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..
Considering this, 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.
Secondly, 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.
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.
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 a constructor be 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 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.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 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 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.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).Can constructors be 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 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.What is the difference between static and private constructor?
A static constructor cannot access non-static members. It executes before the first instance of a class. However, Private Constructor is used to restrict a class to be instantiated and to be inherited. Private Constructor is used whenever a class contains only static members.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 is constructor in OOP?
A constructor is a special method of a class or structure in object-oriented programming that initializes an object of that type. A constructor is an instance method that usually has the same name as the class, and can be used to set the values of the members of an object, either to default or to user-defined values.Can constructor be static in C++?
C++ doesn't have static constructors, as Java or C# does, so you usually have to initialize the static data members one by one (independently). This is a limitation because you may want to initialize several static data members in the same loop or algorithm, for example.What happens if constructor is not used?
Question: What happens if you do not provide a constructor? Answer: Java does not actually require an explicit constructor in the class description. If you do not include a constructor, the Java compiler will create a default constructor in the byte code with an empty argument.What happens when a constructor is called?
THE JVM will first allocate the memory for your object, then initialize all fields, then invoke your constructor. The constructor gets called when a new object is created. Hope this helps. basically constructors are called to initialize the values of the instance variables except the case for default constructors.How do Constructors work?
The purpose of constructor is to initialize the object of a class while the purpose of a method is to perform a task by executing java code. Constructors cannot be abstract, final, static and synchronised while methods can be. Constructors do not have return types while methods do.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. 
