There is no rule that constructor to be public . Generally we define it public just because we would like to instantiate it from other classes too . Private constructor means,"i dont let anyone create my instance except me ". So normally you would do this when you like to have a singleton pattern..
Similarly, it is asked, are constructors public or private?
No, Constructors can be public , private , protected or default (no access modifier at all). Making something private doesn't mean nobody can access it. It just means that nobody outside the class can access it. So private constructor is useful too.
Subsequently, question is, can I declare constructor as private? Yes, class can have a private constructor. It is needed as to disallow to access the constructor from other classes and remain it accessible within defined class. A singleton is a design pattern that allows only one instance of your class to be created, and this can be accomplished by using a private constructor.
Also, when should a constructor be private?
Private constructors are used to prevent creating instances of a class when there are no instance fields or methods, such as the Math class, or when a method is called to obtain an instance of a class. If all the methods in the class are static, consider making the complete class static.
Why constructors are always public?
A constructor isn't always public. Though we want it to be public because of each time we create an object of a class, we know the constructor would be called but if the constructor is defined in the private section we wouldn't be able to call it. Hence we won't be able to create our object of the class.
Related Question Answers
What happens if I make a constructor private?
By providing a private constructor you prevent class instances from being created in any place other than this very class. There are several use cases for providing such constructor. Your class instances are created in a static method. The static method is then declared as public .Can constructor be inherited?
Constructors are not members of classes and only members are inherited. You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses.Can we make constructor final?
No, a constructor can't be made final. A final method cannot be overridden by any subclasses. In other words, constructors cannot be inherited in Java therefore, there is no need to write final before constructors. Therefore, java does not allow final keyword before a constructor.Can a constructor be static?
constructor is implicitly called to initialize an object, so there is no purpose in having a static constructor. Java does not permit to declare a constructor as static. A constructor always belongs to some object. If a constructor is static, an object of subclass cannot access.Can we override a constructor in Java?
By rule in Java, a constructor cannot be overridden but a method can be overridden. a constructor cannot be called with an object but method can be called. To call a constructor, an object must be created. in case of methods, we say, the "subclass method can call super class method".Can we override static method?
Static methods cannot be overridden because they are not dispatched on the object instance at runtime. The compiler decides which method gets called. Static methods can be overloaded (meaning that you can have the same method name for several methods as long as they have different parameter types).Can we override private methods?
No, a private method cannot be overridden since it is not visible from any other class. You have declared a new method for your subclass that has no relation to the superclass method. One way to look at it is to ask yourself whether it would be legal to write super.What is singleton class in Java?
Singleton Class in Java. In object-oriented programming, a singleton class is a class that can have only one object (an instance of the class) at a time. To design a singleton class: Make constructor as private. Write a static method that has return type object of this singleton class.Can we use this () and super () in a method?
this() and super(), both are the constructors that's why must be the first statement. But we can use both in a program. this(): It is used to call, same class Default or Parametrized Constructor. super(): It is used to call, immediate super/parent class Default or Parametrized Constructor.What happens if constructor is private in C++?
If some constructor is private, it means that no one but the class itself (and friends) should be able to create instances of it using that constructor. Therefore, you can provide static methods like getInstance() to create instances of the class or create the instances in some friend class/method.Can constructors have return types?
Constructors in Java do not return anything explicitly. They do not have a return type, not even void. However, as the definition of Constructor goes, it is used to initialize an object of the class. So implicitly, they return the current instance of the class whose constructor it is.What is the difference between static and private constructor?
Static Constructor is called automatically before the first instance of the class or any static data is referenced. It is called only once for any number of classes instance is created. Private constructor is a special constructor that generally used in class that contains only static members.Can constructors be private in C++?
Yes, a constructor can be private. And you can call it with member functions (static or non) or friend functions. For possible use cases, see the Factory Pattern, or the Named Constructor Idiom.Can a class have private and public constructor?
Can a class have both public and private constructor? Yes, it is possible. A private constructor is needed to set the private field whose type is a private inner class.Can a constructor be protected?
protected constructor can be accessed from its own class, its subclasses, all other classes belonging to the same package and subclasses of other packages. A class with both private and protected constructors available can be inherited; but subclass has the accessibility for protected constructors only.Can you have more than one default constructor?
The funny thing is that your class can have multiple constructor overload which could work with no arguments and are therefore valid default constructors: struct A { A() {} A( int i = 0 ) {} }; This is perfectly legal to write. The answer to the question from the title is therefore: Yes.Can constructor be overloaded?
Constructor can be overloaded in a similar way as function overloading. Overloaded constructors have the same name (name of the class) but different number of arguments. Depending upon the number and type of arguments passed, specific constructor is called.Can abstract class have private constructor?
A private constructor in an abstract class can also serve the purpose of sealed classes (like in Scala or Kotlin etc.). Since you can still provide subclasses from within the abstract class, but outsiders cannot extend/implement (as @Marko Topolnik answered).Is constructor public or private in C++?
If a constructor is not provided to a class in C++, the compiler adds a default 'public' constructor. However if a constructor is provided and is declared as private, that would imply that the object of the class cannot be created outside the class scope. 
