Lecture 4. Lecture

Transcription

1 Interfaces Classes Building blocks Methods Arrays Example: BitArray Packages D0010E Object- Oriented Programming and Design InformaGon hiding Graphics and interacgon Example: A blinking signal Access Modifiers - Håkan Jonsson 1 1. Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 1. Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 1

2 1. Building blocks There are three ways to parggon a Java program. A) Methods B) Classes C) Packages Good programmers strive to write parts that are as loosely- connected and independent as possible. Such parts are more easier understood. Such parts are more easily maintained. Such parts can more easily be reused. A) Methods A method is a parameterized chunk of code (usually) with a name. We can invoke (use) the code by simply calling the method using its name and suitable arguments. Don t repeat code you need more than once. Instead, make the code general/independent and put in a method that you then call (from where you needed the code). - Håkan Jonsson 5 A) Methods An example in Eclipse... 2

3 A) Methods Methods brings many advantages: Obviously no need to repeat (almost idengcal) code - Less to write Less to write means less you can mess up. Just one place to make changes, if such are needed. No risk that you forget to make a change in one of the places where the code is repeated. An independent method can easily be reused. No need to reinvent the wheel. You automagcally get future improvements of the method. Meaningful names of methods improves readability. Easier to understand and get an overview of the program. Easier for co- programmers to write correct code. B) Classes A class defines the type of a set of representagons that are something. Methods define what such a something can do. Like with methods, write independent classes. Example: The class Helicopter from the last lecture. Then, specialize the representagons where you use them (not in the class defining them). B) Classes Example: The class Helicopter in Eclipse... 3

4 B) Classes A loosely- coupled and independent class (like Helicopter) has the same advantages as an independent method. We can create any number of helicopters. We can reuse the class in other programs where we like to have helicopters. To change what a helicopter is, we only need to reprogram the class not the engre program. It is easy to understand what a helicopter is just look at the class. No need to understand a potengally enormous program. Improvements of the helicopter class make all programs that use the class beder without any need to change them() Good name on classes (like Helicopter) makes it easier for fellow programmers to read and understand your code. etc. C) Packages A package is a set of classes (and interfaces) that together define a subsystem, or subfunc8onality, of a program. The content of a package are more Gghtly coupled to each other than to content of other packages. Code in packages also have special access to each other via package access. A package is also a separate name space. Names are unique only in their own packages. In Java, each class (and interface) belongs to (exactly) one package. In Unix- based systems, each package is stored in a separate directory. Stored in my home directory down in... /l04/ex5/signal Packages are separate even if their directories lie on a path. The three packages l04 l04.ex5 l04.ex5.signal are independent of each other. - Håkan Jonsson 11 C) Packages Membership in packages is declared on the first row in each file of the package. A package can contain any number of files. A file can contain declaragons of many classes (and interfaces). All declaragons in a file belong to the same package. A file without the package declaragon belongs to the nameless, or default, package. NB The default package can lead to problems so you are advise against using it. (It can not be referred to directly.) An example of a file in the package administra8on: package administration; public class University { /*... declarations... */ class Student { /*... declarations... */ class Teacher { /*... declarations... */ } - Håkan Jonsson 12 4

5 public class Sum2 { static int counter = 0; C) Packages A (member of a) class in one file can be used in another file either in qualified form or imported. Qualified means its full name is wriden every Gme it is used. Not pracgcal in the long run. A must, however, in case of name collisions. Imported means included once for all. Very pracgcal. Imports are done with import declaragons, of whom there can be any number: You can import a single class C from package x.y with import x.y.c; or import C and everything else in package x.y with import x.y.*; In UML, a package is drawn as a large rectangle around its content and with a name tag in the upper- lej corner. packagename Y ~ zebras : Z ~ min(agent : X) : char - Håkan Jonsson 13 package l04.ex3; public class FamousMathFunctions { static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; static int fibonacci(int m) { int prev, prevprev, answer; switch (m) { case 0: answer = 1; break; case 1: answer = 1; break; default: prevprev = 1; prev = 1; answer = prev; for (int i = 1; i < m; i++) { answer = prev + prevprev; prevprev = prev; prev = answer; return answer; public static void main(string[] args) { int num = 6; System.out.println("Fibonacci of " + num + " = " + fibonacci(num)); System.out.println(num + " = " + factorial(num)); } FamousMathFuncGons One stagc object during run- Gme l04.ex3 FamousMathFunctions ~ factorial(n : int) : int ~ fibonacci(n : int) : int + main(string[] args) : void Lecture 3 - Håkan Jonsson 14 package l04.ex3; public class Fibonacci { static int fibonacci(int m) { int prev, prevprev, answer; switch (m) { case 0: answer = 1; break; case 1: answer = 1; break; default: prevprev = 1; prev = 1; answer = prev; for (int i = 1; i < m; i++) { answer = prev + prevprev; prevprev = prev; prev = answer; return answer; } Fibonacci package l04.ex3; public class { static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; Three stagc objects during run- Gme l04.ex3 Fibonacci Facorial ~ fibonacci(n : int) : int ~ factorial(n : int) : int FamousMathFunctions + main(string[] args) : void package l04.ex3; public class FamousMathFunctions { public static void main(string[] args) { int num = 6; System.out.println("Fibonacci of " + num + " = " + Fibonacci.fibonacci(num)); System.out.println(num + " = " +.factorial(num)); FamousMathFuncGons Lecture 3 - Håkan Jonsson 15 5

