Brain Re-Wiring Time. CS121: Computer Programming I. How to Tackle a Problem? Agenda. Making Changes - which localises change?

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1 CS121: Computer Programming I Brain Re-Wiring Time Object-Oriented Primer Dr Olly Gotel ogotel@pace.edu Having problems? -- Come see me or call me in my office hours -- Use the CSIS programming tutors CS121/IS223 Week 9, Slide 1 CS121/IS223 Week 9, Slide 2 Agenda Many ways to tackle a problem Design example and important design concepts Exercise -- OO design -- thinking with objects Important object-oriented (OO) concepts Role-playing demo to illustrate some of these concepts by simulating OO program execution (maybe in a couple of weeks) Revisiting methods and re-looking at classes CS121/IS223 Week 9, Slide 3 How to Tackle a Problem? Decomposition - handling complexity by the principle of divide & conquer Tends to involve Two forms of decomposition constructing Process-oriented: according to hierarchies steps or functions Object-oriented: according to behaviour of autonomous objects Both valid, but current claims for superiority of OO Stronger framework, reuse of common abstractions & resilient under change CS121/IS223 Week 9, Slide 4 2 Types of Decomposition Making Changes - which localises change? Transaction Transaction Get Transaction Open Passbook Checking Open Open Bonus Open Passbook Withdraw Withdraw Checking Withdraw Bonus Withdraw Open Deposit Withdraw Get Transaction Open Passbook Checking Open Open Bonus Open Passbook Withdraw Withdraw Checking Withdraw Bonus Withdraw Open Deposit Withdraw Account Deposit Account Loadsadosh Open Deposit Loadsadosh Withdraw ihs ihs ihs Passbook Deposit Checking Deposit Bonus Deposit Other object(s) ihs ihs ihs Passbook Deposit Checking Deposit Bonus Deposit Other object(s) ihs Loadsadosh Deposit Passbook Account Checking Account Bonus Account Passbook Account Checking Account Week Bonus Slide Account 5 CS121/IS223 9, CS121/IS223 Loadsadosh Week 9, Slide Account 6 1

2 Design Example - starting object think Design Example No right or wrong here! A conceptual design for a small, one-branch bank A simple design for a small one-branch bank The whole thing, not just the computer part What are the THINGS? What can they do? Think - how would someone on the street RECOGNISE this as a small bank? [Sample solution from Cockburn 1998] CS121/IS223 Week 9, Slide 7 CS121/IS223 Week 9, Slide 8 Design Concepts CRC Cards Items required for good design [Cockburn 1998]: Name of key components, or things, in the system Vault What it is called Purpose, basic function, or main *responsibility* of each thing Holds the money Teller Key communication paths between them List of the services provided What it does What it interacts with CS121/IS223 Week 9, Slide 9 CS121/IS223 Week 9, Slide 10 STEP 1 CRC Cards - use them to think! Exercise Thinking With Objects What it does Class name Responsibilities (i.e. services it provides) What it is called Collaborators (i.e. other classes this class relies on for services) What it uses CS121/IS223 Week 9, Slide 11 You are contractors who just won a bid to design a custom coffee vending machine for the employees of Acme Works to use. Arnold, the owner, wants his own, custom design. He is, however, a cheapskate. Arnold tells us he wants a simple machine. All he wants is a machine that serves coffee for 35 cents, with or without sugar & creamer. He expects you to be able to put this little machine together quickly & for little cost You get together & decide there will be a coin slot & coin return, coin return button, & four other buttons: black, white, black with sugar, & white with sugar Source: [Cockburn 1988] CS121/IS223 Week 9, Slide 12 2

3 Design the Coffee Machine Using Objects Candidate Design 1 What are the components? What are their responsibilities? Which components rely on which other components for services? How do they work together to deliver the simple service: give coffee for 35 cents? Which component knows the price of the drink? Which component knows how to make the drink? Create the CRC cards for these Run some scenarios does it work? Enact the CRC cards Draw this! CS121/IS223 Week 9, Slide 13 [Sample solution from Cockburn 1998] CS121/IS223 Week 9, Slide 14 Interaction Diagram for Design 1 Does Your Design Support Change? STEP 2 After 5 machines are installed & have been operating for a while, Arnold comes along & says, "I would like to add bouillon, at twenty-five cents. Change the design." You add one more button for bouillon & one more container for bouillon powder How else do you change your design? [Cockburn 1998] CS121/IS223 Week 9, Slide 15 CS121/IS223 Week 9, Slide 16 Candidate Design 2 Interaction Diagram for Design 2 [Sample solution from Cockburn 1998] CS121/IS223 Week 9, Slide 17 [Cockburn 1998] CS121/IS223 Week 9, Slide 18 3

