Constructor getAnnotation() method in Java with Examples

Last Updated : 27 Nov, 2019

The getAnnotation() method of a Constructor class is used to get this constructor object annotation for the specified type if such an annotation is present, else null. Specified type is passed as parameter. Syntax:

public <T extends Annotation> T
  getAnnotation(Class<T> annotationClass)

Parameters: This method accepts a single parameter annotationClass which represents the Class object corresponding to the annotation type. Return value: This method returns this element's annotation for the specified annotation type if present on this element, else null. Exception: This method throws NullPointerException if the given annotation class is null. Below programs illustrate the getAnnotation() method: Program 1:

Java

// Java program to demonstrate
// Constructor.getAnnotation() method

import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Constructor;

public class GFG {

    public static void main(String... args)
        throws NoSuchMethodException
    {

        // Create Constructor Object
        Constructor[] constructors
            = Demo.class.getConstructors();

        // Create annotation object
        Annotation annotation
            = constructors[0]
                  .getAnnotation(PathVar.class);

        if (annotation instanceof PathVar) {

            PathVar customAnnotation
                = (PathVar)annotation;
            System.out.println(
                "Path: "
                + customAnnotation.Path());
        }
    }
}

// Demo class
class Demo {
    public Demo(@PathVar(Path = "Demo/usr")
                String str) {}
}

// PathVar interface
@Target({ ElementType.TYPE_USE })
@Retention(RetentionPolicy.RUNTIME)
@interface PathVar {
    public String Path();
}

Output:

Path: Demo/usr

Program 2:

Java

// Java program to demonstrate
// Constructor.getAnnotation() method

import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Constructor;

public class GFG {

    public static void main(String... args)
        throws NoSuchMethodException
    {

        // Create Constructor Object
        Constructor[] constructors
            = Maths.class.getConstructors();

        // Create annotation object
        Annotation annotation
            = constructors[0]
                  .getAnnotation(Calculator.class);

        System.out.println(
            "Annotation:"
            + annotation.toString());
    }
}

// Demo class
@Calculator(add = "Adding value",
            subtract = "Subtracting Value")
class Maths {

    @Calculator(add = "Adding value",
                subtract = "Subtracting Value")
    public Maths() {}
}

// Calculator interface
@Retention(RetentionPolicy.RUNTIME)
@interface Calculator {
    public String add();
    public String subtract();
}

Output:

Annotation : @Calculator(add=Adding value, subtract=Subtracting Value)

References: https://docs.oracle.com/javase/10/docs/api/java/lang/reflect/Constructor.html#getAnnotation(java.lang.Class)

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Constructor getAnnotation() method in Java with Examples

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