Clarifications/Suggestions in Java

[Deepak Lal]

Hi Ranchers,
I have below clarifications.


1> I have a Employee class with six employee objects{e1,e2,e3,e4,e5,e6} added to it.How do i sort it using Tree Set ?Code example please
2> What is load factor in HashMap.?
3> Can i subclass a singleton class ?
4> what is lazy loading of Servlets ?
5> how do you intialize a session in Servlets ?
6>what is the pros and cons os using jsp:use bean tag
7> what is scope in use bean tag
8> give an example where you can use DynaAction form.Code example please



Thanks and Regards,
Deepak Lal

[Rob Spoor]

The first 7 questions can be answered each within a few minutes by just a bit of searching. You should DoYourOwnHomework.

[Deepak Lal]

Rob,i will be thanful if you help me this time.because few of the answers to questions are confusing and im unaware of the correct answer.Need help Ranchers

[James Elsey]

Smells like homework ;)

Do as Rob suggested and have a quick search on google, attempt to write some code examples and if they fail post them up and we can help out

There is a good quote : If you give a man a fish, he'll eat for a day, if you teach a man to fish, he'll eat for a lifetime

In short, you would benefit soooo much more than being given answers!

[David Newton]

And it would likely be quicker.

Try posting your answers to the questions once you've done some research--if you have specific questions about anything you discovered we'll be happy to help!

[Deepak Lal]

Hi Ranchers,
Hope you can help me now.




1> I have a Employee class with six employee objects{e1,e2,e3,e4,e5,e6} added to it.How do i sort it using Tree Set ?

Employee e1 = new Employee();
Employee e2 = new Employee();
Employee e3 = new Employee();
Employee e4 = new Employee();
Employee e5 = new Employee();
Employee e6 = new Employee();
SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...)); // i dont know how to proceed at this step.Please help me now.

2> Can i subclass a singleton class ? I have not tried but i *think* we cannot subclass a singleton class.Need example for this

3> How do you intialize a session in Servlets ?

 HttpSession session  = request.getSession(); //please correct me if im wrong.  

4> give an example where you can use DynaActionForm ? Please help me with this..


Thanks and Regards
Deepak Lal

[Gupta Tarun]

Hints: not in the same order as your questions..

Private Constructors

Comparator/Comparable

One of your answers is correct.
Google search..

[Bobby Smallman]

Something about posting eight questions in a single post on topics which are relatively well covered makes me less inclined to give large amounts of time responding. But I will however direct you to another post which 1/8th of your post was inquiring about.

http://www.coderanch.com/t/511303/java/java/Sort-Name

[Deepak Lal]

Bobby Smallman wrote:Something about posting eight questions in a single post on topics which are relatively well covered makes me less inclined to give large amounts of time responding. But I will however direct you to another post which 1/8th of your post was inquiring about.

http://www.coderanch.com/t/511303/java/java/Sort-Name

I did go to website URL but i need to sort employee objects using TreeSet that is the requirement im having,help me out please.
1> Question Answer

import java.util.*;

public class Test{

  public static void main(String args[]){	 
	Employee e1 =  new Employee();
	Employee e2 =  new Employee();
	Employee e3 =  new Employee();
	Employee e4 =  new Employee();
	Employee e5 =  new Employee();
	Employee e6 =  new Employee();
	
	e1.setName("D");
	e2.setName("A");
	e3.setName("S");
	e4.setName("J");
	e5.setName("V");
	e6.setName("Z");
	
	Set ts=new TreeSet();

    	ts.add(e1);
    	ts.add(e2);
    	ts.add(e3);
    	ts.add(e4);
    	ts.add(e5);
    	ts.add(e6);
    
	//I'm stuck here.can you help me out now
        SortedSet s = Collections.synchronizedSortedSet(new TreeSet()); 
        // i dont know how to proceed at this step.Please help me now.  