4 Analysis of Designs 1 & 2 Yet Another Change! STEP 3 Design to localise change Beware of super components distribute the responsibilities Create some CRC cards to help you check that your design works by running some scenarios of use List as many other changes & additions you believe Arnold may ask you to make in the future Arnold comes back with a brilliant idea. He has heard that some companies use their company badges to directly debit the cost of coffee purchases from their employees' paychecks. Since his employees already have badges, he thinks this should be a simple change. You add a badge reader & link to a payroll system What else? CS121/IS223 Week 9, Slide 19 CS121/IS223 Week 9, Slide 20 Candidate Design 3 Move Over Starbucks! STEP 4 [Sample solution from Cockburn 1998] CS121/IS223 Week 9, Slide 21 People are starting to buy Starbucks lattes instead of Arnold's coffees. So Arnold wants the machine modified just slightly, so that he can create a "drink of the week." He wants to be able to add new drinks and change prices any time, to match his competition. He wants to be able to add espresso, cappuccino, hot chocolate, latte, choco-latte, steamed mil, soy chai in short, anything he can mix together. You add a couple more buttons, a milk steamer & dispenser, & some more powder dispensers. You need a really good, robust design, so that you or Arnold can change the products & prices at will, without tearing the machine apart any further. CS121/IS223 Week 9, Slide 22 Candidate Design 4 Interaction Diagram for Design 4 [Sample solution from Cockburn 1998] CS121/IS223 Week 9, Slide 23 [Cockburn 1998] CS121/IS223 Week 9, Slide 24 4

5 Reference for Design Exercise Read all about it! On my web-site Recommended Good Design Takes Time Simple Object-Oriented Analysis & Design, Parts 1 & 2, Alistair Cockburn, CUJ, May & June 1998: /cuj9805cockburn/ /cuj9806cockburn/ Supports easy change Uses abstractions from the domain Satisfies the need today & tomorrow! Rare to get it perfectly the first time you must be prepared to learn and iterate CS121/IS223 Week 9, Slide 25 CS121/IS223 Week 9, Slide 26 C-class; R-methods; C-method calls Classes, Objects & Methods Hello World in Java (Non-OO) MUST call this file HelloAgain.java public class MessageMaker{ public static void main(string[] args) { Message mymessage = new Message(); mymessage.printmessage(); mymessage.changemessage( goodbye ); mymessage.printmessage(); (Message.java) public class Message{ private String m = hello ; public void printmessage() { System.out.println(m); public void changemessage (String newmessage) { m = newmessage; Other methods here // Another Hello World Program // HelloAgain.java public class HelloAgain { public static void main(string[] args) { System.out.println("Hello World!"); REMEMBER THIS? Must name the file after this CS121/IS223 Week 9, Slide 27 CS121/IS223 Week 9, Slide 28 Hello World in Java (OO) TRY THIS NOW!!! Object-Oriented Concepts // Yet Another Hello World Program // HelloWorld.java public class HelloWorld { public static void main(string[] args) { Hello hello = new Hello(); hello.printhello(); Must call this file HelloWorld.java // A Hello class for Yet Another Hello World Program // Hello.java public class Hello { public void printhello() { System.out.println("Hello World!"); MUST call this file Hello.java CS121/IS223 Week 9, Slide 29 Classes Objects Methods Messages & message passing Parameters & arguments Return types & voids Encapsulation & information hiding Instance variables & class variables State Access modifiers Overriding & overloading* Interfaces* Inheritance* Polymorphism* * later We we will look at these in detail and look more at the other concepts in the follow -up course CS121/IS223 Week 9, Slide 30 5

6 Next Few Weeks: what they mean & how to use! Role-playing Demo At some point object state message attribute identity behaviour relationship class abstraction polymorphism interface generalisation & specialisation inheritance encapsulation message passing method dynamic binding instance CS121/IS223 Week 9, Slide 31 How an OO program works - to illustrate concepts in everyday terms CS121/IS223 Week 9, Slide 32 object has state + identity + behaviour In Plain Terms, an Object is Example Objects object Something that represents an atomic entity Something you can uniquely identify (identity) Something that encapsulates data as its state Something you can send messages to, causing it to respond/behave in some way Something whose behaviour depends on its internal state (which may change) Has a public interface & private internals CS121/IS223 Week 9, Slide 33 Passive objects One loaf of bread One packet of herbal tea Invoice sent to A Farm, Malvern Active objects Lorry "M235 BCM ; Van "N683 CNM" Fax machine in Richard Green's office Human agents Richard Green; David Brown (Executive) Structure objects Marketing Department CS121/IS223 Week 9, Slide 34 object has state + identity + behaviour Looking Inside an Object Gordon, Junior Librarian, as object State All the data the object currently encapsulates state M T W T F Attributes Name: Gordon Position: Junior Librarian Age: 25 Salary: 12, Relations Operations Loan out books Rename position Amend salary Data is defined in terms of valued attributes (characteristic features - name / value pairs) The value of some attributes is fixed (immutable) attribute The value of some attributes can change (mutable) Supervisor Timetable Equipment Values of the attributes are the state of the object student_id; registered_courses; weight; date_of_birth CS121/IS223 Week 9, Slide 35 CS121/IS223 Week 9, Slide 36 6

