Garbage Collector In Java

Garbage Collector:

 In c,c++ programing language programmer show very much interest to allocate the memory but

it most of time it does not deallocate the memory due to this negligibility the chance of failing application is more

To overcome this problem:

 There is one component running in backround to destoy useless object due this componenet chances  of failing application is less due to memory promblem.

This component of JVM is nothing but Garbage Collector(GC)

hence main purpose of GC to destoy useless objects

The ways to make object Eligible for garbage collector:

Reassigning reference variable:

Student s=new Student();

Student s1=new Student();

//some line of code here

s=new Student();

//some line of code here

s=s1;

so s is eligible for garbage collector

Nullifing reference variable:

Student s=new Student();

//some line of code  here

s=null;

so s is eligible for garbage collector


once we made object eligible for GC this object may not destroy by GC immediately
whevenever JVM run GC then these object will be destroy and we cannot except exctaly when
jvm run  GC it is JVM vendor dependent.
// this question asked in interview to one of my friend

Method for Requesting JVM to Run GC:

By using:

System.gc();


By Using:

Runtime r=Runtime.getRuntime();

this Runtime class has 3 method:

long freeMemory();
void gc();
long totalMemory();

out of this two 2nd one is best suitable because:
System.gc() is static method and r.gc() is instance method

static method are recommented to used over instance method because here
System.gc() internally called r.gc() method.

Enum in Java

Enum concept introduced in JDK 1.5 version.

Java enum more powerful than other language.

The main purpose of enum is if we want to developed or represent group of constant then we should go for enum.

e.g

      enum Friend

{

       kunal,sachin,jagdish,atish,vaibhav,vishal;

        

}

Internally enum represent as liked this Class concepts

  Every enum by default public static final

  Every constant represent as object of type enum

 

class Friend

{

Public static final Friend sachin=new Friend();

Public static final Friend atish=new Friend();

.

.

.

}

Specialty of java enum:

In old language we represent only constant but in java enum we declare variable,constructor and method also and enum can have main method also we run enum from cmd also

i.e java enum is more powerful than other languages;

//simple.java

enum month                                                                            

{

                jan,feb,march,may;

public static void main(String[] args)

                {

                                System.out.println("Hello World!");

                  month n=month.jan;

       System.out.println(n);

                 

                }

}

o/p:

 

 

Some valid combination java enum:

• ·         It can be inside the class.
• ·         class can be inside  enum.
•      But inside method does not allowed

     

enum is direct child class of java.lang.Enum so we cannot extend any other enum

so that it cannot extend any another enum.

it can implement any number of interface.

One of the most important interview question I faced

                                                                              

Difference between enum Enum and Enumeration..??

enum:-it is used for creating java enum it is keyword

Enum:it is class represent in java.lang.Enum which act as based class for all java enum

Enumaration-it is cursor it used for retrieving element from collection one by one.

 

// EnumDemoExample.java

enum Friend

{

       kunal,sachin,jagdish,atish,vaibhav,vishal;

     int x=10;

       Friend()

  {

         System.out.println("inside the constructor");

  }

}

public class EnumDemoExample {

       public static void main(String[] args) {

              // TODO Auto-generated method stub

             

             

              Friend f=Friend.kunal;

        System.out.println("hellow friend");

        System.out.println(f.x);

       Friend f1[]=Friend.values();

       for(Friend a:f1)

       {

              System.out.println(a+"---"+a.ordinal());

       }

       }

}

O/P-

inside the constructor

inside the constructor

inside the constructor

inside the constructor

inside the constructor

inside the constructor

hellow friend

10

kunal---0

sachin---1

jagdish---2

atish---3

vaibhav---4

vishal---5