I want to write a java class that can be instantiated only 5 times just like you have singleton class which have only one instance.
Apart from this the instances should be picked in round robin basis.
Suppose I have a class A. I should be only able to create 5 instances of this class. Say I have InstanceA_1, InstanceA_2, InstanceA_3, InstanceA_4, InstanceA_5. Whenever I need to use them they should be picked on round robin basis.
Is this a homework?
Why don't you create a Factory that creates 5 instances and store them in a list? When someone calls Factory.getInstance(), it will then get the right instance using the right index of the list.
Create five static instances. Keep an index of the last instance. Make a private constructor. Use a method to get an instance (similar to a singleton);
public class MyObject {
private static final MyObject instance_1 = new MyObject();
private static final MyObject instance_2 = new MyObject();
private static final MyObject instance_3 = new MyObject();
private static final MyObject instance_4 = new MyObject();
private static final MyObject instance_5 = new MyObject();
private static final MyObject[] instances = new MyObject[]{instance_1, instance_2,instance_3, instance_4,instance_5};
private MyObject() {
}
private static volatile int index = 0; //in a multi-threaded evironment, you should use volatile
/**
* The following code is not thread safe
*/
public static MyObject getInstance() {
MyObject instance = instances[index];
if (index == 4) {
index = 0;
} else {
index++;
}
return instance;
}
}
Well, something along these lines:
To be thread-safe you will need to lock within the static method. You could use AtomicInteger, but wrapping is tricky. You could always use AtomicInteger or AtomicLong, increment each time, and index using the % operator... but when the index wrapped, you'd get a discontinuity. That may or may not be a problem for you.
(This is assuming you don't mind creating all five instances up-front. That certainly makes things easier.)
I imagine that all you would need to do is extend the Singleton pattern to use a counter. Something like...
// Declare a list holding the 5 Objects
List<Foo> instances = new ArrayList<Foo>(5);
// Keep track of which Object you are returning next
int curr = 0;
...
static Foo getInstance() {
// Only instantiate when necessary
if (instances.get(curr) == null) {
instances.set(curr, new Foo());
}
Foo toReturn = instances.get(curr);
// Use modulus to loop back to 0 after 4
curr = (curr + 1) % 5;
return toReturn;
}
Disclaimer: I'm assuming you don't need the code to be thread-safe.
You need something like an Object pool. The simplest implementation would be to have a singleton containing a List of 5 objects, and whenever you call getInstance(..), the list should be iterated and one of the objects returned.
Here is a very simplistic solution. Note that this isn't thread safe:
class ClassA {
private final List<ClassA> instances = Arrays.asList(new ClassA(), new ClassA(), new ClassA(), new ClassA(), new ClassA());
private ClassA() {}
public static ClassA getInstance() {
Collections.shuffle(instances);
return instances.get(0);
}
}
class OnlyFiveTime {
private static LinkedList<OnlyFiveTime> instaces = new LinkedList<OnlyFiveTime>();
private static final int count = 5;
public static OnlyFiveTime getInstace() {
if(instaces.size() != count) {
instaces.addLast(new OnlyFiveTime());
}
OnlyFiveTime robinRound = instaces.removeFirst();
instaces.addLast(robinRound);
return robinRound;
}
private OnlyFiveTime() {
}
}
A simplified RoundRobin.
import java.util.*;
public enum RoundRobin {
EENIE, MEENIE, MINY, MO;
private final static RoundRobin[] values = values();
private static int counter = -1;
public static RoundRobin nextInstance() {
counter = (counter + 1) % values.length;
return values[counter];
}
public static void main(String[] args) {
for (int i = 0; i < 10; i++)
System.out.println(RoundRobin.nextInstance());
// EENIE, MEENIE, MINY, MO, EENIE, MEENIE, MINY, MO, ...
}
}