Reflection allows inspection and modification of classes, methods, fields, and constructors at runtime — even private members. It powers frameworks like Spring, Hibernate, and JUnit. Getting Class Objects link // From an object String s = "hello"; Class clazz1 = s.getClass(); // From class name Class clazz2 = Class.forName("java.util.ArrayList"); // From literal Class clazz3 = String.class; Class clazz4 = int.class; // primitive Class clazz5 = int[].class; // array Inspecting Class Information link Class clazz = Class.forName("java.util.ArrayList"); System.out.println(clazz.getName()); // java.util.ArrayList System.out.println(clazz.getSimpleName()); // ArrayList System.out.println(clazz.isInterface()); // false System.out.println(clazz.getSuperclass()); // class java.util.AbstractList System.out.println(clazz.getPackageName()); // java.util Class[] interfaces = clazz.getInterfaces(); for (Class iface : interfaces) { System.out.println("Implements: " + iface.getName()); } Fields link public class Person { private String name; public int age; } Class clazz = Person.class; // Public fields only for (Field field : clazz.getFields()) { System.out.println(field.getName()); } // All declared fields (including private) for (Field field : clazz.getDeclaredFields()) { System.out.println(field.getName() + " : " + field.getType()); } // Access private field Person person = new Person(); Field nameField = clazz.getDeclaredField("name"); nameField.setAccessible(true); nameField.set(person, "Alice"); System.out.println(nameField.get(person)); // Alice Methods link Class clazz = String.class; // Public methods (including inherited) for (Method method : clazz.getMethods()) { System.out.println(method.getName()); } // Declared methods only for (Method method : clazz.getDeclaredMethods()) { System.out.println(method.getName() + " → " + method.getReturnType()); } // Invoke method Method lengthMethod = clazz.getMethod("length"); int len = (int) lengthMethod.invoke("hello"); // 5 Method substringMethod = clazz.getMethod("substring", int.class, int.class); String sub = (String) substringMethod.invoke("hello", 1, 4); // ell Constructors link Class clazz = ArrayList.class; Constructor defaultCtor = clazz.getConstructor(); ArrayList list = (ArrayList) defaultCtor.newInstance(); Constructor sizedCtor = clazz.getConstructor(int.class); ArrayList list2 = (ArrayList) sizedCtor.newInstance(100); Creating Objects via Reflection link Class clazz = Class.forName("java.util.HashMap"); Object map = clazz.getDeclaredConstructor().newInstance(); Annotations via Reflection link for (Method method : clazz.getDeclaredMethods()) { if (method.isAnnotationPresent(Deprecated.class)) { System.out.println(method.getName() + " is deprecated"); } } setAccessible and Module System linkSince Java 9, accessing private members across modules requires opens: Field field = clazz.getDeclaredField("secret"); field.setAccessible(true); // may throw InaccessibleObjectException on Java 17+ For Java 17+, add JVM flag for deep reflection: java --add-opens java.base/java.lang=ALL-UNNAMED MyApp Performance Considerations linkReflection is slower than direct calls: Method lookup on every call (cache Method objects) Boxing/unboxing overhead Cannot be JIT-optimized as aggressively // Cache Method objects private static final Method STRING_LENGTH; static { STRING_LENGTH = String.class.getMethod("length"); } Best Practices link Use reflection only when necessary — frameworks, testing, generic utilities Cache Class, Method, and Field objects — do not look up repeatedly Prefer public APIs over breaking encapsulation with setAccessible Be aware of Java module system restrictions in Java 17+ Consider alternatives: interfaces, annotations processors, or code generation

JUnit 5