==============
Reflection API
==============

-> Using Reflecting api we can analyze our java classes

-> Reflection api is used to analyze the software


package in.ashokit;

public class Student {

	private int id;
	private String name;

	public void display() {
		System.out.println("hi");
	}

	public void m1() {
		System.out.println("this is m1()");
	}

	public void m2() {
		System.out.println("this is m2()");
	}
}

package in.ashokit;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Demo {

	public static void main(String[] args) throws Exception {

		Class clz = Class.forName("in.ashokit.Student");

		Field[] fields = clz.getDeclaredFields();
		for (Field f : fields) {
			System.out.println(f.getName());
		}

		Method[] methods = clz.getDeclaredMethods();
		for (Method m : methods) {
			System.out.println(m.getName());
		}

		Constructor[] constructors = clz.getDeclaredConstructors();
		for (Constructor c : constructors) {
			System.out.println(c.getName());
		}

	}

}

// Accessing private variable outside of the class using Reflection API


package in.ashokit;

public class Student {

	private int age = 20;

	public void getAge() {
		System.out.println("Age : " + age);
	}

}



package in.ashokit;

import java.lang.reflect.Field;

public class Demo {

	public static void main(String[] args) throws Exception {

		// loading class into jvm
		Class clz = Class.forName("in.ashokit.Student");

		// creating object for the loaded class
		Object obj = clz.newInstance();

		// getting the filed whose name is age
		Field field = clz.getDeclaredField("age");

		// making variable accessible outside of the class
		field.setAccessible(true);

		// set value to field
		field.set(obj, 35);

		Student s = (Student) obj;
		s.getAge();

	}

}






/// Invoking the method using Reflection API

package in.ashokit;

import java.lang.reflect.Method;

public class Demo {

	public static void main(String[] args) throws Exception {

		Class<?> clz = Class.forName("in.ashokit.Student");

		Method method = clz.getDeclaredMethod("getAge");

		Object obj = clz.newInstance();

		method.invoke(obj, null);
	}
}


Q) What is the difference between ClassNotFoundException and NoClassDefFoundError ?

-> If we give wrong class name in Class.forName (" ") then we will get ClassNotFoundException

-> NoClassDefFoundError will occur if jvm not able to find .class file of particular class to load



==================
Garbage Collection
==================

-> Garbage Collection is the process of removing un-used / un-referenced objects from JVMs heap area.

-> Garbage Collection is  an in-built process in JVM

-> In JVM, garbage collector available to perform Garbage Collection.

-> Garbage Collector is a daemon thread (runs in background)


package in.ashokit;

public class Student {

	public static void main(String[] args) {

		// Object created
		Student s1 = new Student();

		// nullifying (making obj eligible for GC)
		s1 = null;

		System.gc();

	}

	protected void finalize() throws Throwable {
		System.out.println("finalize( ) called...");
	}

}


=> When we assign s1=null then s1 become un-referenced object and it is elgible for GC

=> When GC identify un-referenced obj then it will call finalize ( ) method and it will remove that object from HEAP area (memory clean up)

========================
How to invoke GC in java?
========================

-> In java we have 2 ways to invoke GC

		System.gc();

		Runtime.getRuntime().gc();

Note:  Even if we call above methods there is no gaurantee that JVM will start GC immediatley.
GC execution process will be managed by JVM only.


===============================
How GC works internally in JVM ?
===============================

-> Garbage Collection works in below phases

1) Stop the world
2) Marking
3) Sweeping
4) Compaction

-> When GC starts it will trigger STOP THE WORLD (all running threads will be stopped for few milli secs)

-> GC will go to JVM Heap area and it will identify un-referenced objects and it will mark them for sweep. 

-> GC will sweep marked objects

-> After sweeping completed memory holes will be created in Heap area to clear that GC will perform Compaction (it will adjust memory holes).


-> After Compaction, GC will give signal to JVM to continue the execution.


Note: GC process will slow down our program execution hence Sun Microsystem didn't give the chance for programmers to perform Garbage Collection. It will be managed by JVM.


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Generics
Inner Classes
Enums
Class Loaders