Java 中的构造函数 getAnnotation()方法，示例
原文:https://www . geeksforgeeks . org/constructor-getannotation-method-in-Java-with-examples/
构造函数类的 getAnnotation() 方法用于获取指定类型的构造函数对象注释，如果存在这样的注释，则为空。指定的类型作为参数传递。
语法:
public <T extends Annotation> T
getAnnotation(Class<T> annotationClass)
参数:该方法接受单个参数标注类，表示标注类型对应的类对象。
返回值:该方法返回指定标注类型的该元素的标注(如果存在于该元素中，否则为空)。
异常:如果给定的注释类为空，该方法抛出空指针异常。
下面的程序说明了 getAnnotation()方法: 程序 1:
// 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();
}
输出:
Path: Demo/usr
程序 2:
// 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();
}
输出:
Annotation : @Calculator(add=Adding value, subtract=Subtracting Value)
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