Reflection in Java

ReflectionExample.java

import java.lang.reflect.Method;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.lang.reflect.Constructor;

class ReflectionExample {
	public static void main(String args[]) throws Exception {
		// Examining & constructing object for Test class at runtime
		Class<?> clazz = Class.forName("Test");
		Test test = (Test) clazz.newInstance();

		// Examininig and changing accessibilty of Test class's fields NUM_1 and assigning value at runtime
		System.out.println("Making field NUM_1 accessible and assigning value 100");
		Field num1Field = test.getClass().getDeclaredField("NUM_1");
		num1Field.setAccessible(true);
		num1Field.set(test, 100);

		// Examining and changing accessibility of Test class's fields NUM_2 and assigning value at runtime
		System.out.println("Making field NUM_2 accessible and assigning value 200");
		Field num2Field = test.getClass().getDeclaredField("NUM_2");
		num2Field.setAccessible(true);
		num2Field.set(test, 200);

		// Changing accessibilty & invoking sum() of test object at runtime
		System.out.println("Making method sum() acccessible and invoking it");
		Method sumMethod = test.getClass().getDeclaredMethod("sum", int.class, int.class);
		sumMethod.setAccessible(true);
		int sum = (Integer) sumMethod.invoke(test, num1Field.get(test), num2Field.get(test));

		// Changing accessibilty & invoking printMessage() of test object at runtime
		System.out.println("Making method printMessage() acccessible and invoking it");
		Method printMessageMethod = test.getClass().getDeclaredMethod("printMessage", int.class);
		printMessageMethod.setAccessible(true);
		printMessageMethod.invoke(test, sum);
	}
}

Test.java

class Test {
	private final int NUM_1 = 10;
	private final int NUM_2 = 20;

	private void printMessage(int sum) {
		System.out.println(String.format("Sum is %d", sum));
	}

	private int sum(int num1, int num2) {
		return num1 + num2;
	}
}