F21SF Software Engineering Foundations

SJF L4 4 More Programming concepts Using the Car class Constants, Math class, boolean, formattingSELECTION Dept of Computer Science F21SFSoftware Engineering Foundations Monica Farrow EM G30 email : M.Farrow@hw.ac.uk Material available on Vision

SJF L4 Topics • Improving car comparisons using • Class constants • Math class including the abs function • If/else • Including • The word ‘static’ • Algorithm • Boolean expressions • Formatting output • Scope

SJF L4 Multiple objects • There are (at least) 2 advantages to dividing a program into classes • The program is modular, with everything connected with a particular object in one place • If the program contains multiple objects of the same type (e.g. more than one car), the methods in the Car class are only written once, but can be used with each Car object. This can reduce code duplication.

SJF L4 myCar model “Ford Ka” tankSize 40 manfMPG 33.6 Multiple objects • One Car class, many objects • Each object has its own instance variables • Each object can use the class’s methods yourCar model “Mercedes....” tankSize 65 manfMPG 22

SJF L4 Multiple objects • In the Car class, we can create 2 cars and compare how far they can travel Car myCar = new Car("Ford Ka", 40, 33.6); Car yourCar = new Car ("Mercedes Benz E280", 65, 22); double myDist = myCar.estimateDistance(); double yourDist = yourCar.estimateDistance(); double difference = myDist – yourDist; System.out.println ("Diff is " + difference);

SJF L4 Issues arising Output:Diff is -18.920000000000016 Using raw numbers (0.22 for gallons per litre) is often bad style Use constants We’d like to print out values to one decimal point Format the output If we don’t know which car goes further, we might print out a negative difference. Use an ‘absolute value’ function in the Math class A slightly more meaningful message would be nice Use if/else structures

SJF L4 No magic numbers! To convert the manufacturer’s MPG to MPL, we need to multiply by 0.22. We might want to use the conversion factor elsewhere in the class, not just in one method We should declare it with the instance variables, then it can be used anywhere in the class, and it’s obvious what the number is The instance variables model, tank size and mpl will vary with each type of car The conversion factor 0.22 is fixed and will never change for ANY car

SJF L4 Class Constants Insert these lines just after the instance variables //gallons per litre private static final double GPL=0.22; Use instead of 0.22 return tankSize * manfMPG * GPL; Never changes during the program Use capitals by convention private static final double GPL= 0.22; Independent of specific objects of the class - The same for all Car objects

SJF L4 Math class The Math class contains a collection of mathematical methods and constants Rounding, power, absolute value, sin/cos, pi etc (The absolute value of a number is the numerical value, disregarding the sign. E.g. it is 4 for both 4 and -4) It is in the java.lang package and so is available to all java programs

SJF L4 Math abs method - documentation Method name and link to slightly fuller description Parameters incl type Indep of obj static double abs(double a) Returns the absolute value of a double value. Return type Description • We’ll use the abs method to disregard the sign when printing the difference in distance • Here is the java documentation on the method

SJF L4 Math abs method - using • We use it like this: double absDiff = Math.abs(myDist - yourDist); Ststem.out.print(“Difference is “ + absDiff); • Math methods are all static methods • i.e. independent of a particular object • No need to instantiate a Math object • Call using the name of the class before the period

SJF L4 Calling methods for an object • Usually, methods are Instance methods associated with an object, which use the values stored in the instance variables belonging to that object. Call using the object name and a full stop (period). • myCar.estimateDistance() uses the value of tankSize associated with the myCar object • yourCar.estimateDistance() uses the value of tankSize associated with the yourCar object

SJF L4 Calling static methods • Static methods typically take data from parameters and compute a value, like a maths function. • They use no instance variables of any object of the class they are defined in. Just call using the name of the class double squareRoot = Math.sqrt(45);

SJF L4 static • ‘static’ means that it is independent of particular objects of that class • It is used for class constants e.g. GPL • It is used for the ‘main’ method, which is not related to any objects public static void main (String arg[]) • All methods in the Math class are static – you can’t make a Math object. • The String class has some static methods (e.g. format, which returns a String, introduced next) and many instance methods (e.g. equals, charAt etc)

SJF L4 A Program may contain these classes • A class containing the main method • Essential. The program starts here. We will use this in a class by itself. It could be used within an object class. • Object classes that • Either we write ourselves e.g. Person, Car, Name • Or are supplied as part of the java API e.g. for collections, for GUIs, for random numbers, etc. • Static classes • These are independent of an individual object E.g. Math – a collection of useful functions (rounding, squaring..)

SJF L4 Formatting output • When printing the distance a car can travel, it’s not possible using the System.out.println method to control the number of decimal places that are displayed. • Originally, input and output was poorly provided for in java, and all textbook writers and many university departments wrote their own classes! • Now there are formatting features similar to C, which look complicated. You specify the type and the spacing.

