Typy anotácií Java

V tomto tutoriáli sa pomocou príkladov dozvieme o rôznych typoch anotácií Java.

Anotácie jazyka Java sú metadáta (údaje o údajoch) pre náš zdrojový kód programu. Java SE poskytuje niekoľko preddefinovaných anotácií. Okrem toho môžeme tiež vytvárať vlastné anotácie podľa našich potrieb.

Ak neviete, čo sú anotácie, navštívte výukový program anotácií Java.

Tieto anotácie možno kategorizovať ako:

1. Preddefinované anotácie

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

2. Vlastné anotácie

3. Meta-anotácie

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

Preddefinované typy anotácií

1. @ zastaraný

@DeprecatedAnotácia je anotácie značka, ktorá označuje prvok (triedy, metódy, polia, atď) je zastaraný a bol nahradený novšou prvkom.

Jeho syntax je:

 @Deprecated accessModifier returnType deprecatedMethodName() (… )

Keď program používa prvok, ktorý bol vyhlásený za zastaraný, kompilátor vygeneruje varovanie.

Na @deprecateddokumentáciu zastaraného prvku používame značku Javadoc .

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

Príklad 1: Príklad zastaranej anotácie

 class Main ( /** * @deprecated * This method is deprecated and has been replaced by newMethod() */ @Deprecated public static void deprecatedMethod() ( System.out.println("Deprecated method"); ) public static void main(String args()) (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

Výkon

 Zastaraná metóda

2. @ Override

Tieto @Overrideanotácie určuje, že metóda podtriedy potlačia metódu rodičovskej triedy s rovnakým názvom metódy, návratový typ a zoznam parametrov.

@OverridePri prepísaní metódy nie je povinné použitie . Ak to však použijeme, kompilátor pri prepísaní metódy zobrazí chybu, ak je niečo zlé (napríklad nesprávny typ parametra).

Príklad 2: @Override príklad anotácie

 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(); ) )

Výkon 

 Ja som pes

V tomto príklade vytvorením objektu dog1 triedy Dog môžeme nazvať jeho metódu printMessage (), ktorá potom vykoná display()príkaz.

Pretože display()je definovaná v oboch triedach, metóda podtriedy Pes má prednosť pred metódou nadtriedy Zviera. Preto display()sa nazýva podtrieda.

3. @SuppressWarnings

Ako naznačuje názov, @SuppressWarningsanotácia dáva pokyn kompilátoru, aby potlačil varovania, ktoré sa generujú počas vykonávania programu.

Môžeme určiť typ upozornení, ktoré sa majú potlačiť. Varovania, ktoré je možné potlačiť, sú špecifické pre kompilátor, existujú však dve kategórie varovaní: zastaranie a nezačiarknuté .

Na potlačenie konkrétnej kategórie varovania používame:

 @SuppressWarnings("warningCategory")

Napríklad,

 @SuppressWarnings("deprecated")

Na potlačenie viacerých kategórií varovaní používame:

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

Napríklad,

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

Kategória deprecateddáva pokyn kompilátoru, aby potlačil varovania, keď použijeme zastaraný prvok.

Kategória uncheckeddáva pokyn kompilátoru, aby potlačil varovania, keď používame nespracované typy.

Nedefinované varovania sa ignorujú. Napríklad,

 @SuppressWarnings("someundefinedwarning")

Príklad 3: Príklad anotácie @SuppressWarnings

 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(); ) )

Výkon 

 Zastaraná metóda

Tu deprecatedMethod()bol označený ako zastaraný a pri jeho použití sa zobrazí upozornenie kompilátora. Použitím @SuppressWarnings("deprecated")anotácie môžeme zabrániť varovaniam kompilátora.

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(); )

Hodnota definovaná v @Repeatableanotácii je anotácia kontajnera. Anotácia kontajnera má premennú hodnotu typu poľa vyššie uvedenej opakovateľnej anotácie. Tu Universitiesje uvedený typ anotácie.

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

Teraz je možné @Universityanotáciu použiť viackrát na jednu deklaráciu.

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

Ak potrebujeme načítať anotačné údaje, môžeme použiť Reflection API.

Na načítanie hodnôt anotácií používame getAnnotationsByType()alebo getAnnotations()metódu definovanú v Reflection API.

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