libs6Reflection

1package pkg13libs;2 3import java.lang.reflect.Constructor;4import java.lang.reflect.Field;5import java.lang.reflect.Method;6import java.util.Arrays;7 8/*9 * libs6Reflection.java10 * --------------------11 * Reflection: inspecting and invoking classes/methods/fields at runtime.12 *13 * DEFINITION:14 *   Reflection (java.lang.reflect) lets code examine and manipulate types it did15 *   not know about at compile time. It powers frameworks (Spring, JUnit, Jackson,16 *   ORMs) that wire up your classes generically.17 *18 * KEY POINTS:19 *   - Class<?> is the entry point: getDeclaredFields/Methods/Constructors.20 *   - setAccessible(true) bypasses access checks (use sparingly).21 *   - Invoke methods/build objects dynamically with invoke()/newInstance().22 *   - Trade-offs: slower, breaks encapsulation, no compile-time safety.23 */24public class libs6Reflection {25 26    static class Person {27        private String name;28        private int age;29        public Person() {}30        public Person(String name, int age) { this.name = name; this.age = age; }31        public String greet() { return "Hi, I'm " + name + " (" + age + ")"; }32        @Override public String toString() { return "Person{" + name + ", " + age + "}"; }33    }34 35    public static void main(String[] args) throws Exception {36        Class<?> clazz = Person.class;37        System.out.println("Class       : " + clazz.getName());38        System.out.println("Simple name : " + clazz.getSimpleName());39 40        // Inspect fields41        System.out.println("\nFields:");42        for (Field f : clazz.getDeclaredFields())43            System.out.printf("  %s %s%n", f.getType().getSimpleName(), f.getName());44 45        // Inspect methods (declared in this class)46        System.out.println("\nMethods:");47        for (Method m : clazz.getDeclaredMethods())48            System.out.printf("  %s %s(%s)%n", m.getReturnType().getSimpleName(),49                    m.getName(),50                    Arrays.stream(m.getParameterTypes()).map(Class::getSimpleName).reduce((x, y) -> x + ", " + y).orElse(""));51 52        // Build an object dynamically53        Constructor<?> ctor = clazz.getConstructor(String.class, int.class);54        Object person = ctor.newInstance("Ada", 36);55        System.out.println("\nCreated     : " + person);56 57        // Invoke a method dynamically58        Method greet = clazz.getMethod("greet");59        System.out.println("Invoked     : " + greet.invoke(person));60 61        // Read/modify a private field (bypassing access control)62        Field nameField = clazz.getDeclaredField("name");63        nameField.setAccessible(true);64        nameField.set(person, "Grace");65        System.out.println("After set   : " + person);66    }67}