Transcript MT311-1
MT311 Java Application Development and Programming Languages Li Tak Sing(李德成) Contact information Email: [email protected] Office: Rm A0936 Tel: 27686816 Course Web Site: http://plbpc001.ouhk.edu.hk/~mt311f Assignment submission: http://plbpc001.ouhk.edu.hk/tma/login.htm Continuous assessment Three assignments each carries 20% of the final continuous assessment score. One test that carries 20% of the final continuous assessment score. 20% comes from the work done during tutorials. According to OUHK rules, you need to pass the continuous assessment in order get a pass in the course. Software Netbeans and JDK 5.0 MIT Scheme SWI Prolog Textbooks Java Programming: Advanced Topics, by Joe Wigglesworth and Paula McMillan, 3rd Edn, Thomson Concepts of Programming Languages, by Robert Sebesta, 6th Edn, Addison Wesley, 2003. Overview of the course This course have two modules – – Java Application development Programming Languages Java Application Development Object-oriented Programming in Java Graphical User Interface Network Programming with Java Java Security Programming Languages Introduction to Programming Languages Variables, Data Types, and Expressions Control Structures, Subprograms, and Implementation of Subprograms Object-Oriented Programming Languages Functional and Logic Programming Languages Object-Oriented Programming in Java Object-oriented programming has the following characteristics – – – – Abstract data type Encapsulation Polymorphism Inheritance Tutorials You will be given a tutorial sheet each time detailing what you have to do. You need to write Java programs in the first semester. You will use Netbeans as the IDE. You need to bring either a floppy or a USB drive to store your works. You need to upload your work to the web site at http://plbpc001.ouhk.edu.hk/~mt311f Abstract data type Abstraction: to describe an object using the most important information only. For example, there are thousands of components of a car. However, if you are asked to describe a car, you would not describes it in terms of all of its component. Rather, you would just select a few important components to describe it. For example, a car is something that has wheels, steering wheels, engine and seats. Java abstract data type public class car { Wheel wheel[]=new Wheel[4]; SteeringWheel steeringWheel; Engine engine; } This is an abstract data type of a car The car is described in terms of three of its main components: wheel, steering wheel and the engine. Java programs are collections of classes All program codes are stored inside classes Java is a strong typing language which means that all errors regarding typing will be reported. You also need to declare the type of all entities before they can be used. Java abstract data type A bank account public class Account { int balance; } This class now only contains data, no program code. This class contains one field called balance which is an integer. Usually when you define a class, you also define some procedures to manipulate the attributes of the class. They are called methods in OO terms. For example, we can have two methods, one to get the balance, the other to set the balance. Bank account with setter and getter public class Account { int balance; void setBalance(int b) { balance=b;} int getBalance() {return balance;} } For method setBalance, a parameter is passed into the method which is an integer named b. In fact, the method also have access to another entity, which is the bank account itself. Each method of a class is associated with an instance of the class. So in the code inside the method setBalance, we have accessed b, which is passed as a parameter, and we have accessed balance, which an attribute of the instance of Account. For method getBalance, it does not have any parameter, so the code inside getBalance can only access the content of the associated Account. A program that uses Account ..... Account account=new Account(); //create a new account account.setBalance(100); //set the balance to 100 System.out.println(account.getBalance()); //print the //balance .... In the above code, a new instance of Account, account, is created. Then the setBalance method of account is called with 100 as the parameter. Then, 100 is assigned to the balance attribute of account. 'this' Every method of a class is associated with an instance of that class. For example, in the method setBalance of Account, we can access the attributes of the object that is associated with the method. However, what can we do to access the object itself instead of just an attribute? 'this' We can do that by using the 'this' keyword. The setBalance method can be rewritten here using the 'this' keyword: void setBalance(int b) { this.balance=b;} Usually, we do not need the 'this' keyword if we just want to access the attributes of the associated object. 'this' You need to use 'this' in the following situation: – there is a name conflict so that you want. example: public class ABC { int a; void meth(int a) { this.a=a; } } 'this' – You want to pass the associated object to another method: public class ABC { ... void method(..) { DEF def=new DEF(); def.meth(this); ... } } Static attributes Attributes model the properties of an instance of a class. However, there are properties that are those of the whole class, not just an instance. For example, assume that we have a class for OUHK student. Static attributes The attribute name, birthday are of course properties of a particular instance of the class Student. However, the average age of all students is not a property of any particular student. Instead it is the property of all students. In Java term, we can say that the average age of all students is the property of the class Student. Static attributes The property of a class is defined as static attribute using the static keyword: public class Student { String name; Date birthday; static int averageAge; } Static attributes A static attribute is the property of a class, not that of a particular instance. So it is referenced using the name of the class: .... Student student=new student(); student.name="Chan Tai Man"; Student.averageAge=23; .... Static attributes Note that it is ok to access a static attribute through either an object or the class. So both of the following two statements are correct: student.averageAge=24; Student.averageAge=32; Static attributes However, even if you access a static attribute using an instance of the class, the accessed attribute is still nothing to do with that instance. The name of the class is not needed if you want to access a static attribute of a class inside a method of the same class. Static attributes public class ABC { static int def; public void meth() { def=4; } } The static attribute def of ABC is referenced inside the method meth of ABC. There is no need to specify the class name ABC here. Static attributes However, if you like, you can replace def in the above program by any of the followings: – – this.def (This is only valid if it is used inside a method of ABC) ABC.def (This is valid anywhere in your program) Static methods Just like static attributes, we can also have static methods. A static method can access all static attributes of the class. A static method cannot access non-static attributes as the static method does not have an associated instance. Static methods Just like static attributes, a static method should be accessed through the name of the class. But you can still access it through the name of an object or using 'this' or even without any qualifier if inside a method of the same class. However, in all cases, the static method is still not associated with any instance of the class. Static methods public class Student { String name; Date birthday; static int averageAge; static void setAverageAge(int i) { averageAge=i; } } Static methods The following code contains errors public class Student { String name; Date birthday; static int averageAge; static void setName(String n) { name=n; } } Static methods However, the following code is correct: public class Student { String name; Date birthday; static int averageAge; static void setName(Student s, String n) { s.name=n; } } A static method cannot directly access nonstatic attributes of the class. But it can access it through an instance of the class. Main program A main program in Java must be a public and static method of a class in the following form: public class Student { .... public static void main(String st[]) { .... } } Main method Since a main method is a static method, it can only access static attributes and methods of the class. If you want to access non-static attributes, you need to create an instance of the class first and then access them through the instance. Main method public class Student { String name; Date birthday; static int averageAge; public static main(String st[]) { Student student=new Student(); student.name="Li Tak Sing"; averageAge=23; student.averageAge=23; Student.averageAge=23; } } Compile a Java program The class must be stored in a file called Student.java compile using the command javac Student.java A file Student.class will be generated To run the program, use the command java Student Java programs When a Java program is compile, the product is a file with extension .class. The file is in the format known as bytecode. Bytecode is not machine code. That is to say, it is not directly executable in any computer. Instead, it needs an interpreter to interpret the code and then execute it accordingly. Java programs The interpreter is called a virtual machine or VM in short. This property has two consequences: – – It runs slower than those programs that are directly compiled to machine code like C++. The same bytecode programs can be executed in many different platforms as long as a VM is available. This is the reason why Java is so popular on the Internet.