As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from theCreateObjectDemo
program creates an object and assigns it to a variable:The first line creates an object of thePoint originOne = new Point(23, 94); Rectangle rectOne = new Rectangle(originOne, 100, 200); Rectangle rectTwo = new Rectangle(50, 100);Point
class, and the second and third lines each create an object of theRectangle
class.Each of these statements has three parts (discussed in detail below):
- Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
- Instantiation: The new keyword is a Java operator that creates the object.
- Initialization: The new operator is followed by a call to a constructor, which initializes the new object.
Declaring a Variable to Refer to an Object
Previously, you learned that to declare a variable, you write:This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.type name;You can also declare a reference variable on its own line. For example:
If you declarePoint originOne;originOne
like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use thenew
operator, as described in the next section. You must assign an object tooriginOne
before you use it in your code. Otherwise, you will get a compiler error.A variable in this state, which currently references no object, can be illustrated as follows (the variable name,
originOne
, plus a reference pointing to nothing):Instantiating a Class
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.
Note: The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:
The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:Point originOne = new Point(23, 94);
This statement will be discussed in the next section.int height = new Rectangle().height;Initializing an Object
Here's the code for the Point class:This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:public class Point { public int x = 0; public int y = 0; //constructor public Point(int a, int b) { x = a; y = b; } }The result of executing this statement can be illustrated in the next figure:Point originOne = new Point(23, 94);Here's the code for the Rectangle class, which contains four constructors: Each constructor lets you provide initial values for the rectangle's size and width, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:public class Rectangle { public int width = 0; public int height = 0; public Point origin; // four constructors public Rectangle() { origin = new Point(0, 0); } public Rectangle(Point p) { origin = p; } public Rectangle(int w, int h) { origin = new Point(0, 0); width = w; height = h; } public Rectangle(Point p, int w, int h) { origin = p; width = w; height = h; } // a method for moving the rectangle public void move(int x, int y) { origin.x = x; origin.y = y; } // a method for computing the area of the rectangle public int getArea() { return width * height; } }This calls one ofRectangle rectOne = new Rectangle(originOne, 100, 200);Rectangle
's constructors that initializesorigin
tooriginOne
. Also, the constructor setswidth
to 100 andheight
to 200. Now there are two references to the same Point object— an object can have multiple references to it, as shown in the next figure:The following line of code calls the Rectangle
constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:The Rectangle constructor used in the following statement doesn't take any arguments, so it's called a no-argument constructor:Rectangle rectTwo = new Rectangle(50, 100);All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or theRectangle rect = new Rectangle();Object
constructor if the class has no other parent. If the parent has no constructor (Object
does have one), the compiler will reject the program.