7 Identity identity Behaviour behaviour Attribute values may change, so don t clearly pin -point the object over time Identity makes it possible to distinguish any object in an unambiguous way & independent from its state Characterises the object to give persistence Like a natural key or unique handle phone number student_id CS121/IS223 Week 9, Slide 37 Operations enable an object to act & react Objects can receive certain messages & act upon them The set of messages the object can understand is generally fixed & defined in its interface Reaction to messages may depend on current attribute values (i.e. state); it may even change the state on flickswitch CS121/IS223 Week 9, Slide 38 off message message passing Message Message Passing Messages are sent to objects to trigger behaviour Structure - keyword selector & optional arguments Arguments are values being passed to the object Identity: myclock Attribute: currenttime Message: resettime(newtime) Message: telltime Object may respond to a message by sending a message to another object! Objects therefore interact by passing messages to one another (i.e. object interaction) A message requests an object to perform an operation or service A message consists of a name & any arguments CS121/IS223 Week 9, Slide 39 CS121/IS223 Week 9, Slide 40 Interface interface Method method An object can have public & private interface An object s public interface defines the messages it will accept & that all can use An object s private interface can only be used by the object itself or those who are privileged Structure - selector, required arguments & what will be returned public -> resettime(newtime: Time) public -> telltime() : Time private -> time : Time CS121/IS223 Week 9, Slide 41 A description that determines how an object reacts to a message (e.g. a piece of code to implement the operation) Methods are called in response to messages sent to objects The operation performed is determined by the the object s class & the value of the object s attributes - so the method has full access to the object s state Matching a message name to a method is called binding CS121/IS223 Week 9, Slide 42 7

8 Dynamic binding dynamic binding Encapsulation encapsulation The same message can be sent to different objects Each object can bind the message to a different method, as defined by its class Dynamic (late) binding refers to the process of identifying what code should be executed as the result of receiving a message Reduces coupling between objects Enables pluggability & substitutability Behaviour & information are encapsulated in objects (we say objects are strongly encapsulated) From outside, an object can only be manipulated via its public interface; the internal state is private & can only be changed by the object itself Other objects can only know operations on the object, not how they work Necessary prerequisite for information hiding & programming by contract CS121/IS223 Week 9, Slide 43 CS121/IS223 Week 9, Slide 44 Class class Class (in UML) class Objects can have a lot in common & can lead to much duplication A class defines the structure & behaviour of a particular kind of object - acting as a template or blueprint Classes represent groups of objects with the same behaviour & information structures Every object belongs to a class (i.e. is an instance of a single class), though a class may have many instance objects Classes represent abstractions ann?timepiece? student CS121/IS223 Week 9, Slide 45 jo We call this representation of a book an abstraction Name compartment Instance variables Method definitions Book title: String copiesonshelf(): int borrow(c:copy) Parameters & their types Class name Name Type Return type CS121/IS223 Week 9, Slide 46 Instance & Instantiation instance Classes & Objects Instantiation - the process of creating a new object belonging to a class Truck Instance - the resulting object Note that this object is a new thing with its own identity Attributes Registration No. Weight Fuel Relations The state of the new object must be initialised (i.e. its instance variables given values) Driver Trailer Operations Create (Truck) Deliveries Goods Refuel Delete (Truck) Values will instance level Classes define structure & behaviour of a system Objects represent the *actual* system relationships classes links objects CS121/IS223 Week 9, Slide 47 CS121/IS223 Week 9, Slide 48 8

9 Key Points You can approach a problem in different ways: Procedurally, by breaking a problem down into smaller problems that compose to solve the overall problem (you create a tree) Using communicating objects (you create a network) OO design is natural & scaleable, but requires thinking with objects, being prepared to iterate & giving up the notion of global control Coming Up Next Banks, coffee machines & trek species are all objects before we look at how to define them (using classes) & create instances (objects), we will look at communication between objects & the services they provide (methods) Java how it deals with classes, objects & methods starting with methods again programming the OO way (you need to have done your prep work!) What is really going on? Program control flow OO programming concepts support this way of thinking & solving problems CS121/IS223 Week 9, Slide 49 CS121/IS223 Week 9, Slide 50 Agenda How to define & use simple classes (with instance variables & methods) may be new How to create instances of classes (objects) & what are these things anyway? may be new How to define & use methods (should be a recap so go through these slides at home for homework): main method & constructors Parameters/arguments & returns Visibility (access modifiers) Method overloading Java Ingredients So Far Variables, assignment, operators Expressions, statements, compound statements Control structures selection, iteration Lots of other useful bits & pieces along the way arrays, files, etc Aim now to write OO programs Need - to create your own objects, which means writing your own class types One step at a time CS121/IS223 Week 9, Slide 51 CS121/IS223 Week 9, Slide 52 How to Write an OO Program in Java (Simplified) Identify classes needed Identify methods & variables needed by objects of those classes Write classes Run program by creating objects in a main method (often in a driver class) & calling the object s methods Objects then call each other s methods CS121/IS223 Week 9, Slide 53 Reminder Class: template for creating objects of a particular type, defining data about the objects (instance variables) & operations they can carry out (methods) Object: instance of a class, given a name & actual data values it can carry out the operations Methods are the building block of OOP they should be small, cohesive & general-purpose KEY WORD Encapsulation: combination of data & operations in a single self-contained object CS121/IS223 Week 9, Slide 54 9

