1.Detect garbage objects
2.Deallocate the memory of garbage objects and make it available for the program.

There are four approaches that a garbage collector may adopt to detect the garbage objects.

A) Reference -counting Collectors :-Reference counting garbage collectors keep a count of the references for the each live object.When an object is created ,the reference count of each object is set to one.When you reference the object ,the reference count is incremented by one.Similarly when a reference to an object is eliminated ,the reference count is decremented by one.An object which has reference count zero is a garbage object when the object is garbage collected ,the references of the object that it refers to are decremented.Therefore garbage collection of the one may lead to the creation of other garbage objects.This method can be executed in small parts with the program ,and the program need not to be interrupted for a long time. However ,there is an overhead of incrementing and decrementing the counter everytime something happens on the references side.

B) Tracing Collectors :-In this technique ,a set of root is defined from where objects are traced. An object is reachable if there are objects that referenced and cannot ,therefore be accessed in the program.Objects that are reachable are marked. At the end of the trace ,all marked objects can be garbage collected.
This is also known as the mark and sweep algorithm. The mark phase marks all the referenced objects. The sweep phase garbage collects the memory of unreachable and unreferenced objects.

C) Compacting Collectors :-These collectors reduce the degree of memory fragmentation by moving the all unused and free space on one side during garbage collection.The free memory is then available as one huge chunk.All references need to be shifted ,objects are the updated to refer to the new memory locations.

D)Adaptive Collectors :-This algorithm makes the use of the fact that different garbage collectors algorithm works better in different situations. The adaptive algorithm monitors the situation and uses the garbage technique that best suits the situation. It may switch from one technique to the other according to the need.

Comments

General Syntax :-

expression 1 ? expression 2 : expression 3

Here expression 1 can be any expression that evaluates to a Boolean value.If expression 1 is true then expression 2 is evaluated else expression 3 is evaluated.Both the expressions expression 1 and expression 2 must have a return type,they can never be void.Make it more clear by understanding below example.

Example:-

public class optest
{
public static void main(String a[])
{
int ratio=0,num=20,denom=10;

/* The ” ? ” operator assignes 0 if condition denom==0 is true else it assigns num/denom to ratio if condituion is false*/

ratio=denom==0?0:num/denom;

System.out.println(“The ratio is “+ratio);
}
}

Output:-
The ratio is 2

Explanation:-When Java evaluates this assignment expression ,it first looks at the expression to the left of the question mark.If denom equals to zero then expression 2 between ? and : is evaluated else last expression is evaluated.The resultant is then assigned to the ratio variable used in expression 1.

Comments

Try to answer these questions on your own but if you cannot recall the concept go refer book,any core Java book will do.

Important Core Java Questions :-

1.How many access modifiers a class can use and which are they?

2.What are the Instance variables?

3.What are local variables?

4.What is the default access modifier and how it is different from protected access modifier?

5.Can you ever mark a class as final?

6.Is it true that we can use abstract and final both on same time and same Java entity?

7.What is the IS-A and HAS-A relationship in Java?

8.What is multiple inheritance and how you can implement multiple inheritance in Java?

9.What is the size of compiler generated Java Bytecode?

10.Does Interfaces follow the Inheritance?

11.A subclass can inherit the private member of superclass is this statement true,give reason?

12.What is the bit depth of Boolean variable in Java?

13.What will happen if finally block itself generates an Exception?

14.What is exception propagation?

15.How Inheritance is affected by object serialization?

These are a few questions which I thought as Important.Further there are some references I am giving in order to solve these questions.

Comments

1.Attempting to access an instance variable from a static context (i.e. from main() method)

Since static variable can only be accessed from static context thus there is no way you can access the non-static instance variable in static context without a reference.For example:-

Class Scopetest{
int x=10;
public static void main(String a[]){
System.out.println(x++);
}
}

This will result in an error
Scopetest.java:4: non-static variable x cannot be referenced from a static context
System.out.println(x++);

Despite this you can use the instance of Scopetest class to access the variable as shown below:-

class Scopetest{
int x=10;
public static void main(String a[]){
System.out.println(new Scopetest().x++);
}
}

This will produce output as 10.
So,remember you can never ever access a non-static instance variable within a static context alone.You have to use the enclosing class instance.