6 package l04.ex3; public class FamousMathFunctions { public static void main(string[] args) { int num = 6; System.out.println("Fibonacci of " + num + " = " + Fibonacci.fibonacci(num)); System.out.println(num + " = " +.factorial(num)); package l04.ex3; public class MyFunctions { static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; static int fibonacci(int m) { int prev, prevprev, answer; switch (m) { case 0: answer = 1; break; case 1: answer = 1; break; default: prevprev = 1; prev = 1; answer = prev; for (int i = 1; i < m; i++) { answer = prev + prevprev; prevprev = prev; prev = answer; return answer; } FamousMathFuncGons l04.ex3 Two stagc objects during run- Gme MyFunctions ~ factorial(n : int) : int ~ fibonacci(n : int) : int FamousMathFunctions + main(string[] args) : void MyFuncGons Lecture 3 - Håkan Jonsson 16 D) Several packages in UML functions central FamousMathFuncGons +void main(string[] args) Fibonacci +fibonacci(m:int) : int +factorial(n:int) : int - Håkan Jonsson 17 package functions; public class { public static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; functions central FamousMathFuncGons +void main(string[] args) Fibonacci +fibonacci(m:int) : int +factorial(n:int) : int - Håkan Jonsson 18 6

7 package functions; public class Fibonacci { public static int fibonacci(int m) { int prev, prevprev, answer; switch (m) { case 0: answer = 1; break; case 1: answer = 1; break; default: prevprev = 1; prev = 1; answer = prev; for (int i = 1; i < m; i++) { answer = prev + prevprev; prevprev = prev; prev = answer; return answer; } central FamousMathFuncGons +void main(string[] args) functions Fibonacci +fibonacci(m:int) : int +factorial(n:int) : int - Håkan Jonsson 19 package central; import functions.*; public class FamousMathFunctions { public static void main(string[] args) { int num = 6; System.out.println("Fibonacci of " + num + " = " + Fibonacci.fibonacci(num)); System.out.println(num + " = " +.factorial(num)); functions central FamousMathFuncGons +void main(string[] args) Fibonacci +fibonacci(m:int) : int +factorial(n:int) : int - Håkan Jonsson Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 7

8 2. Access Modifiers Everything declared in a class is accessible from within that class. In addigon, the accessibility of classes, variables, and methods from other classes is controlled with access modifiers. public, protected, private, and the nameless access modifier. Note that local declaragons in a method are only accessible in the method. The visibility of a declaragon is called scope. - Håkan Jonsson Access Modifiers Public access Keyword: public Can be accessed from all other classes. (Protected access - a topic of upcoming lectures - ignore) Package access Keyword: None, rather the absence of public, protected, and private() Can be accessed from classes stored within the same package only. Private access Keyword: private Can not be accessed from other classes regardless of where they are stored. public <nothing> private - Håkan Jonsson 23 public Here, the method factorial can be used by code in all other classes in all packages. package functions; public class { public static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; +factorial(n:int) : int - Håkan Jonsson 24 8

9 private Here, the method factorial is hidden within the class. Only code in this class (only factorial itself right now) can access it. package functions; public class { private static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; factorial(n:int) : int - Håkan Jonsson 25 Package access By leaving out the access modifier, factorial becomes accessible from code in the same package only. Code in Fibonacci could, for instance, use factorial. (No modifier) package functions; public class { static int factorial(int n) { int i = 1, res = 1; while (i <= n) { res = res * i; i = i + 1; return res; ~ factorial(n:int) : int - Håkan Jonsson Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 9

10 3. Arrays in Java An array is a sequence of cells that are all capable of storing one item of the same type. array of integer, array of string, array of characters etc. Can also store references [to dynamic objects]. The size of an array is the number of cells it has. If s is an array, s.length gives its size. The size can not be change ajer an array has been created. Each cell is referenced by an index. The index is in the range 0 to size Håkan Jonsson CreaGng and IniGalizing arrays Example of declaragons: String [] s; int[] ssn; s is now an array of string references, and ssn an array of integers. CreaGng the arrays: s = new String[10]; ssn = new int[120]; s now refers to an array with space for 10 references to strings indexed from 0 to 9. ssn now refers to an array with 120 integers indexed from 0 to 119. Accessing a cell: s[m] The string reference stored in the m th cell of s, where m is an integer. Trying to use an index out of bounds results in a run- Gme error. (Same for ssn.) - Håkan Jonsson Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 10

