1005ICT Object Oriented Programming Lecture Notes

Transcription

1 1005ICT Object Oriented Programming Lecture Notes School of Information and Communication Technology Griffith University Semester 2,

2 4 Programming In the Large In this section we: consider the levels of programming, programming-in-the-small versus programming-in-the-large; introduce the Java application programming interface (API) and its documentation; and introduce Java packages. Knowing how to read and use the Java API is as important as learning the Java language itself. 1005ICT Object Oriented Programming

3 4.1 Small To large programs The assumed knowledge for this course is of programming-in-the-small. At the core of imperative/procedural programming are its five elements: assignment variables, types, variable declarations, and assignment statements; calls methods, procedures, functions, and calls to them; sequence putting the above actions in a sequence to be carried out in order; iteration loops, recursion, repetition of actions; and selection choosing between alternate actions. 1005ICT Object Oriented Programming

4 With just these five elements, any program could be constructed. All of the generations of programming languages provide them. So why do modern programming languages provide a lot more features than just these? Two words: scale and cost. Cost, because programmers want to get paid, and the people who hire them want to get the most value for that pay; and scale, because real programs range from quite small to huge. 1005ICT Object Oriented Programming

5 A (crude) measure of how big a program is, is source lines of code (SLOC). Literally, how many lines of source code are used to create the program. Here are some SLOC figures quoted on the web (rarely published for closedsource programs): mashc Microsoft word (first version) Microsoft word (current) World of Warcraft Windows XP Mac OS X thousand 27 thousand at least a few million 5.5 million 45 million 86 million Only one of these examples was written by just one person. Most of these lines of code, are the kinds of lines you already know how to write, but there are a lot of them. 1005ICT Object Oriented Programming

6 Not only do all of these lines of code have to be written in the first place, they must: be organised so that multiple people can find and access the parts of the code that they need to; be organised so that multiple people can not destructively interfere with each other s code; be organised and documented so that they can be easily used and reused; and be organised so that they can be maintained (debugged, and modified over time). Maintenance over time can be the most expensive part of a software s total life. 1005ICT Object Oriented Programming

7 A modern programming language must promote: the efficiency with which individual programmers work; the efficiency with which teams of programmers work; the reusability of code; the maintainability of code; and the efficiency with which code from different organisations can be combined. 1005ICT Object Oriented Programming

8 4.2 The standard Java API Java is designed for small and large programming projects and has all the modern features to support programming in the large. The best place to see this demonstrated is by looking at the standard Java API. This is the most important reference source for Java programmers. It documents the pre-existing reusable code that they use to build new applications. 1005ICT Object Oriented Programming

9 4.2.1 Where to find the Java API documentation Java was originally developed by computer scientists working for Sun Microsystems, which has since been taken over by Oracle. Hence: is the current start point for downloading Java tools and documentation. We will be using the Java Platform, Standard Edition (Java SE), at: From here, you can download your own copy of the API. Click on the Downloads tab. 1005ICT Object Oriented Programming

10 Look for Additional Resources, Java SE Documentation and then Download. 1005ICT Object Oriented Programming

11 Click on Accept Licence Agreement, then the download link. 1 2 When you unpack the.zip file, you get the API documentation as a large collection of HTML files. The most important aspect of the licence granted to you is that you do not make these files accessible from the internet. 1005ICT Object Oriented Programming

12 You don t have to download the API and install it on your own machine. You can use the Java API on Oracle s site: I have also installed it on our own web server, but it is only available within Griffith University, to save your NetCheck costs ICT Object Oriented Programming

13 Here is the first page (in three frames) of the API. Everything is arranged in packages. 1005ICT Object Oriented Programming

14 4.3 Small To large programming features Subroutines to methods Subroutines The first step in making code reusable was the subroutine (or procedure, or function). A subroutine enables some statements to be encapsulated and given a name by which they can be called. In old languages like Fortran, programmers could share their subroutines. A collection of useful subroutines was called a library. Note that what was considered important to share and reuse at that time was only the statements and the actions they performed. 1005ICT Object Oriented Programming

15 The problem with names One problem that emerges quickly is that programmers must make sure that the names that they give their routines don t clash with the names used by other programmers, who s code they want to use. This was especially fun in early FORTRAN, where the names could only be 6 characters long. Methods Only in object-oriented languages are subroutines etc. called methods. 1005ICT Object Oriented Programming

16 4.3.2 Modules to classes Modules Later languages were better designed for sharing not just actions, but data. The new languages, C, Algol, Modula, could share new data types, not just methods. Modules were created to encapsulate and share data types, variables, and methods. Modules can control which of the definitions inside them are public or private. An opaque type is one that is public, but the details of its internal structure are not. The module will provide the methods that do everything that can be done with or to that type. 1005ICT Object Oriented Programming

17 A module that defines an opaque type and its methods is said to define an abstract data type. A module that contains private variables and methods to operate on them is said to define an abstract data object. Classes Classes in object oriented languages like Java take this a little further. A class is a module. A class also defines a new type. A class can be used like an abstract data object, but more commonly like an abstract data type. Sometimes a class is just used as a collection of methods. 1005ICT Object Oriented Programming

