Can we override a constructor in Java and can a constructor be private?

Question

I would appreciate an explanation for these questions:

1. Can we Override a constructor in Java?
2. Can a Constructor be private?

Answer 1

No, you can't override a constructor. They're not inherited. However, each subclass constructor has to chain either to another constructor within the subclass or to a constructor in the superclass. So for example:

public class Superclass
{
    public Superclass(int x) {}

    public Superclass(String y) {}
}

public class Subclass extends Superclass
{
    public Subclass()
    {
        super(5); // chain to Superclass(int) constructor
    }
}

The implication of constructors not being inherited is that you can't do this:

// Invalid
Subclass x = new Subclass("hello");

As for your second question, yes, a constructor can be private. It can still be called within the class, or any enclosing class. This is common for things like singletons:

public class Singleton
{
    private static final Singleton instance = new Singleton();

    private Singleton()
    {
        // Prevent instantiation from the outside world (assuming this isn't
        // a nested class)
    }

    public static Singleton getInstance() {
        return instance;
    }
}

Private constructors are also used to prevent any instantiation, if you have a utility class which just has static methods.

Answer 2

Constructor is meant for a class. It cant be overridden under any circumstances. Its like wanting to change Ferrari's factory from BMW's factory (which isn't practical). Surely you can overload to get the functionality you need.

Yes Constructor can be private. By making it private you are not letting the outside world to create an object of it directly through constructor, But singleton pattern uses a public static method to call the constructor of the class and object can be created.