CareerCup

public class MySingleton {
    
    
    //the static singleton object
    private static MySingleton theObject;

    private MySingleton() {
    }
    
    
    /**
     * Checks if the singleton object is created or not,
     * if not it creates the object and then the object is
     * returned.
     *
     * @return the singleton object
     */
    public static MySingleton createMySingleton() {
        
        if (theObject == null)
            theObject = new MySingleton();
        
        return theObject;
    }
}

public class MySingleton {
    
    
    //the static singleton object
    private static MySingleton theObject;
    
    /**
     * private constructor
     */
    private MySingleton() {
    }
    
    
    /**
     * Checks if the singleton object is created or not,
     * if not it creates the object and then the object is
     * returned.
     *
     * @return the singleton object
     */
    public static MySingleton createMySingleton() {
        
        if (theObject == null)
            theObject = new MySingleton();
        
        return theObject;
    }
}

1. Not thread safe
2. Should be final, just to be extra cautious make abstract as well.

// Also an error, synchronization does not prevent
// two calls of constructor.
public static MySingleton getInstance() {
if (_instance==null) {
synchronized (MySingleton.class) {
_instance = new MySingleton();
}
}
return _instance;
}

In the correct solution, seen in Soultion 3, make getInstance() a synchronized method:

Solution 2
// Double-checked locking -- don't use
public static MySingleton getInstance() {
if (_instance==null) {
synchronized (MySingleton.class) {
if (_instance==null) {
_instance = new MySingleton();
}
}
}
}

Solution 3
// correct solution
public static synchronized MySingleton getInstance() {
// . . . continue as in Listing 3

public class MySingleton {
    
    
    //the static singleton object
    private static final MySingleton theObject = new MySingleton();
    
    /**
     * private constructor
     */
    private MySingleton() {
    }
    
    public MySingleton getInstance() { return theObject;}
 }

isn't that the simplest thread-safe way?