Blueprint and objects

A class is a blueprint; an object is a thing built from it. The blueprint declares fields (what it knows) and methods (what it does):

public class Book {
    private String title;
    private int pages;

    public Book(String t, int p) {     // constructor
        title = t;
        pages = p;
    }

    public String getTitle() {         // accessor (getter)
        return title;
    }

    public boolean isLong() {          // behavior
        return pages > 300;
    }
}

new Book("Dune", 412) builds one object: allocates it, runs the constructor, returns a reference to it. Store the reference in a variable:

Book b = new Book("Dune", 412);
b.getTitle()      // "Dune"
b.isLong()        // true

The dot reaches into the object: b.title would work in the same class, but from outside, title is private — unreachable. That is encapsulation: state is private, access goes through methods.

this names the current object. When a parameter shadows a field, this. disambiguates: this.title = title;