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Although Strings are objects, they are not very interesting objects, because
In this chapter, we are going to use two new object types that are part of the Java language, Point and Rectangle. Right from the start, I want to make it clear that these points and rectangles are not graphical objects that appear on the screen. They are variables that contain data, just like ints and doubles. Like other variables, they are used internally to perform computations.
The definitions of the Point and Rectangle classes are in the java.awt package, so we have to import them.
The built-in Java classes are divided into a number of packages, including java.lang, which contains almost all of the classes we have seen so far, and java.awt, which contains classes that pertain to the Java Abstract Window Toolkit (AWT), which contains classes for windows, buttons, graphics, etc.
In order to use a package, you have to import it, which is why the program in Section 4.8 starts with import java.awt.*. The * indicates that we want to import all the classes in the AWT package. If you want, you can name the classes you want to import explicitly, but there is no great advantage. The classes in java.lang are imported automatically, which is why most of our programs haven't required an import statement.
All import statements appear at the beginning of the program, outside the class definition.
At the most basic level, a point is two numbers (coordinates) that we treat collectively as a single object. In mathematical notation, points are often written in parentheses, with a comma separating the coordinates. For example, (0, 0) indicates the origin, and (x, y) indicates the point x units to the right and y units up from the origin.
In Java, a point is represented by a Point object. To create a new point, you have to use the new command:
Point blank;The first line is a conventional variable declaration: blank has type Point. The second line is kind of funny-looking; it invokes the new command, specifies the type of the new object, and provides arguments. It will probably not surprise you that the arguments are the coordinates of the new point, (3, 4).
The result of the new command is a reference to the new point. I'll explain references more later; for now the important thing is that the variable blank contains a reference to the newly-created object. There is a standard way to diagram this assignment, shown in the figure.
As usual, the name of the variable blank appears outside the box and its value appears inside the box. In this case, that value is a reference, which is shown graphically with a dot and an arrow. The arrow points to the object we're referring to.
The big box shows the newly-created object with the two values in it. The names x and y are the names of the instance variables.
Taken together, all the variables, values, and objects in a program are called the state. Diagrams like this that show the state of the program are called state diagrams. As the program runs, the state changes, so you should think of a state diagram as a snapshot of a particular point in the execution.
The pieces of data that make up an object are sometimes called components, records, or fields. In Java they are called instance variables because each object, which is an instance of its type, has its own copy of the instance variables.
It's like the glove compartment of a car. Each car is an instance of the type "car," and each car has its own glove compartment. If you asked me to get something from the glove compartment of your car, you would have to tell me which car is yours.
Similarly, if you want to read a value from an instance variable, you have to specify the object you want to get it from. In Java this is done using "dot notation."
int x = blank.x;The expression blank.x means "go to the object blank refers to, and get the value of x." In this case we assign that value to a local variable named x. Notice that there is no conflict between the local variable named x and the instance variable named x. The purpose of dot notation is to identify which variable you are referring to unambiguously.
You can use dot notation as part of any Java expression, so the following are legal.
System.out.println (blank.x + ", " + blank.y);The first line prints 3, 4; the second line calculates the value 25.
You can pass objects as parameters in the usual way. For example
public static void printPoint (Point p) {is a method that takes a point as an argument and prints it in the standard format. If you invoke printPoint (blank), it will print (3, 4). Actually, Java has a built-in method for printing Points. If you invoke System.out.println (blank), you get
java.awt.Point[x=3,y=5]This is a standard format Java uses for printing objects. It prints the name of the type, followed by the contents of the object, including the names and values of the instance variables.
As a second example, we can rewrite the distance method from Section 5.2 so that it takes two Points as parameters instead of four doubles.
public static double distance (Point p1, Point p2) {The typecasts are not really necessary; I just added them as a reminder that the instance variables in a Point are integers.
Rectangles are similar to points, except that they have four instance variables, named x, y, width and height. Those names should sound familiar; they are the names of the variables we used to specify bounding boxes for the Mickey Mouse fractal in Section 4.14.
Other than that, everything is pretty much the same.
Rectangle box = new Rectangle (0, 0, 100, 200);creates a new Rectangle object and makes box refer to it. The figure shows the effect of this assignment.
