More Than One Constructor Method
When designing your object classes you are not limited to only using one constructor class. You might decide there are a couple of ways an object can be initialized. The only constraint on using more than one constructor method is that they must differ in the parameters they accept.
Imagine that at the time we create the Person object we might not know the username we want to give to a person. Let's add a new constructor method that sets the state of the Person object using only the firstname, lastname and address:
public class Person {
private String firstName;
private String lastName;
private String address;
private String username;
// The constructor method
public Person(String firstName, String lastName, String address, String username)
{
this.firstName = firstName;
this.lastName = lastName;
this.address = address;
this.username = username;
}
// The new constructor method
public Person(String firstName, String lastName, String address)
{
this.firstName = firstName;
this.lastName = lastName;
this.address = address;
this.username = "";
}
// A method to display the state of the object to the screen
public void displayPersonDetails()
{
System.out.println("Name: " + firstName + " " + lastName);
System.out.println("Address: " + address);
System.out.println("Username: " + username);
}
}
Note that the second constructor method is also called "Person" and it also does not return a value. The only difference between it and the first constructor method is the parameters – this time it is only expecting three string values: firstName, lastName and address.
We can now create Person objects in two different ways:
public class PersonExample {
public static void main(String[] args) {
Person dave = new Person("Dave", "Davidson", "12 Pall Mall", "DDavidson");
Person jim = new Person("Jim","Davidson", "15 Kings Road");
dave.displayPersonDetails();
jim.displayPersonDetails();
}
}
dave will be created with a state of: firstname = "Dave", lastname = "Davidson", address = "12 Pall Mall", username = "DDavidson". Jim will not get a username and instead will have a state of: firstname = "Jim", lastname = "Davidson", address = "15 Kings Road", username = "".
A Quick Recap
Constructor methods are only called when a new instance of an object is created. They:
- must have the same name as the class
- do not return a value
- can have none, one, or many parameters
- can number more than one as long as each constructor method has a different set of parameters
- can have parameter names the same as the private fields as long as the "this" keyword is used
- are called using the "new" keyword