10 Classes & Methods & Objects Written a Method, How Do You Use It? public class ExampleClass{ public void methodone(){ Statements public void methodtwo(){ Statements Instance object created ExampleClass myobj = new ExampleClass(); myobj.methodone(); myobj.methodtwo(); A class can declare a number of methods Constructor Methods are called on instance objects of the class CS121/IS223 Week 9, Slide 55 Methods are called on objects but when you start a program running, you have no objects Remember the main method: public static void main(string[] args) { Means method does NOT need to be called on an object main creates object(s) & calls their methods the objects then take over the show main is special write no other static methods on this course CS121/IS223 Week 9, Slide 56 Classes Class name: Car Data: Colour Licence Speed Fuel Methods: Accelerate Brake Colour: orange Licence: J342 Speed: 20 mph Fuel: 2 gallons Colour: red Licence: RACER Speed: 100 mph Fuel: 15 gallons A class is a data type The Same Information in UML Unified Modelling Language a class diagram Car - colour: string - licence: string - speed: double - fuel: double + accelerate(double mss, double t): void + brake(double mss, double t): void + honk(): void Class name Data Methods CS121/IS223 Week 9, Slide 57 CS121/IS223 Week 9, Slide 58 Classes & Objects One class, many objects Writing the Java Class Definition - colour: string - licence: string - speed: double - fuel: double Car + accelerate(double mss, double t): void + brake(double mss, double t): void + honk(): void Each Java class definition should be stored in its own file (filename the same as class name, with.java extension just as you have been doing) e.g. Car.java Your program can have many classes (so many files), but only one has a main method amongst them It is a good idea to put this in a driver or test class Put all your classes in the same directory for now CS121/IS223 Week 9, Slide 59 CS121/IS223 Week 9, Slide 60 10

11 Write the Classes Primary class Car Race public class Car{ private String colour, licence; private double speed, fuel; public void honk(){ System.out.println( Beep Beep! ); Test class with main() method Instance variables private means can only be changed inside the class (by its methods) Method public means that it can be called from outside the class by objects of Car class public class CarTester{ public static void main(string[] args){ Car mycar = new Car(); Make a new car mycar.honk(); object & call it mycar Get mycar to honk its horn CS121/IS223 Week 9, Slide 61 CS121/IS223 Week 9, Slide 62 Objects & Classes Object: distinguishing characteristics data associated with it (attributes or instance variables) can perform certain actions (methods) Class: defines a type of object the blueprint/template/definition Methods Can structure programs as a collection of methods that call one another Methods help to organise code - they should be small, cohesive & general-purpose Methods can have parameters & so take arguments this makes them more general-purpose Methods can return values (or void) Your program always has ONE main method & this is where execution starts Examples of objects & classes? CS121/IS223 Week 9, Slide 63 CS121/IS223 Week 9, Slide 64 From Procedural to OO Programming (OOP) Before: all code was in a class that you gave the same name as your file all the class code was inside a main method this code was executed in sequential manner A program was a sequence of statements Now: still can have a single class / file for hybrid code in the main method creates an object of the class type, then calls its methods (i.e. tells it to do things) the doing code is packaged into named methods no longer any sequence as such more interesting A program is a collection of method-calling objects Onward to OO You can approach a problem in different ways: Procedurally, by breaking a problem down into smaller problems that compose to solve the overall problem (you create a tree) Using communicating objects (you create a network) OO design is natural & scaleable, but requires thinking with objects & being prepared to iterate to improve your thinking OO programming concepts support this way of thinking & solving problems CS121/IS223 Week 9, Slide 65 CS121/IS223 Week 9, Slide 66 11