11 4. InformaGon hiding When you implement a class, hide away as much implementagon detail as possible from other programmers. In pargcular, internal representagons and internal methods. The moment you give others access to your code, they will start using it and you will have a hard Gme changing it. And they will use it in the worst possible way.. Make a 100% clear difference between what you declare as externally accessible and what you keep hidden internally. Restrict access to the members of a class to an absolute minimum. This reduces misuse [by others], plain mistakes [by others], and errors [done by others] caused by you. In this course: You are free to use private as you like. However, if you use another access modifier, public for instance, you have to mo8vate why. Keep your code hidden unless there is a really good reason not to. Don t paint yourself into a corner. - Håkan Jonsson InformaGon hiding In Java, the scope of declaragons can be controlled via packages and access modifiers. Always think carefully about what other parts of a program should be able to access in the code you write. This is ojen agreed upon before the coding starts, and codified in "contracts" where programmers sgpulate what their code does and how it can be used. Exactly how code works is hidden, for instance. Much will be said about this during the second half of the course. For now we will se an example of this principle of informagon hiding. - Håkan Jonsson Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 11

12 5. Example: Bit- Arrays The class BitArray provides arrays of bits that can be of arbitrary length and some operagons on them. - Håkan Jonsson 34 program Main + main(string[] args) : void ~ cointoss() : boolean ~ present(b : BitArray) : void bitarray signal BitArray - bits : Signal[] + BitArray(size : int) + BitArray(String pattern) - zeros(n : int) : String + setbit(n : int) : void + clearbit(m : int) : void + valueofbit(p : int) : int + size() : int + and(ba : BitArray) : void + convert() : BigInteger + tostring() : String Signal - on : boolean + Signal() + ison() : boolean + seton() : void + setoff() : void - Håkan Jonsson Building blocks 2. Access modifiers 3. Arrays in Java 4. InformaGon hiding 5. Example: BitArray 6. Graphics etc first steps 12

13 6. Graphics etc first steps Everything on a computer screen has to be deliberately painted. Text, geometric shapes, pictures, borders, scroll bars etc. Java provides means to make such drawings as in the form of GUI components. With strange names like JPanel, JFrame, and the like. GUI = Graphical User Interfaces. Moreover, GUI:s in Java are event- driven. They react on events like mouse- clicks, keys pressed etc. When events occur, special methods are automagcally called by the operagng system. This might seem like magic but, no, that is not the case All GUI components and their methods are available for us to reprogram them, thereby making the program react in a suitable manner. Let us start with an example to illustrate this. This has to do with lab assignment 2. - Håkan Jonsson 37 Example: A blinking signal We will proceed in three steps to explain this example. a) Observer/Observable design padern b) Timer c) The final program with the blinking signal. - Håkan Jonsson 38 a) Observer Observable The observed: Is observable. AutomaGcally gets the methods setchanged nogfyobservers addobserver Must call setchanged nogfyobservers in this order whenever it has changed. The observer: Is an observer. Must implement the method update This method is called whenever the model calls setchanged nogfyobservers Any code within scope can add an observer to an observable with the method addobserver( reference- to- an- observer ) that the observable has. - Håkan Jonsson 39 13

14 a) Example: Obs Shows how observers are nogfied as soon as the observed states that it has been changed. The funcgonality is traced via print outs. b) Timer In Java, it is fairly simple to add Gmers that acgvate code at regular intervals. The class java.swing.timer provides a Gmer suitable for GUI:s. There are other Timer class too make sure you use the right one, if you like to try Gmers on your own. To each Gmer, listeners can be added. All listerers are called when the Gmer expires. b) Example: TimerDemo 14

15 c) Signal and signal image We separate the signal from its image (view). Make the Signal class an observable. Make Display, the image class, an observer. The idea is that a Signal object does not need to know who is observing. In this case, a Display object. Leads to less dependence between the classes, which is beneficial during the programming. Also, any number of observers can be added without the need to change Signal. Then we add a Gmer and have it call a method at a given rate that flips the signal. - Håkan Jonsson 43 c) Example: DisplayedTimedSignal Signal - on : boolean ~ Signal() ~ ison() : boolean ~ seton() : void ~ setoff() : void Display - s : Signal + Display(s : Signal) + update(observable obs, Object obj) : void + paintcomponent(g : Graphics) : void Driver + run() : void() Listener - s : Signal + Listener(s : Signal)+ actionperformed(actionevent evt) : void DTS DisplayedTimedSignal + DisplayedTimedSignal(s : Signal, title : String, rate : int, x : int, y : int) Observable JPanel <<interface>> Observer Signal - on : boolean ~ Signal() ~ ison() : boolean ~ seton() : void ~ setoff() : void <<interface>> ActionListener Display - s : Signal + Display(s : Signal) + update(observable obs, Object obj) : void + paintcomponent(g : Graphics) : void Driver + run() : void() Listener - s : Signal + Listener(s : Signal)+ actionperformed(actionevent evt) : void DTS DisplayedTimedSignal + DisplayedTimedSignal(s : Signal, title : String, rate : int, x : int, y : int) Timer JFrame 15