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Introduction to GARBAGE COLLECTION

                     In ‘C++’ the programmer is responsible for both creation and  destruction of objects but usually the programmer is giving very much importance for creation of objects and he is ignoring the destruction of objects. Due to this at creation point of time there may not be sufficient memory for the creation of new objects and entire program may fails. But in java programmer is responsible only for creation of objects but sun people has introduced one assistance which is running continuously in the background for destruction of objects. Due to this assistance there is no chance of failing the java program due to memory problem, this assistance is nothing but “Garbage Collector”. The ways to make an object eligible for Garbage Collector Even though the programmer is not responsible for  destruction of objects it’s good programming practice to make an object eligible for Garbage Collector if it is no longer required. The following are different ways for this Nullifying the reference Variable
If an object is no longer required assign null to all its reference variables.

Ex:

Student s1 = new Student();
Student s2 = new Student();
s1 = null;
s2 = null;

Reassigning the reference Variable

Ex:

Student s1 = new Student();
Student s2 = new Student();
s1 = s2;



The Objects Created inside a method. The objects which are created in a method are by default eligible for Garbage Collector once the method completes

Ex:
1) class Test
{
public static void main(String arg[])
{
m1();
}
public static void m1()
{
Student s1 = new Student();
Student s1 = new Student();
}
}

2) class Test
{
public static void main(String arg[])
{
Student s = m1();
}
public static Student m1()
{
Student s1 = new Student();
Student s1 = new Student();
return s1;
}
}

3) class Test
{
public static void main(String arg[])
{
m1();
}
public static Student m1()
{
Student s1 = new Student();
 Student s1 = new Student();
return s1;
}
}

The Island of Isolation
Ex:
class Test
{
Test i;
public static void main(String[] args)
{
Test t1 = new Test();
Test t2 = new Test();
Test t3 = new Test();
t1.i = t2;
t2.i = t3;
t3.i = t1;
t1 = null;
t2 = null;
t3 = null;
}
}
Note: 1) If an object doesn’t have any reference variable that object is always eligible for Garbage Collection
2) Even though object having the reference variable still there may be a chance of that object eligible for Garbage Collection (Island of Isolation …Here ‘i’ is internal reference)



The methods to request JVM to run Garbage Collector

We can request JVM to run Garbage Collector but there is no uarantee whether JVM accepts our request or not. We can do this by using the following ways.

By System class

‘System’ class contains a static ‘gc’ method for requesting JVM to run Garbage Collector.
                      System.gc();

By Using Runtime Class

A java application can communicate with JVM by using Runtime Object. We can get Runtime Object as follows.

Runtime r = Runtime.getRuntime();

Once we get Runtime Object we can apply the following methods on that object.

freeMemory(): returns the free memory available in the loop

totalMemory(): returns heap size

gc(): for requesting JVM to run GarbageCollector

Ex:

import java.util.*;
class RuntimeDemo
{
public static void main(String arg[])
{
Runtime r = Runtime.getRuntime();
System.out.println(r.totalMemory()); -> 2031616
System.out.println(r.freeMemory());  ->1870416
for(int i= 0; i<= 10000; i++)
{
Date d = new Date();
d = null;
}
System.out.println(r.freeMemory()); -> 1633032
r.gc();
System.out.println(r.freeMemory()); -> 1923992
}
}

Note:- gc() method available in the system class is static method but gc() method available in Runtime class is an instance method.


Q) Which of the following is the valid way for requesting JVM to run gc?

System.gc(); \/
Runtime.gc(); X -> not a static
(new Runtime()).ge(); X -> we can’t create Object
Runtime.getRuntime().gc(); \/


Finalization

Just before destroying any object Garbage Collector always calls finalize() to perform clean up activities.finalize() is available in the Object class which is declared as follows.

protected void finalize() throws Throwable
{
}

case1: Garbage Collector always calls finalize() on the Object which is eligible for Garbage Collector and the corresponding class finalize method will be executed.


Ex:
class Test
{
public static void main(String arg[])
{
String s = new String("raju");
//Test s = new Test();
s = null;
System.gc();
System.out.println("end of main method");
}
public void finalize()
{
System.out.println("finalize method called");
}
}
O/P:- end of main method.

In this case String Object is eligible for G.C and hence String class finalize() method has been executed. In the above program if we are replacing String Object with Test Object then Test class finalize() will be
executed.

In this case O/P is end of main method
finalize method called
or
finalize method called
end of main method

case2: we can call finalize() explicitly in that case it will execute just like a normal method and object won’t be destroyed.

While executing finalize() method if any exception is uncaught it is simply ignored by the JVM but if we are calling finalize method explicitly and if an exceptions is uncaught then the program will be terminated
abnormally.


Ex:
class Test
{
public static void main(String arg[])
{
Test s = new Test();
//s.finalize();
s = null;
System.gc();
System.out.println("End of main method");
}
public void finalize()
{
System.out.println("finalize method");
System.out.println(10/0);
}
}
O/P:- finalize method
end of main method

Q) which of the following statements are true

1) JVM ignores all exceptions which are raised while executing finalize()
2) JVM ignores only uncaught exceptions which are raised during execution of
finalize()

case3:-
 
1) Garbage Collector calls finalize() only once on any object i.e it won’t call more than once.
2) While executing finalize() there maybe a chance of object getting reference variable at that timeG.C won’t destroy that object after completing finalize()
3) If the same object is eligible for G.C second time with out executing finalize() method G.C will destroy that object.



Ex:
class FinalizeDemo
{
//Test s;
public static void main(String arg[])
{
FinalizeDemo f = new FinalizeDemo();
System.out.println(f.hashcode());
f = null;
System.gc();
Thread.sleep(5000);
System.out.println(s.hashCode());
s = null;
System.gc();
Thread.sleep(5000);
System.out.println("End of main method");
}
public void finalize()
{
System.out.println("finalize method called");
s = this;
}
}

The behavior of G.C is vendor dependent hence there is no guarantee for the following

1) Whether the G.C follows mark & sweep algorithm or not
2) What exact Algorithm followed by Garbage Collector
3) In which order Garbage Collector destroys Object
4) Whether Garbage destroys all eligible objects or not
5) At what time exactly Garbage Collector will run.

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