Programming Process

1
Programming Process

• Programming in Java is an exercise in using
  pre-defined classes and writing new classes to
  fill in the gaps
• A procedure for determining the relevant objects
  for design and application
• Determine primary objects by examining the nouns
  of the problem statement
• Decide upon the primary behavior of the classes
  corresponding to these objects
• Define the interface
• Introduce instance variables to maintain state
• Implement the method bodies

2
Designing Classes an overview

• Statement of problem
• Describes the object or system to be modeled
• Sketch a sample scenario illustrating the
  solution
• Find the primary objects
• Key elements of the model
• Collect all the nouns in the problem statement
• Demonstrate the desired behavior of the objects
• Produce basic set of required methods
• Multiple objects, work one object at a time
• Determine the interface
• Prototype each method arguments and return types
• Write sample code to see how methods are invoked
• Define necessary instance variables
• Implement the methods

3
Numeric processing(page 64-70)

• Method implementation requires a wide variety of
  tools, including the ability to process numbers
• Java defines primitive data types including
  numeric types
• Integer and floating point types
• int, short, byte, long, float, double
• Not classes or objects, utilize underlying
  hardware
• Cannot receive messages have no associated
  methods
• There are associated classes with these types, to
  be looked at later
• Form expressions as operands of operators
• Static methods
• Written to supplement the basic arithmetic
  operations
• Can accept primitive data type as an argument and
  return value

4
The int data type

• Built-in data type
• Models the behavior of integers (whole numbers)
• Data type provides basic arithmetic operations
• (addition), - (subtraction),
  (multiplication), / (division)
• Used to construct standard arithmetic expressions
• x y z
• Integers can be printed by the overloaded print
  and println methods of the PrintStream class
• System.out.println(total / count)

5
Working with int

• Declarations are like reference variables
• int count
• int number, size
• Have no methods or instance variables
• An integer value is the only thing that can be
  associated with a variable of type int
• The value may be assigned when the variable is
  declared
• int count 0
• Or through an assignment statement
• total cost count
• No constructors, and no new instruction

6
Basic arithmetic

• Integer division results in integer results
• - the results are whole numbers

7
Constants literals

• Constants
• There are some values that will not change over
  the life of the program, they have a constant
  value
• Constants are declared using the form
• static final type identifier value
• Example
• static final int numStudents 20
• final constants value is final, it will never
  change
• static constant belongs to the class, not an
  instance of the class
• Literals
• In the expression
• int total 2 cost
• The 2 is a literal because it is a literal
  representation of its value, (which can never be
  changed)

8
Precedence

• What is the result of this expression?
• 2 6 / 8
• 1
• 2
• An ambiguous expression, unless precedence rules
  are available to resolve ambiguities
• Multiplication and division have higher
  precedence than addition subtraction
• If more than one operator of the same precedence,
  the expression is evaluated from the left to the
  right
• Parentheses can be used to clarify ambiguity or
  to overwrite precedence rules

9
Compound Assignment Operators

• What does the following mean? Is it nonsense?
• total total 2 // doubling total
• count count 1 // incrementing count
• These kind of operations happen so often that
  there are special operator shorthand for them

10
String objects and the operator

• String concatenation is a common operation in
  Java
• Therefore the operator is overloaded for
  concatenating string objects
• string1 string2
• is Java shorthand for
• string1.concat(string2)

11
int input

• To read an int value into an int variable (from
  keyboard or file)
• Read the line from the data file into a String
  object, using readLine()
• Convert the String value into the int value using
  the static parseInt() method of the predefined
  class Integer
• Example
• String s br.readLine()
• int num Integer.parseInt(s)
• Or
• int num Integer.parseInt(br.readLine())

12
Other integer types

• long
• The int type models a range of integers from 2
  billion to 2 billion
• When we need a larger range of values we use the
  long type, which models a range of integers from
  8 quintillion to 8 quintillion
• long is identical to ints in terms of values,
  operators, and behaviors
• However long literals have an L appended to them
• long num 4000000000L
• Should all integers be long?
• ints require 32 bits of memory, longs 64 bits
• Most computers today are 32 bit computers, and
  they cannot carry out 64 bit arithmetic as
  efficiently

