Comp 201 - Lab 03: Java Syntax, UML Go to... Home Information Resources Exam Resources OwlSpace site Comp201: Principles of Object-Oriented Programming I Spring 2008 -- Lab 03: Java Syntax, Calendar Class, StarUML This lab will cover: using StarUML to create UML class diagrams basic Java syntax and exercises on syntax, usage of the class Calendar, a predefined abstract class, and one of its concrete subclass, namely the GregorianCalendar class. StarUML Please see the tutorial on using StarUML. Summary of Java Syntax; Using the Calendar class From now on, we will work with the Advanced language level. So, run DrJava and select Language Level/Advanced. I. Java Syntax (for Advanced Level) A Java program consists of one or more classes/interfaces. Comments syntax: // Line-oriented - comment goes to end of the current line. /* block-oriented can span several lines. */ Class definition syntax: [
] means optional. [public] class class-name [inheritance-specification] { [field-list] [constructor-list] [method-list] } In the above, class-name must be a string of one or more alpha-numeric character starting with an alphabet character. For example, Agent007Bond is a legal class name, while 007Bond is not a legal class name. Java is case-sensitive, meaning it distinguishes upper cases from lower cases. For example, agent007 is distinct from Agent007. A Java source (i.e. code) file can have one or more classes. However, only one class in a file can be public. The name of the file must be identical to the name of the public class it contains. Field list syntax: A field list consists of zero or more field declarations of the form [static] [final] [public | private | protected] field-type field-name [assignment]; Examples: private int _width; private double _radius = 5.0; Constructor list syntax: A constructor list consists of zero or more constructor definitions of the form [public | private | protected] class-name ([parameter-list]){ [statement-list;] } NOTE: The constructor's name is the same as the class name. Constructors are used for initialization of the object during the object's instantiation only. If you do not define a constructor, by default the compiler will generate a "default" constructor that takes no parameters at all. This is to make it possible for you to instantiate (by calling new on the constructor). The Elementary, Intermediate and Advanced Language levels of DrJava will automatically create a constructor for you that takes as many input parameters as necessary to initialize all the fields of a class. Do not worry about the access specifiers private and protected for now. Examples: public Rectangle(double width, double height) { _width = width; _height = height; } public Nothing() { // no code at all! } Java Statement syntax: NOTE: Each Java statement must terminate with a semi-colon. Method list syntax: A method list consists of zero or more method definitions of the form [static] [final] [public | private | protected]return-type method-name([param-list]) { [statement-list;] } A return type void means the method does not return any value. param-list looks like: type1 param1, type2 param2,
, typeN paramN Examples: public double get Area () { return _width * _height; } public double circleArea(double radius) { return Math.PI * radius * radius; } To call a method on an object, use the object variable's name followed by a period and then the method name with parentheses surrounding the input parameter values, if any: aRectangleObject.getArea(); // you still need the parentheses even if there are no input parameters! // That's how Java knows its a method and not a field. aGeometryObject.circleArea(42.94); // no type identifier used! 42.94 is a double already. Syntax Exercise: Click here to download a zip file containing four Java source files: IFruit.dj2, Mango.dj2, Lemon.dj2, and TestFruit.dj2. These are all Advanced Level files. Unzip the file into your lab03 directory and use DrJava to open all of them. Try to compile all! You will see a multitude of error messages, many of which are perhaps unintelligible. This may appear overwhelming to the uninitiated. One way to minimize the number of error messages is to compile one file at a time starting from the top of the hierarchy or with the file that can make sense alone and does not depend on the other files. When you are developing a program, always make sure the current file you are working on compiles before moving on to a new file. Later, you will be taught how to use the project facility to manage programs with multiple files. To compile a single file, right-click the filename and select "Compile Current Document". This option is also on the main menu under "Tools". So first, compile IFruit.dj2. After you fix all the errors there, go on to Mango.dj2 and fix all the bugs there. Then compile Lemon.dj2 and fix all the bugs there. Finally compile TestFruit.dj2 and fix all the bugs there. II. Using the Calendar Class The purpose of this exercise is to learn how to use existing classes to perform certain useful tasks. Basically, it entails reading the documentation, called the API, and try out calling a few methods using DrJava. The classes we choose are Calendar and GregorianCalendar. They will prove to be useful for homework #3. Go to the course main web page and find and click the link to the Java Resource Web Site (in the Additional Resources section). From this page find and click the link to the Java 2 API. From this page find and click the link to the class Calendar in the All Classes pane on the left hand side. You should see the documentation for Calendar . Calendar is an abstract class used to represent and perform common computations on date and time. The people who develop Java have written the code to carry out most of the low-level computations, but have left a few methods "abstract" (i.e. with no code body) for the user to inherit and implement. This way, you can re-use the existing code without having to know anything about them. You will learn more about abstract class in the next lecture. The Java developers also have provided us with a concrete subclass of Calendar called GregorianCalendarto represent the standard calendar that we (and most of the world) are using these days. At the top level description of Calendar, you should see a description of direct known subclasses and the class GregorianCalendarlisted there. Click on this link to see the documentation for GregorianCalendar. Packages in Java: Packages in Java are a way to organize many different classes into manageable subsets. Packages are set up as directories (folders). For instance the package "java.util" is a subdirectory (subfolder) called "util" in the subdirectory "java". Technically, in Java, the true name of a class, its "fully qualified class name", is the class name plus its package, e.g. java.util.Calendar. We will discuss packages in more detail later. Use the Interactions pane of DrJava to explore the Calendar and GregorianCalendar classes. To use the above classes, you must enter the following statement: import java.util.*; To instantiate a GregorianCalendar object containing today's date, enter Calendar today = new GregorianCalendar(); Now enter: today.getTime() What do you see? Enter: today.get(Calendar.YEAR) today.get(Calendar.MONTH) today.get(Calendar.DATE) Can you see what it doing? Here is how you create a GregorianCalendar object that corresponds to March 03, 2000: Calendar mar03_02 = new GregorianCalendar(2000, Calendar.MARCH, 3); Now you can check the time, year, month and date of mar03_02 to see if it contains the information that you have wanted to record. Try these also: Calendar.JANUARY Calendar.DECEMBER today.after(mar03_02) today.before(mar03_02) The only way you deal with a Calendar object is to call on the public methods or to access the public fields (such as Calendar.YEAR). To paraphrase a famous quote: Ask not what you can do with an Object, ask what the Object can do for you! You will see the use of the Calendar and GregorianCalendar class in homework #3. Last Revised Thursday, 03-Jun-2010 09:50:27 CDT ©2008 Stephen Wong and Dung Nguyen
