I read about the Singleton Pattern from here http://ift.tt/1juRdSp and it has described some common implementation mistakes. I just want to check if these programs I have written do demonstrate their incorrectness.
In the first program, I have created an inherited class and called the protected constructor of the base class. This allows me to create 2 instances of Singleton.
import java.util.*;
import java.lang.*;
import java.io.*;
class Singleton {
public static Singleton Instance() {
if (_instance == null) {
_instance = new Singleton();
return _instance;
}
return _instance;
}
protected Singleton() {}
private static Singleton _instance = null;
}
class SingletonBasher extends Singleton {
public SingletonBasher() {
}
}
class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{
Singleton x = new Singleton();
Singleton y = SingletonBasher.Instance();
if(x != y)
System.out.println("fail!");
}
}
In the second program I have simply created two objects using new operator and this works because the constructor is protected. Had it been private, I would have been forced to use the Instance method which will only allow me to create 1 instance.
import java.util.*;
import java.lang.*;
import java.io.*;
class Singleton {
public static Singleton Instance() {
if (_instance == null) {
_instance = new Singleton();
return _instance;
}
return _instance;
}
private Singleton() {}
private static Singleton _instance = null;
}
class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{
Singleton x = Singleton.Instance();
Singleton y = Singleton.Instance();
if(x != y)
System.out.println("fail!");
}
}
