F21SF Software Engineering Foundations

1
F21SFSoftware Engineering Foundations

• 4 Basics B
• Constants, Math class, selection, boolean
• Code available on web
• Car CarMain classes printed today

Monica Farrow EM G30 email
monica_at_macs.hw.ac.uk Material available on Vision
and my website
2
Topic overview

• Multiple objects. Comparing 2 cars and thinking
  about
• static constants
• Math class
• selection
• Boolean expressions

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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 yourDist myDist
• System.out.println ("Diff is "
  difference)

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Multiple objects

• One Car class, many objects
• Each object has its own instance variables
• Each object can use the classs methods

yourCar
model
Mercedes....
tankSize
65
manfMPG
22
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Issues arising

• Using raw numbers (0.22 for gallons per litre) is
  often bad style
• If we dont know which car goes further, we might
  print out a negative difference
• Wed like to print out values to one decimal
  point (covered in last lecture)

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No magic numbers!

• To convert the manufacturers MPG to MPL, we need
  to multiply by 0.22.
• We might want to use the conversion factor
  elsewhere in the class
• We should declare it with the instance variables,
  then it can be used anywhere in the class, and
  its 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 litre
• private static final double GPL0.22
• Use in the methods
• 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
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Math class

• 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 available to all java programs
• The complete specification can be found at
  http//java.sun.com/javase/6/docs/api/
• You can find the method signatures and a brief
  explanation of the methods purpose

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The API

• Java itself is a small language
• To program in Java, you need to use the Java API
  (Application Programming Interface)
• The API is divided into packages
• A package contains a group of related classes
• Every class in the API is documented

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Class documentation

• The documentation will show the classs
  interface
• the name of the class
• a general description of the class and its use
• a list of methods, with parameters and return
  types
• a description of each method

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Math abs method - documentation

• Well use the abs method to disregard the sign
  when printing the difference in distance
• Here is the java documentation on the method

Method name and link to slightly fuller
description
Parameters incl type
Indep of obj
Return type
Description
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No implementation

• In some cases, you might spot a variable, though
  almost all of them are private and not in the
  interface
• You wont see any implementation
• For example, you wont see how the method works
• As a rule, you dont need to know this
• You need to know what a method does (not how it
  does it)

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Math abs method - using

• We use it like this
• double absDiff Math.abs(difference)String
  absString
• String.format(.1f, absDiff)
• Ststem.out.print(Difference is
• absString)
• 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

• Usually, methods are Instance methods associated
  with an object which use 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
• 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)

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static

• static means that it is independent of
  particular objects of that class
• It is used for class constants
• 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
  cant make a Math object.
• As the module proceeds, well meet more static
  constants and methods

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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.
• Sample textThe Ford Ka can travel 296 miles,
  which is 19 miles less than the Mercedes Benz
  E280.
• Algorithm (set of instructions) In English, for
  output
• Output text for first car up to which is
• Output the absolute difference and milesif
  first distance is lt second, output
  lessotherwise output moreOutput remaining
  text for second car.What is the logic error
  here?!
• Is this the only possible algorithm?

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If-else statement

• The if-else statement allows you to execute
  alternative blocks of code depending on a
  condition
• if (myDist lt yourDist)
• diffMessage absString
• " miles less than the "
• else if (myDist gt yourDist)
• diffMessage absString
• " miles more than the "
• else //equal
• diffMessage "the same as the "

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Other if statements

• You can also just have if aloneSystem.out.prin
  tln("coin")if (coinNumber gt 1)
  System.out.println("s")
• Or one if..else
• if (coinNumber 1)
• System.out.println("coin")
• else //coinNumber gt1
• System.out.println("coins")

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More about if

• Always use curly brackets to enclose the blocks
• Like my last 2 examples, not like my first
• You dont NEED to use them if the block has only
  one statement
• The chances are that you might change your code
  to add a second statement and forget to add the
  curly brackets so start off with them!
• The if statement evaluates the condition (which
  is given within the rounded brackets) to a
  boolean value, True or False

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Final output

• //note the insertion of spaces 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
• Run it with the existing code, where the first
  distance is lt the second
• Run again, changing the 2nd car so that the
  distances are the same
• Run again, changing the data so that the first
  distance is gt the second
• This is messy! We should be able to do all the
  testing in one run. Improvements suggested in
  another lecture

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Boolean expressions

• These can become complicated. The basic
  operators are
• lt (less than)
• lt (less than or equal to)
• gt (greater than)
• gt (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 cant say if (0ltnumlt12)
• We have to say If the number is greater than 0
  AND ALSO less than or equal to 12
• if (num gt 0 numlt12)
• 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 trueboolean found false
• Methods can return a boolean type e.g.
• public boolean tankBigger(int tSize) return
  tankSize gt tSize
• Use booleans rather than integer flags with
  value 0 or 1
• if (found) is more readable than if (flag 1)

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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 2 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 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 local variables

• 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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Debugger

• Instead of Run As Java Application you can run
  the eclipse debugger
• First set a breakpoint by the first statement in
  the main method by right-clicking in the left
  hand margin next to the statement, and choose
  Toggle Breakpoint. You should now see a little
  circle in the margin
• Then choose Debug As
• The perspective changes
• The code is displayed with a solid line over the
  statement with the breakpoint, it has stopped
  there
• Continue by choosing RunStep-over or click the
  icon
• You can see the variables and their values as
  they are created
• There are buttons to Step Into other classes,
  resume and terminate
• StepInto doesnt work for me at the moment

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Debugger screenshot
variables
Control buttons