What is a constructor and what is its purpose?

A constructor is a special method called when an object is created, used to initialize the object's state. It has the same name as the class and no return type. Default constructor (no arguments) is provided by compiler if none defined. Parameterized constructors accept arguments for initialization. Constructor overloading allows multiple constructors with different parameters. In Java, 'this()' calls another constructor; 'super()' calls parent constructor. Constructors ensure objects are in valid state from creation.