12 Benefit of Programming this Way? Key Points Class: template for creating objects of a particular type, defining data about the objects (instance variables) & operations they can carry out (methods) Object: instance of a class, given a name & actual data values it can carry out the operations Methods are the building block of OOP they should be small, cohesive & general-purpose KEY WORD Encapsulation: combination of data & operations in a single self-contained object CS121/IS223 Week 9, Slide 67 CS121/IS223 Week 9, Slide 68 EXTRA SLIDES TO HOME A Quick Reminder on Methods Go through this material again (presented earlier in the course) and see if you can try a few things out in code, coupled with support from your text book Writing Java Programs Only written very small programs to date using one class (the name of your program) OO, but not really OO How do your programs work? How do you write really BIG programs? How do you write OO ones? One big class, with lots of statements, loops & selections inside? repeats bits of code jumps all over the place It would just be a mess needs structure CS121/IS223 Week 9, Slide 69 CS121/IS223 Week 9, Slide 70 Helping With Digestion Organising Code Managing complexity Which is easier to talk about or refer to: a sandwich, a packet of chips, a cookie, some fruit, a carton of juice? a box lunch? Package stuff up and give it a label to refer to! We group programming statements to make the task of programming more comprehensible Statements are generally grouped when, together, they perform some task A name is assigned to such blocks of code (functions, procedures, subroutines, modules, methods) so this name can be referred to in place of all the statements CS121/IS223 Week 9, Slide 71 Every programming language has its own way for doing these things CS121/IS223 Week 9, Slide 72 12

13 Chunky Variable a chunk of data with a name Method a chunk of code with a name The foundations of reuse CS121/IS223 Week 9, Slide 73 Packaging Code printsquare: Refer to printsquare() whenever we want to use the sequence of statements (this is abstraction) Write once, debug once, use many times, update in one place In Java, we call this a method & declare it inside a class CS121/IS223 Week 9, Slide 74 Methods in Java A method is a named sequence of programming statements Clarification Some programming languages have functions or procedures Python has functions for e.g. To use a method, you call it but you have to state where the method can be found (i.e. which class the code is inside) You do this by calling the method on an object of the class: General - objectname.methodname(); Specific - myobject.printsquare(); Static methods are called on a class no objects needed CS121/IS223 Week 9, Slide 75 Java has object methods a bit like a function that an object can perform terry.forwardmarch(); object of class turtle method Variable of a class type names an object (e.g. terry) CS121/IS223 Week 9, Slide 76 Code Structure Classes & Methods Source file (ClassName.java) Method 1 Statement Statement Method 2 Statement Class CS121/IS223 Week 9, Slide 77 public class ExampleClass{ public void methodone(){ Statements public void methodtwo(){ Statements Instance object created - one particular ExampleClass ExampleClass myobject = new ExampleClass(); myobj.methodone(); myobj.methodtwo(); A class can declare a number of methods (responsibilities, services, behaviours) Constructor - a method with same name as class to make an instance object of this class Methods are called on instance objects of the class (dot notation) CS121/IS223 Week 9, Slide 78 13

14 Classes & Methods - Example public class SquareClass{ Method name public void printsquare(){ for (int i = 0; i < 5; i++) System.out.println( ); Method code in braces Methods are just a nice way of packaging your code now just call the method rather than repeat the code Class name CS121/IS223 Week 9, Slide 79 What Do These Mean? public class SquareClass{ public void printsquare(){ for (int i = 0; i < 5; i++) System.out.println( ); public can call the method from anywhere in a program (if object is accessible) a visibility/access modifier void method returns no value back to the place of the method call, just does something handy CS121/IS223 Week 9, Slide 80 Written a Method, How Do You Use It? Hybrid OO - meaning I am making an instance of a class in the class itself better to have a separate DRIVER class that does this but easier for small things for now Using Our Example Methods are called on objects but when you start a program running, you have no objects Remember the main method: public static void main(string[] args){ Means method does NOT need to be called on an object main creates object(s) & calls their methods the objects then take over the show Only one main method in a program, even if lots of java files CS121/IS223 Week 9, Slide 81 public class SquareClass{ public void printsquare(){ for (int i = 0; i < 5; i++) System.out.println( ); public static void main(string[] args){ SquareClass myobj = new SquareClas s(); myobj.printsquare(); Constructor CS121/IS223 Week 9, Slide 82 Classes, Objects & Methods Not the best way Future > class types & driver classes Program Flow Like being interrupted by a phone call but you carry on doing what you were previously doing when it is over Instance variable - gpa - each student object has own value for gpa (Student.java) public class Student { private int gpa=0; public int getgpa() { return this.gpa; public void setgpa(int newgpa) { this.gpa=newgpa; public static void main(string[] args) { Student s1 = new Student(); System.out.println(s1.getGpa()): Other methods here CS121/IS223 Week 9, Slide 83 A method call changes the flow of a program The program jumps & control passes to the method (it may be in an external class/file) Control returns to the point it left when the method has finished With conditionals (branches), loops (repeats) & methods (side-tracks) we can get quite interesting program flow CS121/IS223 Week 9, Slide 84 14

