Constructor

What does Constructor mean?

A constructor is a special method of a class or structure in OOP 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.

Constructors are not called explicitly and are invoked only once during their lifetime. In the case of a hierarchy of classes where a derived class inherits from a parent class, the execution sequence of the constructor is a call to the constructor of the parent class first and then that of the derived class.

Constructors cannot be inherited.

A constructor can be declared using any of the access modifiers. It is mandatory to have a constructor with the right access modifier. However, the compiler supplies a default if an access modifier is not defined in the class. If a constructor is declared as private, the class cannot be created or derived and hence cannot be instantiated. Such a constructor, however, can be overloaded with different sets of parameters. 

Constructor design:

• When using derived class constructors, the parent class constructor should be passed the correct parameters.

• Better code maintainability comes from having the initialization and other related logic in one main constructor and cross-calling this constructor from other overloaded constructors.
• Logic involving specific operations that need to be executed at a particular event in an application – such as opening a database connection – should not be written in a constructor.
• Because a constructor cannot return a value to the calling code, it is a good practice to throw an exception when a failure is encountered.