Читать книгу OCP Oracle Certified Professional Java SE 17 Developer Study Guide - Jeanne Boyarsky - Страница 87

To create an instance of a class, all you have to do is write new before the class name and add parentheses after it. Here's an example:

Park p = new Park();

First you declare the type that you'll be creating (Park) and give the variable a name (p). This gives Java a place to store a reference to the object. Then you write new Park() to actually create the object.

Park() looks like a method since it is followed by parentheses. It's called a constructor, which is a special type of method that creates a new object. Now it's time to define a constructor of your own:

public class Chick { public Chick() { System.out.println("in constructor"); } }

There are two key points to note about the constructor: the name of the constructor matches the name of the class, and there's no return type. You may see a method like this on the exam:

public class Chick { public void Chick() { } // NOT A CONSTRUCTOR }

When you see a method name beginning with a capital letter and having a return type, pay special attention to it. It is not a constructor since there's a return type. It's a regular method that does compile but will not be called when you write new Chick().

The purpose of a constructor is to initialize fields, although you can put any code in there. Another way to initialize fields is to do so directly on the line on which they're declared. This example shows both approaches:

public class Chicken { int numEggs = 12; // initialize on line String name; public Chicken() { name = "Duke"; // initialize in constructor } }

For most classes, you don't have to code a constructor—the compiler will supply a “do nothing” default constructor for you. There are some scenarios that do require you to declare a constructor. You learn all about them in Chapter 6.