2.Attempting to use a block variable after the code block is completed.
How often we commit this error,I guess everyone of us has faced it.So remember never ever try to invoke a variable which is ended its life.That is block variable lifetime is until the code block completes.As block finished execution the variable will also perish from memory,thus any reference out of this block result in a compiler slap!!!
Lets take a look at following example:-

class Scopetest{
public static void main(String a[]){
for(i=0;i<2;i++) style=”font-style: italic;”>.java:5: cannot find symbol
symbol : variable i
location: class Scopetest
if(i<3)>

3.Attempting to access a local variable from nested method.
When a method invokes another method then the calling method does not have access to the called methods variables and vice versa.For example:-If we have a method do1() which called by another method do2() then do1() cannot access the variables local to do2().It is evident from the following example:-

class Scopetest{
public static void main(String[] a){
Scopetest s = new Scopetest();
s.do2();
}
public void do2(){
int x = 4;
do1();
++x;
}
public void do1(){
x++;
}
}

The error which appears is:-
Scopetest.java:12: cannot find symbol
symbol : variable x
location: class Scopetest
x++;

So next time you go to the battlefield(programming) keep yourself armed with better knowledge and strategies of course here is to reduce the bug and make your program more readable.Keep in mind above mentioned errors and be a good programmer,and keep firing on the COMPILER!!!.

Comments

Features of UCertify SCJP exam preparation kit:-

1.Very good collection of qulaity questions.which you can expect in the exam.

2.Tests in the preparation kit are timed tests.

3.Clearly mentioned exam objectives.

4.Exajm specific notes.

5.Practice questions with every exam objective notes.

6.You can create your own custom tests timed or not.

7.There is also an interactive tests which adds interactivity in testing your skills.

8.Nine high quality timed tests to test your caliber.

9.Tips to success small little tips but are beneficial in huge way.

10.Articles present to make basic concepts very clear.

11.There is a final test which you can take finally if you think you studied enough to take the exam.

12.The quiz engine is a learning machine which keeps on storing your past scores and let you compare your previous performances to the current one.

13.You can check out if you are ready for the exam by checking the exam readiness report.

14.UCertify exam preparation is available for free.

15.About 700 high quality certification questions covering all objectives.

Shortcomings of UCertify SCJP exam preparation kit:-

1.The detailed description is not given in the notes.For that you have to refer some book.
2.Study notes which includes objective specific questions are too simple.

All the above features I think are good enough to try out the UCertify SCJP exam preparation guide. The advantages clearly outweighs the shortcomings.
There is much more You can try the quiz engine for free to download click below:-

UCertify SCJP 310-065 exam preparation kit free download Trail

Limited Period Offer:-

Special offer for Java World Readers get 10% off on every product you buy from the UCertify By just mentioning the Coupon Code “UCPREP”. What’s more all products of UCertify comes with a money back guarantee. If you think that the product is not good enough to let you crack the exam you can take your money back.

UCertify SCJP preparation kit :best SCJP exam preparation Kit download now for free!!!

Comments

Now we come to the Examination part. Most guys must know that India is corrupt. Let me tell you it’s not as the media has framed India’s growth. Still it is major factor. I also heard that you can pass the exam by paying extra money. You will get whole bunch of papers. I found the cost was about the same for the exam. This is pathetic and true injustice with the hardworking students. People are just degrading the value of the Sun Certificate, nothing else.

You might be living in any part of the world but the silly thing is that you will never get a value for your thing. Like you will never get a expected job according to your qualifications.

I have another case for you that one of my friend knows nothing about programming and still managed to get a job in a giant IT company @ Rs. 21, 000/- per month($ 1, 050), almost 1. 75 times the person who has qualified the SCJP. when I look at them I need to think well its luck or may be something else, maybe he’s not able to show up his skills or anything. In terms of qualification its really unfair.

If you are outside India you may find yourself lucky that there are not that much Sun Certified programmers but India is a place full of certified programmers. It doesn’t matter whether you are an Indian or not , but looking forward to SCJP. Then keep pulling weights even more, otherwise you will be just another SCJP.

Most important thing never get out of touch. That is keep revising and testing your concepts with new ideas and codes. This way you will keep away from being rusted and at the same time when you will get a call for interview you will turn up positively, full of confidence. Big boys know how important confidence is in Interviews.

I am not discouraging you from giving SCJP but I want you to know the ground reality of SCJP. It would be better if you try for Cisco Certification , Oracle Certification etc. Their demand is much higher than SCJP. It is the market trend. SCJP is a bit saturated field. You have to be a bit different that other Sun certification holders.

All the best ;-]

Comments