Java Reflection API

Java Reflection is a process of examining or modifying the run time behavior of a class at run time.

The java.lang.Class class provides many methods that can be used to get metadata, examine and change the run time behavior of a class.

The java.lang and java.lang.reflect packages provide classes for java reflection.

Where it is used

The Reflection API is mainly used in:

IDE (Integrated Development Environment) e.g. Eclipse, MyEclipse, NetBeans etc.
Debugger
Test Tools etc.

Do You Know ?

How many ways we can get the instance of Class class ?
How to create the javap tool ?
How to create the appletviewer tool ?
How to access the private method from outside the class ?

java.lang.Class class

The java.lang.Class class performs mainly two tasks:

provides methods to get the metadata of a class at run time.
provides methods to examine and change the run time behavior of a class.

Getting the object of Class class

There are 3 ways to get the instance of Class class. They are as follows:

forName() method of Class class
getClass() method of Object class
the .class syntax

forName() method of Class class

is used to load the class dynamically.
returns the instance of Class class.
It should be used if you know the fully qualified name of class.This cannot be used for primitive types.

Let's see the simple example of forName() method.

Example

snippet

class Simple{}

class Test{
 public static void main(String args[]){
  Class c=Class.forName("Simple");
  System.out.println(c.getName());
 }
}

Output

Simple

2) getClass() method of Object class

It returns the instance of Class class. It should be used if you know the type. Moreover, it can be used with primitives.

Example

snippet

class Simple{}

class Test{
  void printName(Object obj){
  Class c=obj.getClass();  
  System.out.println(c.getName());
  }
  public static void main(String args[]){
   Simple s=new Simple();
 
   Test t=new Test();
   t.printName(s);
 }
}

Output

Simple

3) The .class syntax

If a type is available but there is no instance then it is possible to obtain a Class by appending ".class" to the name of the type.It can be used for primitive data type also.

Example

snippet

class Test{
  public static void main(String args[]){
   Class c = boolean.class; 
   System.out.println(c.getName());

   Class c2 = Test.class; 
   System.out.println(c2.getName());
 }
}

Output

boolean Test

Determining the class object

Following methods of Class class is used to determine the class object:

1) public boolean isInterface(): determines if the specified Class object represents an interface type.

2) public boolean isArray(): determines if this Class object represents an array class.

3) public boolean isPrimitive(): determines if the specified Class object represents a primitive type.

Let's see the simple example of reflection api to determine the object type.

Example

snippet

class Simple{}
interface My{}

class Test{
 public static void main(String args[]){
  try{
   Class c=Class.forName("Simple");
   System.out.println(c.isInterface());
   
   Class c2=Class.forName("My");
   System.out.println(c2.isInterface());
  
  }catch(Exception e){System.out.println(e);}

 }
}

Output

false true