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Trail: Learning the Java Language
Lesson: Object Basics and Simple Data Objects

Using Objects

Once you've created an object, you probably want to use it for something. You may need information from it, want to change its state, or have it perform some action. You can use an object in one of two ways:

Referencing an Object's Variables

The following is the general form of a qualified name, which is also known as a long name:
You may use a simple name for an instance variable when the instance variable is in scope - that is, within code for the object's class. Code that is outside the object's class must use a qualified name. For example, the code in the CreateObjectDemo class is outside the code for the Rectangle class. So to refer to the origin, width, and height variables within the Rectangle object named rectOne, the CreateObjectDemo class must use the names rectOne.origin, rectOne.width, and rectOne.height, respectively. The program uses two of these names to display the width and the height of rectOne:
System.out.println("Width of rectOne: " + rectOne.width);
System.out.println("Height of rectOne: " + rectOne.height);
Attempting to use the simple names width and height from the code in the CreateObjectDemo class doesn't make sense—those variables exist only within an object—and results in a compiler error.

Later, the program uses similar code to display information about rectTwo. Objects of the same type have their own copy of the same instance variables. Thus, each Rectangle object has variables named origin, width, and height. When you access an instance variable through an object reference, you reference that particular object's variable. The two objects rectOne and rectTwo in the CreateObjectDemo program have different origin, width, and height variables.

The first part of the variable's qualified name, objectReference, must be a reference to an object. You can use the name of a reference variable here, as in the previous examples, or you can use any expression that returns an object reference. Recall that the new operator returns a reference to an object. So you could use the value returned from new to access a new object's variables:

int height = new Rectangle().height;
This statement creates a new Rectangle object and immediately gets its height. In essence, the statement calculates the default height of a Rectangle. Note that after this statement has been executed, the program no longer has a reference to the created Rectangle, because the program never stored the reference in a variable. The object is unreferenced, and its resources are free to be recycled by the Java platform.

A Word About Variable Access

The direct manipulation of an object's variables by other objects and classes is discouraged because it's possible to set the variables to values that don't make sense. For example, consider the Rectangle class from the previous section. Using that class, you can create a rectangle whose width and height are negative, which, for some applications, doesn't make sense.

Ideally, instead of allowing direct manipulation of variables, a class would provide methods through which other objects could inspect or change variables. These methods ensure that the values of the variables make sense for objects of that type. Thus, the Rectangle class would provide methods called setWidth, setHeight, getWidth, and getHeight for setting and getting the width and the height. The methods for setting the variables would report an error if the caller tried to set the width or the height to a negative number. The other advantage of using methods instead of direct variable access is that the class can change the type and the names of the variables it uses for storing the width and the height without affecting its clients.

However, in practical situations, it sometimes makes sense to allow direct access to an object's variables. For example, both the Point class and the Rectangle class allow free access to their member variables by declaring them public. This keeps these classes small and simple. Also, it keeps them generally useful. Some applications might allow rectangles with negative widths and heights.

The Java programming language provides an access control mechanism whereby classes can determine what other classes can have direct access to its variables. A class should protect variables against direct manipulation by other objects if those manipulations could result in values that don't make sense for objects of that type. Changes to these variables should be controlled by method invocations. If a class grants access to its variables, you can assume that you can inspect and change those variables without adverse effects. To learn more about the access control mechanism, refer to Controlling Access to Members of a Class (in the Learning the Java Language trail). Also, by making the variables accessible, they become part of the class's API, which means that the writer of the class should not change their names or their types.

Calling an Object's Methods

You also use qualified names to invoke an object's method. To form the qualified name of a method, you append the method name to an object reference, with an intervening period (.). Also, you provide, within enclosing parentheses, any arguments to the method. If the method does not require any arguments, use empty parentheses.
The Rectangle class has two methods: area to compute the rectangle's area and move to change the rectangle's origin. Here's the CreateObjectDemo code that invokes these two methods:
System.out.println("Area of rectOne: " + rectOne.area());
rectTwo.move(40, 72);
The first statement invokes rectOne's area method and displays the results. The second line moves rectTwo because the move method assigns new values to the object's origin.x and origin.y.

As with instance variables, objectReference must be a reference to an object. You can use a variable name, but you also can use any expression that returns an object reference. The new operator returns an object reference, so you can use the value returned from new to invoke a new object's methods:

new Rectangle(100, 50).area()
The expression new Rectangle(100, 50) returns an object reference that refers to a Rectangle object. As shown, you can use the dot notation to invoke the new Rectangleís area method to compute the area of the new rectangle.

Some methods, such as area, return a value. For methods that return a value, you can use the method invocation in expressions. You can assign the return value to a variable, use it to make decisions, or control a loop. This code assigns the value returned by area to a variable:

int areaOfRectangle = new Rectangle(100, 50).area();
Remember, invoking a method on a particular object is the same as sending a message to that object. In this case, the object that area is invoked on is the rectangle returned by the constructor.

A Word about Method Access

The methods in our Point and Rectangle classes are all declared public, so they are accessible to any other class. Sometimes, a class needs to restrict access to its methods. For example, a class might have a method that only subclasses are allowed to invoke. A class can use the same mechanism to control access to its methods as it uses to control access to its variables. To learn more about the access control mechanism, refer to Controlling Access to Members of a Class (in the Learning the Java Language trail).

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