  }
	
}

import java.util.Comparator;

public class Employee implements Comparator {

private String name;
public String getName() {
	return name;
}
public void setName(String name) {
	this.name = name;
}

@Override
public int compare(Object emp1, Object emp2){
	//parameter are of type Object, so we have to downcast it to Employee objects
	String emp1Name = ( (Employee) emp1 ).getName();
	String emp2Name = ( (Employee) emp2 ).getName();
	//uses compareTo method of String class to compare names of the employee
	return emp1Name.compareTo(emp2Name);
}
}

Please help me,how should i proceeed further.

2> Can we subclass singleton class ...Please help me ?

[David Newton]

I'm still not satisfied with the level of effort shown, and this still smacks *loudly* of homework and/or interview questions.

Regarding TreeSet: have you read the documentation? If not, go do so now. If you have, what does it say about ordering and the iterator?

[Joanne Neal]

Deepak Lal wrote:2> Can we subclass singleton class ...Please help me ?

Why don't you write a simple singleton class - there are plenty of examples of how to do this here on javaranch and elsewhere - and then write a second class that extends it. Then see if it compiles. If it does then see if it runs. Then you have your answer.

[Deepak Lal]

David Newton wrote:I'm still not satisfied with the level of effort shown, and this still smacks *loudly* of homework and/or interview questions.

Regarding TreeSet: have you read the documentation? If not, go do so now. If you have, what does it say about ordering and the iterator?

1> David i have read the documentation of TreeSet and it says you need to implement Comparator interface and thats what i have done in the code..Could you please please render help.

2> Regarding subclassing a singleton class,it gets subclassed.but i want to know why it is possible because a singleton by definition itself gives a single instance of class which has a private constructor,so whats the use of
subclassing a singleton class.Please clarify me on this point ?

[Campbell Ritchie]

Please only use code tags for code. And avoid very long lines.

[Kiaamaa Liammes]

//I'm stuck here.can you help me out now
SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
// i dont know how to proceed at this step.Please help me now.

you were looking for a example of sorting using TreeSet, but the line you are stuck ,indicates you are trying to synchronize access to your treeSet

[David Newton]

Deepak Lal wrote: 1> David i have read the documentation of TreeSet and it says you need to implement Comparator interface and thats what i have done in the code..Could you please please render help.

Help with what? If you have a comparator, and you have a TreeSet, it's sorted. Re-read the documentation.

[Joanne Neal]

Deepak Lal wrote:Regarding subclassing a singleton class,it gets subclassed.but i want to know why it is possible because a singleton by definition itself gives a single instance of class which has a private constructor,so whats the use of
subclassing a singleton class.Please clarify me on this point ?

Show us the code for your singleton class and subclass.

[Deepak Lal]

Singleton class and subclass -->Can you check whats wrong.

class SingletonClass{
  private SingletonClass(){}
  private static SingletonClass singletonclass; 
  public static synchronized SingletonClass getInstance(){
	  if(singletonclass == null){
		  singletonclass = new SingletonClass();
	  } 
		  return singletonclass;      
  }
}

class XYZ extends SingletonClass{
	private XYZ(){} //im getting an error at this line..please help me
} //subclassing a singleton class

  public class SingletonClassTest{
	public static void main(String args[]){
		@SuppressWarnings("unused")
		SingletonClass singleton = SingletonClass.getInstance();
		System.out.println("Only one instance created ");			
	}	
}

[Rob Spoor]

A constructor will always call a constructor from its super class. This is called constructor chaining. But to be able to call that constructor it must be visible for the subclass. Private constructors aren't.

[Joanne Neal]

Deepak Lal wrote:Singleton class and subclass -->Can you check whats wrong.

class SingletonClass{
  private SingletonClass(){}
  private static SingletonClass singletonclass; 
  public static synchronized SingletonClass getInstance(){
	  if(singletonclass == null){
		  singletonclass = new SingletonClass();
	  } 
		  return singletonclass;      
  }
}

class XYZ extends SingletonClass{
	private XYZ(){} //im getting an error at this line..please help me
} //subclassing a singleton class

  public class SingletonClassTest{
	public static void main(String args[]){
		@SuppressWarnings("unused")
		SingletonClass singleton = SingletonClass.getInstance();
		System.out.println("Only one instance created ");			
	}	
}

You've correctly implemented a singleton class and when you tried to write another class that extended it you got an error.
Your original question was

Deepak Lal wrote:Can i subclass a singleton class ?