18 The names problem solved nearly If variables and methods belong to modules or classes, then we can solve the problem with names that clash, using qualification. If class A defines a method called dothing(), but so does class B, we can refer to them uniquely as A.doThing() and B.doThing(). But what happens if another programmer wants to use the name A or B for her class? 1005ICT Object Oriented Programming

19 4.3.3 Packages Packages are the modern way to share code. A package is an even higher level container for classes. A package corresponds physically to a folder that contains all of the class files in that package. The names problem solved finally Qualification is taken further with packages. The name of a class can be qualified with the name of the package that contains it. Packages can be nested (folders inside folders), so enough qualification can be added to break any name clashes. To be sure to avoid clashes, businesses use their web domain (backwards) to name their packages. 1005ICT Object Oriented Programming

20 For example, two programmers in two different companies can use the same names without any problems with clashes if their code is bought together into the same system. com.eyebeem.tax.deductableitem.writeoff() com.esaypee.tax.deductableitem.writeoff() 1005ICT Object Oriented Programming

21 4.4 Java package declarations All Java code exists within packages, either unnamed or named. Any Java class exists in just one package. All of the classes we have written so far belong to the unnamed package, because we didn t declare otherwise with a package declaration. The compilation unit is the smallest amount of code accepted by the compiler. compilationunit packagedeclaration importdeclaration typedeclaration The type declaration is usually a normal class declaration. 1005ICT Object Oriented Programming

22 A compilation unit starts with an optional package declaration. packagedeclaration package packagename If it is omitted, we are specifying the unnamed package. Example with a package declaration: package au.edu.griffith.oop; public class PackageTest { } public static void main(string[] args) { System.out.println("Go, packages!"); } 1005ICT Object Oriented Programming ;

23 As in these examples, package names should start with lower case letters, so they may be distinguished from class names. The downside is that to run this program, it must be in the folder hierarchy that matches the package name. ] javac au/edu/griffith/oop/packagetest.java ] java au.edu.griffith.oop.packagetest Go, packages! ] For casual development and learning we don t usually bother with packages, and that is the reason that the unnamed package exists. 1005ICT Object Oriented Programming

24 4.5 Java import declarations The converse to declaring that your code belongs in a package is to declare that you want to use someone else s code that is in a package. This is done with an import declaration, of which there are two kinds. importdeclaration singletypeimportdeclaration typeimportondemanddeclaration The first kind imports just one class from a package. singletypeimportdeclaration import typename ; 1005ICT Object Oriented Programming

25 The second kind imports all of the classes from a package (as they are needed). typeimportondemanddeclaration import packageortypename The second kind is more commonly used (unless you like typing a lot).. * ; 1005ICT Object Oriented Programming

26 4.5.1 When you need import declarations If you want to use a class in a package, say com.eyebeem.coolstuff.missilelauncher, you would either import just that class: import com.eyebeem.coolstuff.missilelauncher; or import all the classes in that package, as needed. import com.eyebeem.coolstuff.*; Then you can use the name MissileLauncher in your program, and the compiler will know where it comes from. 1005ICT Object Oriented Programming

27 4.5.2 When you don t need import declarations Here are the times you don t need to import a class: when the class is in the same package as the class you re writing (maybe the unnamed package); when the class is in the special package java.lang, which is always imported automatically; or when you fully qualify the class every time you use its name. There have been no import declarations in any of our example programs so far, because we have not used any classes not in java.lang. 1005ICT Object Oriented Programming

28 4.5.3 Avoid collisions While using fully qualified names always allows you to avoid the collisions between the names you choose for your classes, it is inconvenient. Avoid any of the names already used in package java.lang and any other standard packages that you are likely to use frequently. The compiler will report errors if you pick a name that collides with an already existing one. 1005ICT Object Oriented Programming

29 4.5.4 Example with imports This is an example program that demonstrates the use of import declarations. It does something cool and useful. Warning: the lines commented with //!!! contain Java language features we have not covered yet. /* ** file: OpenURI.java ** author: Andrew Rock ** use: java OpenURI <URI> ** opens the <URI> in your default web ** browser. */ 1005ICT Object Oriented Programming

30 import java.net.*; // for class URI import java.awt.*; // for class Desktop public class OpenURI { } public static void main(string [] args) throws Exception { //!!! Desktop.getDesktop() //!!!.browse(new URI(args[0])); //!!! } We are not far from understanding all of this code. We need two things, objects and exceptions, both coming soon. 1005ICT Object Oriented Programming

31 4.6 Section summary This section covered: a review of the elements of imperative programming; the reasons why programming languages have features beyond those elements; where to find the standard Java API; levels of encapsulation and how they help with name classes; and Java packages and package and import declarations. 1005ICT Object Oriented Programming

32 4.7 End of section feedback questions Send us your answers to these questions any time you like by clicking on them. What was the most useful topic in this section? What was the least useful topic in this section? What was the least clear topic in this section? What topic in this section would you like to like to know more about? Did you find an error in this section? 1005ICT Object Oriented Programming