Reflection enables runtime examination and dynamic invocation of class members. This technique can be applied to invoke methods defined by an interface through a concrete implementation class.

Core Implementation Steps

The process involves several distinct stages.

Step	Action
1	Define the interface.
2	Create a class that implements the interface.
3	Use reflection to instantiate the implementing class.
4	Dynamically invoke the target method on the instance.

Implementation Details

Step 1: Define the Interface

Create an interface containing the method signature to be invoked.

public interface Processor {
    void execute();
}

Step 2: Create the Implementation Class

Implement the interface in a concrete class.

public class TaskRunner implements Processor {
    @Override
    public void execute() {
        System.out.println("Task is now executing.");
    }
}

Step 3: Instantiate the Class Using Reflection

Use Class.forName() and newInstance() (or getDeclaredConstructor().newInstance() for newer JDKs) to create an object. Cast it to the interface type.

try {
    // Load the class
    Class<?> implClass = Class.forName("TaskRunner");
    // Create an instance and cast to the interface
    Processor service = (Processor) implClass.getDeclaredConstructor().newInstance();
    // Proceed to method invocation
} catch (ClassNotFoundException | InstantiationException | 
         IllegalAccessException | java.lang.reflect.InvocationTargetException | 
         NoSuchMethodException e) {
    e.printStackTrace();
}

Note: For Java 9+, prefer getDeclaredConstructor().newInstance() over the deprecated Class.newInstance().

Step 4: Invoke the Method

Call the interface method on the instantiated object. This can be done directly or via reflection's Method.invoke().

Direct Invocation (Simple):

service.execute(); // Outputs: Task is now executing.

Reflective Invocation (More Flexible):

try {
    // Get the Method object for 'execute'
    java.lang.reflect.Method method = service.getClass().getMethod("execute");
    // Invoke the method on the instance
    method.invoke(service); // Outputs: Task is now executing.
} catch (NoSuchMethodException | IllegalAccessException | java.lang.reflect.InvocationTargetException e) {
    e.printStackTrace();
}

Diagram: Class Relationship

classDiagram
    direction LR
    class Processor {
        <<interface>>
        +execute() void
    }
    class TaskRunner {
        +execute() void
    }
    Processor <|.. TaskRunner : implements

Key Use Cases and Considerations

• Plugin Architectures: Load and execute modules without compile-time dependencies.
• Framework Development: Allow user-defined implementations to be integrated dynamically.
• Testing: Mock or stub implementations can be injected at runtime.
• Performance Overhead: Reflection involves runtime checks and is slower than direct method calls. Cache Method or Constructor objects when used repeatedly.
• Secuirty: Reflection bypasses compile-time access checks. Using it may require setAccessible(true) for private members, which is controlled by the SecurityManager.