Java reflection

For example, imagine you have a class in Java, and you want to find out what methods it has or what fields it contains at runtime. Reflection enables you to do this. However, using reflection extensively can have performance implications because it involves dynamic discovery and manipulation of the program’s structure, and it might not be as efficient as direct, statically-typed code.

Example §

public class Person {
    private String name;
    private int age;
 
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
 
    public void sayHello() {
        System.out.println("Hello, my name is " + name + " and I am " + age + " years old.");
    }
}

Now, let’s use reflection to inspect and interact with the Person class at runtime:

import java.lang.reflect.Field;
import java.lang.reflect.Method;
 
public class ReflectionExample {
    public static void main(String[] args) throws Exception {
        // Create an instance of the Person class
        Person person = new Person("John Doe", 25);
 
        // Use reflection to get the class of the object
        Class<?> personClass = person.getClass();
        System.out.println("Class Name: " + personClass.getName());
 
        // Get and print the fields of the class
        Field[] fields = personClass.getDeclaredFields();
        System.out.println("\nFields:");
        for (Field field : fields) {
            System.out.println("Field Name: " + field.getName() + ", Type: " + field.getType());
        }
 
        // Get and print the methods of the class
        Method[] methods = personClass.getDeclaredMethods();
        System.out.println("\nMethods:");
        for (Method method : methods) {
            System.out.println("Method Name: " + method.getName() + ", Return Type: " + method.getReturnType());
        }
 
        // Use reflection to set the private field 'name'
        Field nameField = personClass.getDeclaredField("name");
        nameField.setAccessible(true); // Allow access to private field
        nameField.set(person, "Jane Doe"); // Set the value
 
        // Use reflection to invoke the 'sayHello' method
        Method sayHelloMethod = personClass.getDeclaredMethod("sayHello");
        sayHelloMethod.invoke(person); // Invoke the method
    }
}
 

Explanation:

1. We create an instance of the Person class.
2. We use reflection to get the class of the object (Person class).
3. We retrieve and print information about the fields and methods of the class using reflection.
4. We use reflection to set the private field name to a new value (“Jane Doe”).
5. We use reflection to invoke the sayHello method, which prints a message using the updated name.

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Related §

Reflection Python reflection

Refs §