这里的构造函数是什么？

Question

The text of an exercise from a well known and basic Java programming textbook follows:

'Sometimes we would like a class that has just a single unique instance. Create a class Merlin that has one attribute, theWizard, which is static and of type Merlin. The class has only one constructor and two methods, as follows: * Merlin —a private constructor. Only this class can invoke this construc- tor; no other class or program can create an instance of Merlin * summon —a static method that returns theWizard if it is not null; if theWizard is null, this method creates an instance of Merlin using the private constructor and assigns it to theWizard before returning it. * consult—a non-static method that returns the string "Pull the sword from the stone" '

Were you able to understand what the author is asking in the point 'Merlin -a private constructor' ? I know what a constructor is as well as what a private method is. But here what's the answer ? I thought of something like

public class Merlin {
    private static Merlin theWizard;

    /*public Merlin()
    {
        ???
    }*/

    private Merlin()
    {
         this();
    }

    public static Merlin summon(Merlin theWizard) { 
        if (theWizard == null) {
            theWizard = new Merlin(); 
        return theWizard; 
    }

    public String consult() {
        return "Pull the sword from the stone"; 
    }
}

and of course JavaBeans says 'private Merlin()' is recursive. So what do I do ? The next exercise is related to the above one and could help you to understand what the answer is; here it is:

9. Create a program that tests the class Merlin described in the previous exercise.
   • Use the toString method to verify that a unique instance has been created.

Thank you very much for all that you will be able to tell me.

Answer 1

This is an instance of the Singleton Pattern; in your case, just replace this() with super() to use the superclass's constructor (technically, if you don't place a super constructor call, the compiler will insert one for you; so you could just remove it).

Food for thought:

• why is the Merlin class private ?
• why is the Merlin class static ?
• where's the rest of your code?

Answer 2

Get rid of this() :

public class Merlin {

    private static Merlin theWizard;

    private Merlin() {}
    ...
}

this followed by parens is the syntax used to call a constructor of the same class, since you are not specifying any arguments inside the parens then you're referring to the no-argument instructor, which is the constructor this is being called from, so the constructor would call itself recursively until it runs out of stack space. The IDE is checking for this problem and trying to warn you about it.

Calling super() in the constructor is allowed (it calls the no-argument constructor in the superclass, which here is java.lang.Object) but not required, the compiler will insert a call to super if you don't.

By the way you can't have multiple overloads of a constructor or method with the same argument list. You can't have a public no-arg constructor and a private no-arg constructor.

The author is trying to describe a class that has control over how its instances are created, it makes its constructor private so that only the class itself is allowed to instantiate an object of that class.