hello all,
i just wan to know about how can i restrict object creation ... ( not by hard coding...)
suppose i want to create only two objects can use single tone design pattern for this( i know that we can create only one by using this design pattern)... is there any possibility to use this design pattern.... for this task..
Have you understood how the Singleton pattern works? What is your problem when you try to modify it to accept two instances instead of one?
The best design depends on the details of your requirements. Like, for a client, whether it matters which of the two instances it gets. How magic the number 2 is; if there's any possibility that in a future application of your class, it might be 1 or 3 or 200 instances, or it always will be 2.
> You can create a int static variable and check the
> the value in the constructor.
In the constructor? Probably a poor idea. What would you do there if the count hits 3?
Better do it in the getInstance method (or what you call the public method (or methods?) that returns one of the two instances. If the count is already two, return one of the already created instances (or null, depending on the requirements).
And declare getInstance can throw an exception?
Or throw an unchecked exception and risk your program crashes when somebody tries to create a 3rd object?
I like your counter idea very much. I still prefer to check the counter in getInstance() rather than in the constructor.
Yes, there are more options than the two I mentioned. They share a tendency to make the design more complicated.
thanks to my community people for your response...
Hello OleW,
here is one single tone class, i also got that but unable to implement.... see this whether it's able to get or not....
public class SingleT{
private static SingleT Object.uniqueInst[] ;
private int count=0;
private SingleT() {}
public static final SingleT getInstance() {
for(i=0;i<=count;i++){
if (uniqueInst == null )
uniqueInst = new SingleT();
count++;
}
return uniqueInst;
}
}
sricharan
I'm not sure if it works. Give it a try
public class ClassicSingleton {
private static ClassicSingleton instance0 = null;
private static ClassicSingleton instance1 = null;
private ClassicSingleton() {
}
public static ClassicSingleton getInstance() {
if(instance0 == null) {
instance0 = new ClassicSingleton();
return instance0;
}else if(instance1 == null{
instance1 = new ClassicSingleton();
return instance1;
}else{
//return null or throw some exception
}
}
}
Manuel Leiria
> That is not thread safe
Now is thread safe
public class ClassicSingleton {
private static ClassicSingleton instance0 = null;
private static ClassicSingleton instance1 = null;
private ClassicSingleton() {
}
public synchronized static ClassicSingleton getInstance() {
if(instance0 == null) {
instance0 = new ClassicSingleton();
return instance0;
}else if(instance1 == null{
instance1 = new ClassicSingleton();
return instance1;
}else{
//return null or throw some exception
//return instance0 or instance1 ?
}
}
}
>and even if it were it seems
> you could only call getInstance twice.
I guess you're right!
> This whole idea of having two instances seems
> ridiculous to me.
I agree with you, having two instances smells like bad design
Message was edited by:
manuel.leiria
> That is not thread safe and even if it were it seems
> you could only call getInstance twice.
Fill in either return instance0; or return instance1; in the latter else part, and you can call as many times as you want.
> This whole idea of having two instances seems
> ridiculous to me.
I was hoping the OP would explain the requirements and the reason for them a little more. We cannot rule out there's a point it it. Not that I have any idea what the point might be.
"4. Permits a variable number of instances. The pattern makes it easy to change your mind and allow more than one instance of a Singleton class. " (Gamma et al.: Design Patterns, page 128, subsection "Consequences")
hello java community,
Thanks for all to spend your valuable time .... as one of the member said (in example program) it checks whether the first object is created or not then it goes to the second one... but as per my requirment it must create the two objects when invoked.....
please throw your suggestions... please....