15 Built-in Methods Black-box programming How Many Built-in Methods? Methods provided by the programming language to carry out routine or complex things: System.out.println( bla ); We know nothing about how built-in methods like println are coded We don t need to know anything, other than: how to use the method (i.e. how to call it & whether to pass it any arguments - we are passing println a string here) the type of value the method returns println returns nothing - it is void) A sort of protocol CS121/IS223 Week 9, Slide 85 There are an enormous number of built-in methods to take advantage of (see the API)! However, you are not expected to know about or use all of these you will find things as & when you need them They can save you some work! CS121/IS223 Week 9, Slide 86 User-Defined Methods When there is no built-in method to do what you want to do, you have to roll your own It is exactly like a built-in method, but YOU write the code Your method may do one specific thing, but it is often smarter to make it general purpose Example We want to write a program to print out this sign ******************************************************* ******************************************************* Welcome ******************************************************* ******************************************************* ******************************************************* ******************************************************* Example from Dr Courtney CS121/IS223 Week 9, Slide 87 CS121/IS223 Week 9, Slide 88 A Class & Its Methods - Declaring Driver Class Making an Object; Calling its Methods public class SignClass{ public void printtwolines(){ System.out.println(" *******************************"); System.out.println(" *******************************"); public void printwelcome(){ System.out.println(" Welcome"); public void printfourlines(){ for(int i=1; i <=4; i++) System.out.println("********************************"); i is local / temporary variable with restricted scope Why use methods? CS121/IS223 Week 9, Slide 89 public class SignDemo{ public static void main(string[] args){ SignClass welcomesign = new SignClass(); //instantiating an object welcomesign.printtwolines(); //invocation of method in object s class definition welcomesign.printwelcome(); //invocation of method welcomesign.printfourlines(); //invocation of method Why do this? CS121/IS223 Week 9, Slide 90 15

16 Try it! Making Methods More General (i) Go back 2 slides make a SignClass class exactly as shown Go back 1 slide make a SignDemo class exactly as shown 2 java files in the same source folder - compile and run see what happens Then, look at the code changes to these 2 classes in the following slides, make the changes and keep re -running your code CS121/IS223 Week 9, Slide 91 public void printlines(int number){ Parameter for(int i=1; i <=number; i++) System.out.println("*********************"); Create the above method in the SignClass public class SignDemo{ public static void main (String[] args){ SignClass welcomesign = new SignClass(); welcomesign.printlines(2); welcomesign.printwelcome(); welcomesign.printlines(4); Values for parameters to pass into methods should agree in type, quantity, and position CS121/IS223 Week 9, Slide 92 Aside This is called the method SIGNATURE or method declaration public void printlines(int number) The return type Visibility/access modifier Method name Parameter - variable name and its type Making Methods More General (ii) public class SignDemo{ public static void main(string[] args){ SignClass forsalesign = new SignClass(); forsalesign.printlines(2); forsalesign.decideonmessage(); forsalesign.printmessage(); forsalesign.printlines(4); Need to declare some instance data for the SignClass, so each actual sign object we make can store its own unique message CS121/IS223 Week 9, Slide 93 CS121/IS223 Week 9, Slide 94 Making Methods More General (iii) public class SignClass{ private String message= ; Making a General Purpose Method Try the next example too public void printlines(int number){ for (int i=1; i<=number; i++) System.out.println(" ******************"); public void printmessage(){ System.out.println(" " + message"); public void decideonmessage(){ System.out.println( What message do you want?"); message = scan.nextline(); Make your Scanner object the usual way CS121/IS223 Week 9, Slide 95 CS121/IS223 Week 9, Slide 96 16

17 Returning to our Example Hybrid OO Squares of Any Size? Varied behaviour a better abstraction public class SquareClass{ Revisiting a few concepts here Parameter to method public void printsquare(){ for (int i = 0; i < 5; i++) System.out.println( ); public static void main(string[] args){ SquareClass myobj = new SquareClas s(); myobj.printsquare(); Constructor Change the method to print a square of ANY size CS121/IS223 Week 9, Slide 97 Parameter variable public void printsquare(int side){ for (int row = 0; row < side; row++){ for (int col = 0; col < side; col++){ Scope of side? System.out.print( + ); System.out.println( \n ); To call: myobj.printsquare(8); Change the method to print a rectangle of ANY size New line character Argument to method initialises side (the parameter variable) CS121/IS223 Week 9, Slide 98 Rectangles of Any Size? public void printrect(int rows, int cols){ for (int row = 0; row < rows; row++){ for (int col = 0; col < cols; col++){ System.out.print( + ); System.out.println( \n ); An even better abstraction To call: myobj.printrect(2, 4); 2 parameters to method 2 arguments to method types MUST match, so order is important Rectangles Using Any Character? Better abstraction yet! 3 parameters to method public void printrect(int rows, int cols, char ch){ for (int row = 0; row < rows; row++){ for (int col = 0; col < cols; col++){ System.out.print(ch); System.out.println( \n ); 3 arguments to method types MUST match, so order is important Change the method to print a rectangle of ANY size using any character! CS121/IS223 Week 9, Slide 99 To call: myobj.printrect(2, 4, * ); CS121/IS223 Week 9, Slide 100 Methods That Return Values void methods return no value The return Statement in Java Does a calculation & returns a value you must do something with this return value! Parameter is number to square, Type of the return value declared with its type declared public int square(int number){ number = number * number; return number; return statement returns a value & ends the method MUST be present if method declaration/signature returns a type To call & use: int x = myobj.square(3); CS121/IS223 Week 9, Slide 101 Returns a value & ends a method jumps back to where the method was called & inserts its value Methods only return 1 value, but this could be a collection! public int square(int number){ return number * number; statement; NEVER executed Note, constructors have no return type, not even void! CS121/IS223 Week 9, Slide

