Java in a Nutshell, 5th Edition

Types, Reflection, and Dynamic Loading

The java.lang.Class class represents data types in Java and, along with the classes in the java.lang.reflect package, gives Java programs the capability of introspection (or self-reflection); a Java class can look at itself, or any other class, and determine its superclass, what methods it defines, and so on.

Class Objects

You can obtain a Class object in Java in several ways:

// Obtain the Class of an arbitrary object o
Class c = o.getClass();

// Obtain a Class object for primitive types with various predefined constants
c = Void.TYPE;          // The special "no-return-value" type
c = Byte.TYPE;          // Class object that represents a byte
c = Integer.TYPE;       // Class object that represents an int
c = Double.TYPE;        // etc; see also Short, Character, Long, Float 

// Express a class literal as a type name followed by ".class"
c = int.class;          // Same as Integer.TYPE
c = String.class;       // Same as "dummystring".getClass()
c = byte[].class;       // Type of byte arrays
c = Class[][].class;    // Type of array of arrays of Class objects

Reflecting on a Class

Once you have a Class object, you can perform some interesting reflective operations with it:

import java.lang.reflect.*; Object o; // Some unknown object to investigate Class c = o.getClass(); // Get its type // If it is an array, figure out its base type while (c.isArray()) c = c.getComponentType(); // If c is not a primitive type, print its class hierarchy if (!c.isPrimitive()) { for(Class s = c; s != null; s = s.getSuperclass()) ...