<aside> 💡 Reflective programming or Reflection is the ability of a process to examine, introspect, and modify its structure and behaviour.

</aside>

• Two Components:
  • Introspection
    • A program can observe, and thus reason about its own state.
  • Intercession
    • A program can modify its execution state or alter its own interpretation or meaning.

• Given a code below:

  Employee e = new Manager(...);
  ...
  
  if (e instanceof Manager){
  	...
  }
  

• Problem: What if we don’t know the type that we want to check in advance?

  • Like: If we want to write a function that check whether two objects give as arguments are both instances of the same class…

• Solution: Use getClass() to extract the class of an object.

  • Import package: java.lang.reflect

  import java.lang.reflect.*;
  
  class MyReflectionClass{
  	...
  	public static boolean(Object o1, Object o2){
  		return (o1.getClass() == o2.getClass())
  	}
  }
  

• getClass() method

  • Returns an object of type Class that encodes class information.

The class Class

• Here is a code that explicitly uses the type Class to compare two classes.

  import java.lang.reflect.*;
  
  class MyReflectionClass{
  	...
  	public static boolean classequal(Object o1, Object o2){
  		Class c1, c2;
  		c1 = o1.getClass();
  		c2 = o2.getClass();
  		return (o1==o2);
  	}
  }
  

• For each currently loaded class C ,Java creates an object of type Class with information about C

<aside> 💡 Encoding execution state as data is called Reification.

</aside>

Representing an abstract idea in a concrete form.

Using the Class object

• Can create new instances of a class at runtime.

  ....
  Class c = obj.getClass();
  Object o = c.newInstance(); 
  //This will create an object o of same type as that of obj.
  ....
  

  ---

• Can get hold of the class object using the name of the class.

  ...
  String s = "Manager";
  Class c = Class.forName(s);
  Object o =c.newInstance();
  ...
  

  • Write more compactly as:

    Object o = Class.forName("Manager").newInstance()
    

• From the Class object for class C , we can extract details about constructors, methods and fields of C

  • Structure of these entities:

    • Constructors: arguments
    • Methods: arguments and return type
    • All three: modifiers static, private ,etc.

  • Additional classes Constructor, Method, Field

  • Use getConstructors(), getMethods(), getFields() to obtain constructors , methods , and fields of C in an array.

  • Extracting information about constructors , methods, fields.

    ...
    Class c = obj.getClass();
    Constructor[] constructors = c.getConstructors();
    Method[] methods = c.getMethods();
    Fields[] fields = c.getFields();
    ...
    

  • Now, Constructors, Method, Field in turn have functions to get further details.