18 What s the Difference? Parameters / Arguments Scope, access, lifetime Arguments & parameters: two sides of the same coin -- it is just terminology (often used interchangeably) You specify a method signature using parameters You call a method using arguments (i.e. you give actual values for the parameters) When a method is called, arguments are passed into (substituted for) the parameters Be careful of scope - where do these variables exist? CS121/IS223 Week 9, Slide 103 Special kind of variable used to pass data to a method 2 ways Pass by reference: parameter variable in the method refers to the same location in memory as the original variable in the method call change the parameter variable, you change the original variable Placeholder! Pass by value: method makes a temporary copy of the data in the original variable if you change the parameter variable it has NO effect on the original variable CS121/IS223 Week 9, Slide 104 More Terminology Recap Good to make methods as general -purpose & reusable as possible Encapsulation taking some code statements, turning them into a method, then using them as a whole: simpler to refer to & remember as a whole can be used many times, in many places (reuse) other people can use your code & you can use other people s (a clear contract of use) Generalisation writing methods that can do more than one particular thing: a method that can find the factorial of any number is more useful than a method that only finds 5! Methods in Java: parameters specify the type & number of variables used by a method (can have any number) sometimes called formal parameters arguments are the actual values of these variables & they are passed to the method when it is called sometimes called actual parameters scope determines the lifetime of a variable (often delimited by the braces of compound statements & sometimes nested) local / temporary variables are variables declared in a method, & have restricted scope created when scope entered, destroyed when scope exited (names can be reused if scopes disjoint) if your method specifies a non-void return type, it needs a return statement CS121/IS223 Week 9, Slide 105 CS121/IS223 Week 9, Slide 106 Writing Java Programs Using Methods Methods in Java No longer a long sequential list of statements Instead, loosely coupled methods that call each other Create small cohesive methods that do a single thing: if it does many things, split into multiple methods if it is long, split into multiple methods Parameterise your methods to make them general -purpose CS121/IS223 Week 9, Slide 107 A method is a named sequence of programming statements To use a method, you call it but you have to state where the method can be found (i.e. which class the code is inside) You do this by calling the method on an object of the class: General - objectname.methodname(); Specific - obj.printsquare(); Methods are called for an object CS121/IS223 Week 9, Slide

19 Writing Java Programs Using Methods Example Method design try this at home No longer a long sequential list of statements Instead, loosely coupled methods that call each other Create small cohesive methods that do a single thing: if it does many things, split into multiple methods if it is long, split into multiple methods Parameterise your methods to make them general -purpose What is too long or too much? CS121/IS223 Week 9, Slide 109 Assume we will use a single class what would be a good way of splitting up the work involved in getting a sequence of 100 integers from a user & printing them out in ascending order (i.e. what methods would you write)? 3 methods maybe? Do the input thing Do the sorting thing Do the output thing Logical problem decomposition CS121/IS223 Week 9, Slide 110 Example Continued public class Sorter{ Declare the 3 methods in the same class here FOR NOW public static void main(string[] args){ Sorter myobj = new Sorter(); myobj.inputint(); myobj.sortint(); myobj.outputint(); Constructor (a special method) Lazy OO Writing the Methods (i) inputint(): Need to input 100 integers, so need a loop Need a suitable data structure to store them in (use an array) Method will need to return an integer array CS121/IS223 Week 9, Slide 111 CS121/IS223 Week 9, Slide 112 Code public int[] inputint(){ int[] myints = new int[100]; // loop to read in integers return myints; Writing the Methods (ii) sortint(): Need to take an integer array as an argument Need to sort the array (later) Method will need to return a sorted integer array A better method would be flexible & allow the user to say how many integers they want to input CS121/IS223 Week 9, Slide 113 CS121/IS223 Week 9, Slide

