Anatomy of a Class: Fields, Encapsulation, Constructors

A class is a blueprint; an object is one house built from it

A class definition has three kinds of members: instance variables (the state each object carries), constructors (code that runs once, when the object is created), and methods (the behaviour). Instance variables are declared private; constructors and the methods that outside code needs are declared public. That combination is encapsulation: the object owns its data and no other class can reach in and corrupt it.

A constructor has the same name as the class and no return type at all — not even void. Writing public void Book(String t) does not make a constructor; it silently makes an ordinary method named Book, and the compiler then supplies a no-argument default constructor instead.

Overloading means two constructors (or two methods) share a name but differ in their parameter lists. Java picks the version whose parameter types match the call. One constructor may delegate to another with this(...), which must be the very first statement in the body.

The keyword this is a reference to the object currently executing the method. Its everyday job is to un-shadow: when a parameter has the same name as an instance variable, the parameter wins inside the body, so this.title means the field and plain title means the parameter.

If you write no constructor, Java supplies a no-argument default that leaves every field at its zero value (0, 0.0, false, null). If you write any constructor, that free default disappears.