FAQ: Classes & Objects

Transcription

1 Question 1: How do I define a class as a data type? Answer 1: Data types in Java can be simple data types such as integers and floating point numbers. Data types can also be complex, collecting many different types of data. A class in Java is a data type that can gather different types of data. Classes also have methods that contain executable code that manages the data in the class. This is best understood with an example. Suppose that you were going to write a Television simulation program and you needed to declare a Television class. Think about the obvious attributes and functionality of a television, known as methods in Java. The obvious attributes, or instance variables, that a television possesses are the following: screen volume control tuner (for changing channels) The methods of the Television reflect what a Television does, which is its behaviors. The following are behaviors: changing channels adjusting volume displaying channel contents (to watch a show and commercials) Question 2: How do I define an object as an instance of a class? Answer 2: Variables in Java are similar to variables in algebra they are a symbolic name or a place holder for a value. In Java and other programming languages, a variable is a symbolic name for the number of bytes of computer memory needed to hold data. For example, the Java declaration of an int variable type tells the Java program to reserve 4 bytes of storage to hold a simple integer. As another example consider the following Java declaration: 1

2 int fourbytes = 8; This tells the Java program to reserve 4 bytes of memory (storage) for a variable called fourbytes and to set the value of those 4 bytes to the value of 8 or 0008, showing that the first 3 bytes are set to 0 and the least significant byte is 8. Classes in Java are data types, similar to how ints are data types. Java classes can contain many different types of data, including the humble int. If int is a data type and fourbytes is a variable of type int, then for a Television class and variable tv the following is true: Television tv = new Television(); The tv variable, also known as an object or an instance of a television, will be the symbolic name for all of the bytes for all of the data members declared in a Television class. For example, the Television class can be declared as follows: class Television{ VolumeControl vc = null; //holds 4 bytes for a VolumeControl reference Tuner tuner = null; //holds 4 bytes enough space for Tuner reference int[ ] screenpixels = new int[500*1000][1000*1000]; The statement is executed in a Java program: Television tv = new Television(); This tells the Java program to reserve 4 bytes for a VolumeControl object, another 4 bytes for a TunerObject, and enough bytes for a bidimensional array (e.g., matrix) that has 500,000 rows and 1,000,000 columns to reserve for the pixels on the screen. The Java program must reserve this space someplace in memory. That is what the new operator does. New says to the Java program, Go get me enough bytes to make a Television object based on the layout of the Television class. Java returns the address of those bytes in memory (e.g., 5000) after new Television() executes and stores it in the variable or instance or object of type Television, tv. Similar to the way that fourbytes is an instance or variable of an int, tv is now an instance of a Television. 2

3 Question 3: How do I declare a class with an instance variable? What is the corresponding memory layout? Answer 3: public class Person{ // declares the class and has the beginning brace public String name="joe Black"; public static void main(string[] args){ Person aperson = new Person(); //print the Person s name System.out.println("Name is "+ aperson.name); Address of Person Object & Data Name Variable 1000 J 1002 o 1004 e B 1010 l 1012 a 1014 c 1015 k Question 4: How do I declare four classes with one class containing the other three classes as instance variables? Answer 4: Due to the fact that we want to have the Television class to contain objects representing the Tuner, Volume Control, and Screen, it will be a good idea to define them before we define the Television class itself. 3

4 //Declare a minimal Tuner class class Tuner { /** Creates a new instance of Tuner */ public Tuner() { public void changechannel(){ //Declare a minimal Screen class class Screen { private int[][] screen = new int[500*1000][1000*10000]; /** Creates a new instance of Screen */ public Screen() { //Declare a minimal VolumeControl Class class VolumeControl { /** Creates a new instance of VolumeControl */ public VolumeControl() { public void adjustvolume(){ //Declare a minimal Television class with a main method to //make a Television object public class Television { //Declares the instance variables maintained in the Television class Screen screen= null; //Empty screen variable. Tuner tuner = null; //Empty tuner variable. VolumeControl volumecontrol = null; //Empty volume control variable. 4

5 /** Creates a new instance of Television when the constructor is called*/ public Television() { System.out.println("Television Object Created"); screen= new Screen(); //screen now refers to an object of type Screen tuner = new Tuner(); //tuner now refers to an object of type Screen volumecontrol = new VolumeControl();//volumeControl refers to a VolumeControl object //end constructor public static void main(string[] args){ //call the Television Constructor method Television tv = new Television(); //create Television Object and store it in tv //end Television All of the preceding classes could be copied and pasted into a single Java file Television.java. They then can be compiled and executed. 5

6 Question 5: How do you draw a UML diagram that represents four classes with one class containing the other three classes as instance variables? Answer 5: Use a UML drawing tool, such as Sun Java Enterprise, to draw the UML diagram: The diagram shows that the Television class has variables for Tuner, VolumeControl, and Screen as shown in the Java code. The line with the diamond at the Television end is the UML graphical language that says that Television is composed by one of each of the other three classes. Question 6: What is inheritance, and how is it shown in a UML diagram? Answer 6: 6

7 Inheritance in object-oriented programming and Java is much like inheritance in human beings. As humans, we inherit attributes and behaviors from our parents. In Java, classes can inherit attributes (instance variables) and behaviors (methods) from parent classes. Suppose the code for Television class is declared as follows: //Declare a minimal Television Class with a main method to //make a Television Object public class Television { //Declare the instance variables maintained in the Television Class Screen screen= null; //Empty screen variable. Tuner tuner = null; //Empty tuner variable. VolumeControl volumecontrol = null; //Empty volume control variable. /** Creates a new instance of Television when the constructor is called*/ public Television() { System.out.println("Television Object Created"); screen= new Screen(); //screen now refers to an object of type Screen tuner = new Tuner(); //tuner now refers to an object of type Tuner volumecontrol = new VolumeControl();//volumeControl refers to a VolumeControl object //end constructor public static void main(string[] args){ //call the Television Constructor method Television tv = new Television(); //create Television Object and store it in tv //end Television Then, suppose that the code for the PortableTelevision class is declared as follows: public class PortableTV extends Television { int numberofbatteries; //A portable TV adds batteries to a normal TV public PortableTV() { super(); //call the parent Television constructor numberofbatteries=8; 7

8 public int getnumberofbatteries() { return numberofbatteries; public void setnumberofbatteries(int val) { this.numberofbatteries = val; Due to the fact PortableTV inherits from, or extends, Television, an object of type PortableTelevision can use the Screen, Tuner, and VolumeControl objects that it gets from its parent Television object. The PortableTV is a specialization of a normal Television because it adds a battery power supply represented by the numberofbatteries with get and set methods, getnumberofbatteries and setnumberofbatteries, respectively. When the following line of code executes, PortableTV pt = new PortableTV(); the PortableTV constructor is called. public PortableTV() { super(); //call the parent Television constructor numberofbatteries=8; When executing code in the constructor, the first thing that happens in a constructor of the inheriting class is that it calls the constructor of the parent (or superclass). In this case, this causes the inherited Television part of PortableTV to be created, which causes the Screen, VolumeControl, and Tuner objects to be created. See the code. The PortableTelevision constructor then can attend to setting the numberofbatteries variable to the value of 8. Given the preceding code, it is time to show a UML diagram that shows PortableTV inheriting from Television. 8

9 (UML Modeling, n.d.) The UML graphical language uses the open arrow from PortableTV to Television to indicate that PortableTV inherits from or extends Television. Reference UML modeling: Reverse engineering Java applications. (n.d.). Retrieved November 13, 2006, from Sun Microsystems Web site: ml_re.html#javaproject 9