In today's episode, we will focus on a particular constructor : the copy constructor. Thus, we will check what happens when an object is initialized with a copy of another object of the same class. Indeed, Java lets us create instance copies such as here, for example. Here, we have a first "r1" instance of the "Rectangle" class which we initialize through a call to a constructor taking two values. This leads to the following schema. "r1" is initialized to the values 12.3 and 24.5 Then, we have another "r2" instance of the "Rectangle" class. r2 is initialized with a copy of r1. We will thus copy r1's height into r2 and copy r1's width into r2. However, r1 and r2 and two separated instances; r2 has simply been initialized with a copy of r1. The copy constructor is tasked with this kind of initializations. The copy constructor is tasked with initializing an instance with copy of another instance of the same class. Its header is perfectly defined since it is a constructor. It has thus the same name as the name of class. And since it receives another instance of the same class, its parameter list is reduced to one single parameter : another instance of the same class. For example, in the case our "Rectangle" class, we would have a constructor, which is obviously in the interface of the "Rectangle" class. This constructor is thus called "Rectangle". Also, it receives, as its unique parameter, another rectangle, another instance of the "Rectangle" class. This instance would, for example, be the copy of the height of the rectangle received as parameter into the height of of the instance we are currently initializing. And similarly, a copy of the other rectangle's width into the width of the instance we are currently initializing. Let us go back to the previous example where we had an r1 rectangle with a height of 12.3 and a width of 24.5 and an r2 rectangle initialized with the copy constructor. In this line we indeed have a call to the copy constructor. Here, the construction of r2 will trigger a call to the copy constructor passing, as parameter, r1 which is here "autre rectangle" (TN: means "other rectangle"). In this case, it is indeed r1 passed to the copy constructor of r2. What happens is that, here, the height of r2 will receive the height of r1 in copy. Through this line, we will copy the height of r1 into the width of r2. In the following line, what happens is that we will copy the width of r1 (the rectangle argument) into the width of r2 (the rectangle we are currently constructing). Here, we copy the width of r1 into the width of r2. By the way please note that, unlike other languages, Java does not provide us with a default version of the copy constructor. This means that, if we do not do anything particular we are not allowed to do copies, like these. This constructor does not exist. Therefore, if we want to do copy constructions, we need to write it explicitly. Also, the copy constructor is not the only way to do copies in Java. There is another way which is more widely used. It is the "clone" method, as you can see here. Through a call to the "clone" method, we will clone the r1 instance which we had previously initialized with a usual constructor. We will clone it into r2, upon the declaration-initalization of the r2 instance of the "Rectangle" class. This "clone" method will be detailed later in another video sequences during the upcoming weeks. This concludes this short episode on copy constructors in Java.