If you print box, you get
java.awt.Rectangle[x=0,y=0,width=100,height=200]Again, this is the result of a built-in Java method that knows how to print Rectangle objects.
You can write methods that return objects. For example, findCenter takes a Rectangle as an argument and returns a Point that contains the coordinates of the center of the Rectangle:
public static Point findCenter (Rectangle box) {Notice that you can use new to create a new object, and then immediately use the result as a return value.
You can change the contents of an object by making an assignment to one of its instance variables. For example, to "move" a rectangle without changing its size, you could modify the x and y values:
box.x = box.x + 50;The result is shown in the figure:
We could take this code and encapsulate it in a method, and generalize it to move the rectangle by any amount:
public static void moveRect (Rectangle box, int dx, int dy) {The variables dx and dy indicate how far to move the rectangle in each direction. Invoking this method has the effect of modifying the Rectangle that is passed as an argument.
Rectangle box = new Rectangle (0, 0, 100, 200);prints java.awt.Rectangle[x=50,y=100,width=100,height=200].
Modifying objects by passing them as arguments to methods can be useful, but it can also make debugging more difficult because it is not always clear which method invocations do or do not modify their arguments. Later, I will discuss some pros and cons of this programming style.
In the meantime, we can enjoy the luxury of Java's built-in methods, which include translate, which does exactly the same thing as moveRect, although the syntax for invoking it is a little different. Instead of passing the Rectangle as an argument, we invoke translate on the Rectangle and pass only dx and dy as arguments.
box.translate (50, 100);The effect is exactly the same.
Remember that when you make an assignment to an object variable, you are assigning a reference to an object. It is possible to have multiple variables that refer to the same object. For example, this code:
Rectangle box1 = new Rectangle (0, 0, 100, 200);generates a state diagram that looks like this:
Both box1 and box2 refer or "point" to the same object. In other words, this object has two names, box1 and box2. When a person uses two names, it's called aliasing. Same thing with objects.
When two variables are aliased, any changes that affect one variable also affect the other. For example:
System.out.println (box2.width);The first line prints 100, which is the width of the Rectangle referred to by box2. The second line invokes the grow method on box1, which expands the Rectangle by 50 pixels in every direction (see the documentation for more details). The effect is shown in the figure:
As should be clear from this figure, whatever changes are made to box1 also apply to box2. Thus, the value printed by the third line is 200, the width of the expanded rectangle. (As an aside, it is perfectly legal for the coordinates of a Rectangle to be negative.)
As you can tell even from this simple example, code that involves aliasing can get confusing fast, and it can be very difficult to debug. In general, aliasing should be avoided or used with care.
When you create an object variable, remember that you are creating a reference to an object. Until you make the variable point to an object, the value of the variable is null. null is a special value in Java (and a Java keyword) that is used to mean "no object."
The declaration Point blank; is equivalent to this initialization
Point blank = null;and is shown in the following state diagram:
The value null is represented by a dot with no arrow.
If you try to use a null object, either by accessing an instance variable or invoking a method, you will get a NullPointerException. The system will print an error message and terminate the program.
Point blank = null;On the other hand, it is legal to pass a null object as an argument or receive one as a return value. In fact, it is common to do so, for example to represent an empty set or indicate an error condition.
In Section 8.9 we talked about what happens when more than one variable refers to the same object. What happens when no variable refers to an object? For example:
Point blank = new Point (3, 4);The first line creates a new Point object and makes blank refer to it. The second line changes blank so that instead of referring to the object, it refers to nothing (the null object).
If no one refers to an object, then no one can read or write any of its values, or invoke a method on it. In effect, it ceases to exist. We could keep the object in memory, but it would only waste space, so periodically as your program runs, the Java system looks for stranded objects and reclaims them, in a process called garbage collection. Later, the memory space occupied by the object will be available to be used as part of a new object.
You don't have to do anything to make garbage collection work, and in general you will not be aware of it.
There are two kinds of types in Java, primitive types and object types. Primitives, like int and boolean begin with lower-case letters; object types begin with upper-case letters. This distinction is useful because it reminds us of some of the differences between them:
There is one other difference between primitives and object types. You cannot add new primitives to the Java language (unless you get yourself on the standards committee), but you can create new object types! We'll see how in the next chapter.
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