Question

I cannot grasp the usage of those constructors.

I understand that one is as follows:

public book(){
private string author;
private string title;
private int reference;
}

And a parametrised constructor is as follows:

public book( string author, string title, int reference){ 
}

However how would this be used in a main method?

Answer 1

There are three ways you can have a constructor:

1) You declare a no-parameter one

.

public class book
{
  string author;
  string title;
  int reference;
  public book() {
    // initialize member variables with default values
    // etc.
  }
}

2) You declare one with parameters

.

public class book
{
  string author;
  string title;
  int reference
  public book(string author, string title, int reference) {
    // initialize member variables based on parameters
    // etc.
  }
}

3) You let the compiler declare one for you — this requires you do not declare one of your own. In this case the member variables will be provided a default value based on their type (essentially their no-parameter constructor called)

You can mix options 1) and 2), but 3) is stand-alone (cannot mix it with any of the other two). You can also have more than one constructors with parameters (the parameter types/numbers must be different)

An example use:

public static void main(String[] args) {
  String author = ...;
  String title = ...;
  int ref = ...;
  book book1 = new book(); // create book object calling no-parameter version
  book book2 = new book(author, title, ref); // create book object calling one-parameter
                                // (of type String) version
}