注释元素是 Java 中的一个 notationPresent()方法,示例

原文:https://www . geeksforgeeks . org/annotatedelement-is notationpresent-method-in-Java-with-examples/

Java . lang . annotatedeleement 类is notationpresent()方法用于检查实现此接口的类中是否存在指定注释类型的注释。方法返回一个表示相同内容的布尔值。

语法:

public boolean isAnnotationPresent(Class<T> annotationClass)

参数:该方法接受一个参数标注类,即要获取的标注类型。

返回值:该方法返回布尔值表示相同。

异常:此方法抛出:

  • NullPointRexception:如果给定的注释类为空。

下面的程序演示了 isAnnotationPresent()方法。

例 1:

// Java program to demonstrate
// isAnnotationPresent() method

import java.util.*;
import java.lang.reflect.*;
import java.lang.annotation.*;

// create a custom Annotation
@Retention(RetentionPolicy.RUNTIME)
@interface Annotation {

    // This annotation has two attributes.
    public String key();

    public String value();
}

// call Annotation for method
// and pass values for annotation
@Annotation(key = "GFG", value = "GeeksForGeeks")
public class Test {

    public static void main(String[] args)
        throws ClassNotFoundException
    {

        // returns the Class object
        // for this myAnnotatedElement
        AnnotatedElement myAnnotatedElement
            = Test.class;

        System.out.println(
            "AnnotatedElement represented"
            + " by myAnnotatedElement: "
            + myAnnotatedElement.toString());

        // Check if there is any annotation
        // using isAnnotationPresent() method
        System.out.println(
            "If there is any annotation"
            + " in myAnnotatedElement: "
            + myAnnotatedElement
                  .isAnnotationPresent(
                      Annotation.class));
    }
}

Output:

用 my annotationdelete 表示的注解:类测试 如果 my annotationdelete 中有注解:true

例 2:

// Java program to demonstrate
// isAnnotationPresent() method

import java.util.*;
import java.lang.reflect.*;
import java.lang.annotation.*;

// Class with no annotations
public class Test {

    public static void main(String[] args)
        throws ClassNotFoundException
    {

        // returns the Class object
        // for this myAnnotatedElement
        AnnotatedElement myAnnotatedElement
            = Test.class;

        System.out.println(
            "AnnotatedElement represented"
            + " by myAnnotatedElement: "
            + myAnnotatedElement.toString());

        // Get the annotation
        // using isAnnotationPresent() method
        System.out.println(
            "If there is any annotation"
            + " in myAnnotatedElement: "
            + myAnnotatedElement
                  .isAnnotationPresent(
                      Annotation.class));
    }
}

Output:

用 my annotationdelete 表示的注解:类测试 如果 my annotationdelete 中有注解:false

参考:https://docs . Oracle . com/javase/9/docs/API/Java/lang/reflect/annotatedelement . html # is notationpresent-Java . lang . class-