20 Code public int[] sortint(int[] myints){ // sort integers return myints; Writing the Methods (iii) outputint(): Need to take an integer array as an argument Need to print out the array elements, so will need a loop Method needs to return nothing, so is void CS121/IS223 Week 9, Slide 115 CS121/IS223 Week 9, Slide 116 Code public void outputint(int[] myints){ // loop to print out integers Changing the main Method public class Sorter{ public static void main(string[] args){ Sorter myobj = new Sorter(); int[] myints = myobj.inputint(); myints = myobj.sortint(myints); myobj.outputint(myints); Many ways to do this CS121/IS223 Week 9, Slide 117 CS121/IS223 Week 9, Slide 118 Also called data members Instance Variables (aka Attributes or Fields) Code Structure - Recap An instance variable belongs to an object & has class scope (i.e. can be used by any instance methods) public class Sorter{ private int[] myints = new int[100]; Declare instance variables public static void main(string[] args){ Sorter myobj = new Sorter(); int[] myints = myobj.inputint(); myints This = now myobj.sortint(myints); simplifies! Declare myobj.outputint(myints); methods Constants: final int MAX = 100; CS121/IS223 Week 9, Slide 119 Source file (ClassName.java) int x; Method 1 Statement Statement Method 2 Statement Class Data declarations - Make these private Method declarations - Make these operate on the declared data CS121/IS223 Week 9, Slide

21 Much Simpler Interfaces Control ordering when methods can be called from outside a class public int[] inputint(){ int[] myints = new int[100]; // loop to read in integers return myints; public void inputint(){ // myints = what is read in public void sortint(){ // sort integers public void outputint(){ // loop to print out integers No need to return it can be void Don t need myints is a variable in class scope, so can access it here SAFE? CS121/IS223 Week 9, Slide 121 The set of methods an object makes available to others to use private void inputint() { private void sortint() { private void outputint() { public void dosorting(){ inputint(); sortint(); outputint(); public static void main(string[] args){ Sorter myobj = new Sorter(); myobj.dosorting(); CS121/IS223 Week 9, Slide 122 How Would Other Objects Use This? Public & Private Called visibility modifiers or access modifiers Contracts to ensure used correctly & in right order Specify order of method call but can t enforce it, so better to wrap things up a bit make all these public methods private & provide a single public method to control what an object is able to do Private variables means can be used by methods in the same class Private methods means can be used by other methods in the same class CS121/IS223 Week 9, Slide 123 Use methods to write small cohesive units of code, but try to make a minimum number of methods public (i.e. public interface should be thin) Public methods should act as access methods providing control over the use of private methods & data Variables can also be declared public or private inside a class it is good practice to make these private & use methods to set & change their values (i.e. getters & setters also known as accessors & mutators) Protected #? Default is public so be careful! CS121/IS223 Week 9, Slide 124 Constructors public class Olly { private int age = 0; public Olly(int newage){ age = newage; public int getage() { return age; Constructors have no return public Olly(){ age = 0; Default constructor Method Overloading To perform the same method on different types of data Same method name with multiple parameter lists (method signature) In Java 5.0, return types can be overloaded too public static void main(string[] args) { Olly babyolly = new Olly(); Olly youngolly = new Olly(21); Overloaded constructor System.out.println(youngOlly.getAge()): CS121/IS223 Week 9, Slide 125 E.g. print(counter); print( Hello! ); Constructors are often overloaded CS121/IS223 Week 9, Slide

22 Key Points Programming gets horribly complex if you don t organise your code Methods provide a way to structure your programs There are built-in methods More interestingly, you can create your own methods we call these user-defined CS121/IS223 Week 9, Slide 127 Things to Try Go through the lecture slides again & ask me questions if you have them Try working with the code fragments in these slides and see how you get on Read your book chapters on methods Finish exercise sheet 2 Work on Lab 1 individually or in pairs Work on LAB1 get as far as you can seek help if you need it CS121/IS223 Week 9, Slide 128 What You Should ALWAYS Do! Coming up Next If you want to learn to program in an OO way (with Java), you need to read the course text not just once, but again & again the weekly sessions can only help steer a course for you through this material You should try all the exercises & as many as the programming projects as humanely possible -- it requires your commitment to practice! To accompany this week s class, & if you want to keep up, it would be a good idea to ensure you can master the following exercises: - If you have 3rd edition of text: exercises & programming projects 4.1, 4.2, 4.3 & If you have 4-6th edition of text: exercises , 6.2, 6.3, 6.4 & programming projects 4.1, 4.3 & 6.1 I will always look at your work & help you with problems if you come to me during office hours CS121/IS223 Week 9, Slide 129 Lots of practice with designing your own classes Objects & object reference variables - what are these object things anyway? Let s look underneath Understanding parameter passing there is more going on behind the scenes than you may think! CS121/IS223 Week 9, Slide