SJF L4 Formatting output • This lecture covers the absolute minimum for printing a number within a sentence. • The lecture Formatting.ppt covers this and more details for later, for printing numbers, characters and Strings in a table. • There is a static format method within the String class. • Call using the class name, like a Math function • The parameters are formatting instructions and the item to be formatted. • A formatted String is returned

SJF L4 Formatting real numbers • In the String format method, the value of the 2nd parameter is returned using the format specified in the 1st parameter String myDistString = String.format("%.1f", distance); If distance contained a double of 296.6666recurring, myDistString = “296.7” • "%.1f" means format a real number to 1 decimal place • % means the string contains formatting information • .1 is the number of decimal places after the . • f means it is a real number

SJF L4 Car formatted output //get estimated distance double distance = myCar.estimateDistance(); //get distance as a string to 1 decimal place String myDistString = String.format("%.1f", distance); //print the details to standard output System.out.println(model + " can travel " + myDistString + " miles");

SJF L4 Formatting the absolute difference • Similarly, we can format the absolute difference in distance: double absDiff = Math.abs(myDist-yourDist);String absString = String.format(“%.1f”, absDiff); System.out.print(“Difference is “ + absString);

SJF L4 SELECTION • To improve our message about different distances, we need to print different text depending on which car can go further. • We’d like our code to work in all situations, no matter which 2 cars are compared. • Sample textThe Ford Ka can travel 296 miles, which is 19 miles less than the Mercedes Benz E280.

SJF L4 Algorithms • It’s often useful to plan using an algorithm (set of instructions). • Use programming structures (if, while, for) but keep the rest in English • Use indentation in the same way as in code • To show what happens inside block

SJF L4 Algorithm for printing the line • Algorithm (set of instructions) In English, for output Output text for first car up to ‘which is’ Output the absolute difference and ‘miles’If first distance is < second, output ‘less’else / otherwise output ‘more’Output remaining text for second car.What is the logic error here?! Is this the only possible algorithm?

SJF L4 UML Activity diagram Shows actions, transition lines, decision diamonds with conditions, and merge diamonds, Output text for 1st car up to ‘which is’ = 1 Output absolute diff and ‘miles’= 1 [2nd car dist < 1st car dist] [1st car dist < 2nd car dist] [n== 0, n== 1] [= dist] Output ‘ miles less than the’ ) Output ‘ miles more than the’ Output ‘ the same as the’ result = counter * result counter += 1 [counter <=n] Output text for 2nd car

SJF L4 If-else-else statement This if/else-if/else structure allows for 3 alternatives. You can have as many ‘else-if’ blocks as necessary. String diffMessage = ""; if (myDist < yourDist) diffMessage = absString + " miles less than the "; else if (myDist > yourDist) diffMessage = absString + " miles more than the "; else diffMessage = "the same as the ";

SJF L4 Final output //note the insertion of spaces within the string literals where necessary System.out.println("The " + myCar.getModel() + " can travel " + myDistString + " miles on a full tank, which is " + diffMessage + yourCar.getModel() );

SJF L4 Testing the improved Car class • We need to make sure that the improvements work in all situations • So in the main method, we should • Test where the first distance is < the second • Test when distances are the same • Test when the first distance is > the second Finishing with a little theory

SJF L4 Boolean expressions Boolean expressions return true or false. The basic operators are < (less than) <= (less than or equal to) > (greater than) >= (greater than or equal to) == (equal to) *** != (not equal to) *** Note there are TWO equals signs. (The single = is the assignment operator.)

SJF L4 More about booleans Sometimes you need to make more than one comparison E.g. a number is in the range 1 and 12 We can’t write if (0<num<=12) We have to write “If the number is greater than 0 AND ALSO less than or equal to 12” if (num > 0 && num<=12) Comparison operators: Use && for AND - both conditions must be true Use || for OR – at least one condition must be true Use ! for NOT – condition must be false

SJF L4 boolean type • boolean is another of the primitive types in java • Variables can be declared of type boolean e.g. • boolean isEmployee = true;boolean found = false; • Methods can return a boolean type e.g. //returns true if the tankSize of the car //is bigger than that supplied in the parameter, returns false otherwise public boolean tankBigger(int tSize) {return tankSize > tSize; }

SJF L4 Boolean exercises • Write boolean expressions (which could go inside the brackets of an if statement), using the variables and methods on the previous slides • num is not equal to 6 • num is not in the range 1-12 • found is true • found is false • myCar has a tank bigger than 50 • myCar has a tank less than or equal to 50

SJF L4 Scope – instance variables • Instance variables are declared within a class, not within a method • they can be used in the constructor and any of the methods in that class • If they are declared private, they are not accessible to any other classes • Other classes get at this data via ‘get’ methods • This is the norm in OOP • We will see some variations on this later in the module

SJF L4 Scope – local variables • Variables which are used as useful storage within a method are called local variables • they can only be used within the method in which they are declared • The main method contains a number of local variables • The estimateDistance method could have been written with local variables. This is useful if you want to check values as you go along, either by printing them out or looking at them in a debugger. double mpl = manfMPG * GPL; double distance = tankSize * mpl; return distance;

SJF L4 Scope – parameters • Parameters are like local variables • they can only be used inside the method public Name(String fName, String mName, String lName) { firstName = fName; middleName = mName; lastName = lName; }

SJF L4 QUIZ - terminology • Given the code on the next slide, can you identify: • A local variable • A parameter • An instance variable • A static method call • An object • A statement • A literal • Using a constructor

SJF L4 (in a main method) Car myCar = new Car("Ford Ka", 40, 33.6); String model = myCar.getModel(); double distance = myCar.estimateDistance(); String myDistString = String.format("%.1f",distance); . . . double difference = myDist - yourDist;

SJF L4 To Do Now • Read over lecture, do quizzes • Download code and try it out • Change it so that the distance for ‘yourCar’ is also formatted.

F21SF Software Engineering Foundations