Reflection Basics

Example

The order of getDeclaredFields()/getDeclaredMethods() is not guaranteed, so the example sorts by name.

import java.lang.reflect.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> cls = Class.forName("Customer");
        System.out.println("Class: " + cls.getSimpleName() + ", super: " + cls.getSuperclass().getSimpleName());

        Arrays.stream(cls.getDeclaredFields())
                .sorted(Comparator.comparing(Field::getName))
                .forEach(f -> System.out.println("Field:  " + Modifier.toString(f.getModifiers()) + " "
                        + f.getType().getSimpleName() + " " + f.getName()));
        Arrays.stream(cls.getDeclaredMethods())
                .sorted(Comparator.comparing(Method::getName))
                .forEach(m -> System.out.println("Method: " + m.getName() + " params=" + m.getParameterCount()));

        Object c = cls.getDeclaredConstructor(String.class, int.class).newInstance("Ravi", 1200);
        Method greet = cls.getDeclaredMethod("greet", String.class);
        System.out.println(greet.invoke(c, "Namaste"));

        Field points = cls.getDeclaredField("points");
        points.setAccessible(true);                       // bypass private
        points.setInt(c, 5000);
        System.out.println("After reflection: " + c);
    }
}

class Customer {
    private final String name;
    private int points;
    Customer(String name, int points) { this.name = name; this.points = points; }
    public String greet(String word) { return word + ", " + name + "!"; }
    private boolean isGold() { return points > 1000; }
    public String toString() { return name + " (" + points + " pts)"; }
}

Output

Class: Customer, super: Object
Field:  private final String name
Field:  private int points
Method: greet params=1
Method: isGold params=0
Method: toString params=0
Namaste, Ravi!
After reflection: Ravi (5000 pts)