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F21SFSoftware Engineering Foundations

4 More Programming concepts

Using the Car class

Constants, Math class, boolean, formattingSELECTION

Monica Farrow EM G30 email : M.Farrow@hw.ac.ukMaterial available on Vision

Dept ofComputerScience

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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

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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.

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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

myCar

model “Ford Ka”

tankSize 40

manfMPG 33.6

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Multiple objects

In the Car class, we can create 2 cars and compare how far they can travelCar 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);

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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

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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

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Class Constants

Insert these lines just after the instance variables//gallons per litreprivate static final double GPL=0.22;

Use instead of 0.22return tankSize * manfMPG * GPL;

private static final double GPL= 0.22;

Never changes during the program

Independent of specific objects of the class- The same for all Car objects

Use capitals by convention

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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

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Math abs method - documentation

We’ll use the abs method to disregard the sign when printing the difference in distance

Here is the java documentation on the method

abs(double a) Returns the absolute value of a double value.

static 

double

Method name and link to slightly fuller description

Parameters incl type

Return type

Indep of obj

Description

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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

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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

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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 classdouble squareRoot = Math.sqrt(45);

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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)

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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..)

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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.

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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

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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

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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");

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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);

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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.

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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

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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?

UML Activity diagram

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Output text for 1st car up to ‘which is’ = 1

result = counter * resultcounter += 1

[n== 0, n== 1]

[counter <=n]

Output ‘ miles less than the’ )

Output text for 2nd car

Output absolute diff and ‘miles’= 1

Output ‘ miles more than the’

[2nd car dist < 1st car dist] [1st car dist < 2nd car dist]

[= dist]

Output ‘ the same as the’

Shows actions, transition lines, decision diamonds with conditions,and merge diamonds,

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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 ";

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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() );

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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

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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.)

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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

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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;

}

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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

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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

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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;

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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; }

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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

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(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;

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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.