So, based on what you found, what do you think the answer is ?

[Deepak Lal]

Since we have private Constructors in a subclass which extends a Singleton class,we cannot subclass a Singleton class.

[Joanne Neal]

Deepak Lal wrote:Since we have private Constructors in a subclass which extends a Singleton class,we cannot subclass a Singleton class.

Try making the constructor in the subclass public and see if anything changes.

[Rob Spoor]

Although that allows you to subclass a class it will no longer be a singleton - the class can be instantiated from anywhere. Singletons that are designed to be subclassed usually have protected constructors.

[Joanne Neal]

Rob Prime wrote:Although that allows you to subclass a class it will no longer be a singleton - the class can be instantiated from anywhere. Singletons that are designed to be subclassed usually have protected constructors.

I wasn't talking about the constructor in the singleton class.
Deepak said that the reason you couldn't subclass a singleton was because the constructor in the subclass was private. I was just suggesting he tried making it public and he would have noticed that it made no difference i.e. the access modifier of the subclass constructor has nothing to do with not being able to extend a singleton class. It is, as you said earlier, the fact that the singleton class constructor is private that means you can't extend it.

[Rob Spoor]

I missed the "subclass" there

[Deepak Lal]

I hope Im rightly concluded that "The singleton class constructor is private that means you can't extend/subclass it."

[Rob Spoor]

That conclusion is 100% correct. Because of constructor chaining the super class constructor must be visible, and with private it simply isn't.

[Deepak Lal]

Hi Rancers and Bartenders,
I have few queries in Java Collections framework as well.

1>Suppose i have 5 elements added to a ArrayList and Vector.? suppose i now add 6th element to ArrayList and Vector.How will it be internally added in both the cases.Please explain

2>In which scenario would i use a List and a Map except for the normal differences being that list is being used for sequential addition of elements and Map is for (Key,Value) Pair ?

[Rob Spoor]

Deepak Lal wrote:1>Suppose i have 5 elements added to a ArrayList and Vector.? suppose i now add 6th element to ArrayList and Vector.How will it be internally added in both the cases.Please explain

In your JDK folder there is a file called src.zip. Unpack it, then look for files java/util/ArrayList.java and java/util/LinkedList.java. That's the source of these classes.

2>In which scenario would i use a List and a Map except for the normal differences being that list is being used for sequential addition of elements and Map is for (Key,Value) Pair ?

You've already given the answer yourself, haven't you?

[Deepak Lal]

1> i asked for ArrayList which implies ArrayList.java and i asked for Vector ,does the Vector implementation refer to LinkedList.java as you have pointed out.
2> More differences,The interviewer was not happy with this answer,He wanted more differences from a realistic point of view.he said thats a common answer.

[Rob Spoor]

Deepak Lal wrote:1> i asked for ArrayList which implies ArrayList.java and i asked for Vector ,does the Vector implementation refer to LinkedList.java as you have pointed out.

Sorry, that should have been java/util/Vector.java of course.

[Deepak Lal]

Rob,
what about the answer to second question.COuld you help me with that please. More differences required with respect to List and a Map.
Regards
Deepak Lal

[removed excessive use of font and color]

[Rob Spoor]

I believe that searching around in this forum, Beginning Java, the Java tutorials and Google, you should be able to find more than enough answers on that question. We do require everyone to ShowSomeEffort.

[Campbell Ritchie]

Rob Prime wrote: . . . We do require everyone to ShowSomeEffort.

Deepak Lal: this site is not some sort of substitute for your reading your textbook, turning up to lectures, looking at web tutorials, or doing any work at all.