilatoren om å undertrykke advarsler når vi bruker et utdatert element.

Kategori uncheckedinstruerer kompilatoren om å undertrykke advarsler når vi bruker rå typer.

Og udefinerte advarsler ignoreres. For eksempel,

 @SuppressWarnings("someundefinedwarning")

Eksempel 3: @SuppressWarnings kommentareksempel

 class Main ( @Deprecated public static void deprecatedMethod() ( System.out.println("Deprecated method"); ) @SuppressWarnings("deprecated") public static void main(String args()) ( Main depObj = new Main(); depObj. deprecatedMethod(); ) )

Produksjon 

 Utfaset metode

Her deprecatedMethod()er det merket som utfaset og vil gi kompilator advarsler når det brukes. Ved å bruke @SuppressWarnings("deprecated")kommentaren kan vi unngå kompilatoradvarsler.

4. @SafeVarargs

The @SafeVarargs annotation asserts that the annotated method or constructor does not perform unsafe operations on its varargs (variable number of arguments).

We can only use this annotation on methods or constructors that cannot be overridden. This is because the methods that override them might perform unsafe operations.

Before Java 9, we could use this annotation only on final or static methods because they cannot be overridden. We can now use this annotation for private methods as well.

Example 4: @SafeVarargs annotation example

 import java.util.*; class Main ( private void displayList(List… lists) ( for (List list : lists) ( System.out.println(list); ) ) public static void main(String args()) ( Main obj = new Main(); List universityList = Arrays.asList("Tribhuvan University", "Kathmandu University"); obj.displayList(universityList); List programmingLanguages = Arrays.asList("Java", "C"); obj.displayList(universityList, programmingLanguages); ) )

Warnings 

 Type safety: Potential heap pollution via varargs parameter lists Type safety: A generic array of List is created for a varargs parameter

Output

 Note: Main.java uses unchecked or unsafe operations. (Tribhuvan University, Kathmandu University) (Tribhuvan University, Kathmandu University) (Java, C)

Here, List … lists specifies a variable-length argument of type List. This means that the method displayList() can have zero or more arguments.

The above program compiles without errors but gives warnings when @SafeVarargs annotation isn't used.

When we use @SafeVarargs annotation in the above example,

 @SafeVarargs private void displayList(List… lists) (… )

We get the same output but without any warnings. Unchecked warnings are also suppressed when we use this annotation.

5. @FunctionalInterface

Java 8 first introduced this @FunctionalInterface annotation. This annotation indicates that the type declaration on which it is used is a functional interface. A functional interface can have only one abstract method.

Example 5: @FunctionalInterface annotation example

 @FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method )

If we add another abstract method, let's say

 @FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method public void secondMethod(); // this throws compile error )

Now, when we run the program, we will get the following warning:

 Unexpected @FunctionalInterface annotation @FunctionalInterface MyFuncInterface is not a functional interface multiple non-overriding abstract methods found in interface MyFuncInterface

It is not mandatory to use @FunctionalInterface annotation. The compiler will consider any interface that meets the functional interface definition as a functional interface.

We use this annotation to make sure that the functional interface has only one abstract method.

However, it can have any number of default and static methods because they have an implementation.

 @FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method default void secondMethod() (… ) default void thirdMethod() (… ) )

Custom Annotations

It is also possible to create our own custom annotations.

Its syntax is:

 (Access Specifier) @interface ( DataType () (default value); )

Here is what you need to know about custom annotation:

  • Annotations can be created by using @interface followed by the annotation name.
  • The annotation can have elements that look like methods but they do not have an implementation.
  • The default value is optional. The parameters cannot have a null value.
  • The return type of the method can be primitive, enum, string, class name or array of these types.

Example 6: Custom annotation example

 @interface MyCustomAnnotation ( String value() default "default value"; ) class Main ( @MyCustomAnnotation(value = "programiz") public void method1() ( System.out.println("Test method 1"); ) public static void main(String() args) throws Exception ( Main obj = new Main(); obj.method1(); ) )

Output 

 Test method 1

Meta Annotations

Meta-annotations are annotations that are applied to other annotations.

1. @Retention

The @Retention annotation specifies the level up to which the annotation will be available.

Its syntax is:

 @Retention(RetentionPolicy)

There are 3 types of retention policies:

  • RetentionPolicy.SOURCE - The annotation is available only at the source level and is ignored by the compiler.
  • RetentionPolicy.CLASS - The annotation is available to the compiler at compile-time, but is ignored by the Java Virtual Machine (JVM).
  • RetentionPolicy.RUNTIME - The annotation is available to the JVM.

For example,

 @Retention(RetentionPolicy.RUNTIME) public @interface MyCustomAnnotation(… )

2. @Documented

By default, custom annotations are not included in the official Java documentation. To include our annotation in the Javadoc documentation, we use the @Documented annotation.

For example,

 @Documented public @interface MyCustomAnnotation(… )

3. @Target

We can restrict an annotation to be applied to specific targets using the @Target annotation.

Its syntax is:

 @Target(ElementType)

The ElementType can have one of the following types:

Element Type Target
ElementType.ANNOTATION_TYPE Annotation type
ElementType.CONSTRUCTOR Constructors
ElementType.FIELD Fields
ElementType.LOCAL_VARIABLE Local variables
ElementType.METHOD Methods
ElementType.PACKAGE Package
ElementType.PARAMETER Parameter
ElementType.TYPE Any element of class

For example,

 @Target(ElementType.METHOD) public @interface MyCustomAnnotation(… )

In this example, we have restricted the use of this annotation to methods only.

Note: If the target type is not defined, the annotation can be used for any element.

4. @Inherited

By default, an annotation type cannot be inherited from a superclass. However, if we need to inherit an annotation from a superclass to a subclass, we use the @Inherited annotation.

Its syntax is:

 @Inherited

For example,

 @Inherited public @interface MyCustomAnnotation (… ) @MyCustomAnnotation public class ParentClass(… ) public class ChildClass extends ParentClass (… )

5. @Repeatable

An annotation that has been marked by @Repeatable can be applied multiple times to the same declaration.

 @Repeatable(Universities.class) public @interface University ( String name(); )

Verdien som er definert i @Repeatablekommentaren er container-merknaden. Container-kommentaren har en variabel verdi av array-typen til den repeterbare kommentaren ovenfor. Her Universitieser den inneholder merknadstypen.

 public @interface Universities ( University() value(); )

Nå kan @Universitymerknaden brukes flere ganger på samme erklæring.

 @University(name = "TU") @University(name = "KU") private String uniName;

Hvis vi trenger å hente merknadsdataene, kan vi bruke Reflection API.

For å hente merknadsverdier bruker vi getAnnotationsByType()eller getAnnotations()metoden definert i Reflection API.

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