Metaclasses

• Each object belongs to a class.
• But, Everything is an object.
• So, classes are objects apart from being classes.

String s = "Hello";
Class stringClassObj = s.getClass();

• So the class object (stringClassObj) is an instance of which class ?
• It is an instance of java.lang.Class.
• java.lang.Class extends java.lang.Object since classes are objects.
• So java.lang.Class is a metaclass. Instances of java.lang.Class are classes.
• A class defines the behaviour of its instances. Similarly, a metaclass defines the behavior of the classes that are its instances.
• E.g. java.lang.Class has methods that defined the behavior of its instances.
  • e.g. getName(), getDeclaredFields(), getMethods(),

• e.g. getMethods return objects which are of type Method class. Instances of java.lang.reflect.Method are methods.
• The method class defines behavior of all methods, so it is a meta-method. for e.g. getReturnType(), getModifiers() etc.

• java.lang.Class is not a complete class, since it is declared as final. We cannot add behavior to it or specialize it.

• This whole meta thing is reflection - a program can reflect on itself.
• Useful for tools. e.g. Eclipse. Automatic method drop down possibly reflects on the Class object and gets all its declared methods.