Java in a Nutshell, 5th Edition

Creating and Initializing Objects

Now that we’ve covered fields and methods, we move on to other important members of a class. Constructors and initializers are class members whose job is to initialize the fields of a class.

Take another look at how we’ve been creating Circle objects:

Circle c = new Circle();

What are those parentheses doing there? They make it look like we’re calling a method. In fact, that is exactly what we’re doing. Every class in Java has at least one constructor , which is a method that has the same name as the class and whose purpose is to perform any necessary initialization for a new object. Since we didn’t explicitly define a constructor for our Circle class in Example 3-1, Java gave us a default constructor that takes no arguments and performs no special initialization.

Here’s how a constructor works. The new operator creates a new, but uninitialized, instance of the class. The constructor method is then called, with the new object passed implicitly (a this reference, as we saw earlier) as well as whatever arguments that are specified between parentheses passed explicitly. The constructor can use these arguments to do whatever initialization is necessary.

Defining a Constructor

There is some obvious initialization we could do for our circle objects, so let’s define a constructor. Example 3-2 shows a new definition for Circle that contains a constructor that lets us specify the radius of a new Circle object. The constructor also uses the this reference ...