13
Mixed type arithmetic

• int values may be assigned to long variables
• No chance of information being lost
• However, if we want to assign a long value to an
  int, even if it is a legitimate int value
  (range), we must cast the value as an int
• long bigNum 1000000
• int s (int)bigNum

14
Other integer types

• Represent smaller range of integers, require less
  memory
• short
• -32768 to 32767 (16 bits)
• Byte
• -128 to 127 (8 bits)
• Mixed arithmetic rules follow the same pattern
  set by long and int
• If information might be lost as a result of an
  assignment, an explicit cast is required

15
Numbers of Measurement Floating Point numbers

• Numbers with fractional parts precision is
  always an issue
• Two types that model floating point behavior
• float
• Models floating point numbers with approximately
  7 digits of precision
• double
• Models floating point numbers with approximately
  15 digits of precision

16
Using floating point numbers

• Print() and println() are overloaded as with ints
• Format for printing floats and doubles borrows
  from scientific notation
• 3.4028235 X 1038 ? 3.4028235E38
• Literals can be written using the scientific
  notation style
• The decimal point, the fraction, the exponent may
  all be omitted
• To distinguish float from double, float literals
  must have a trailing f
• 3.14159f
• To distinguish double literals that look like int
  literals, they must have a trailing d
• 98d

17
Using floating point numbers (2)

• Declaring doubles and floats is similar to
  declaring ints
• double area, perimiter
• static final double pi 3.14159
• double price 1.99
• Operators for addition (), subtraction (-),
  multiplication (), and division (/) works with
  floting point numbers
• You cannot find the remainder () of floating
  point division, however
• Shortcut assignment operators (, -, , /)
  work
• Increment does not

18
Reading float and double

• The book says there is no parseDouble() in the
  Double class, and no parseFloat() in the Float
  class
• However the API shows them both, and I believe
  that they are now implemented
• Reading a double or float value should be like
  reading an int
• Example
• String s br.readLine()
• double value Double.parseDouble(s)
• Or
• double num Double.parseDouble(br.readLine())
• If this doesnt work we will use the style showed
  in the text
• double num Double.valueOf(br.readLine()).double
  Value()

19
Mixed type arithmetic

• double to float follow the same pattern set by
  long and int
• If information might be lost as a result of an
  assignment, an explicit cast is required
• Mixing integer types and floating point types
• Integers may be assigned to floating points
• Floating point to integers requires a cast
• Assigning a long to a float or double may result
  in loss of precision

20
Mixed type expressions

• What is the result of this expression?
• int count 4
• int total 25
• double answer total / count
• 6
• 6.25
• Analysis
• num / x is an integer expression with an integer
  result
• The integer result (6) of the integer expression
  is assigned to a double
• Cast one of the int variables to get the result
  you want
• double answer (double) total / count

21
Designing a class Collecting tolls

• Problem statement
• A county is installing a toll collecting
  system.Trucks pulling up to a tollbooth are
  required to pay a toll of 5 per axle plus 10
  per half-ton of the trucks total weight. A
  display in the booth shows the toll receipts and
  the number of truck arrivals since the last
  collection.
• Primary objects
• Trucks
• Axle, weight
• Tollbooth
• Receipts, display, toll, number

22
Designing a class Truck

• Behavior
• Construct a Truck object with weight and number
  of axles
• Get the weight
• Get the number of axles
• Interface
• public Truck(int weight, int numAxles)
• public int getWeight() // could be double
• public int getAxles()
• Instance Variables
• private int myWeight // could be double
• private int myNumAxles

23
Designing a class Tollbooth

• Behavior
• Construct a Toolbooth object
• Calculate the toll
• Display the data
• Receipt collection
• Interface
• public Toolbooth()
• public void calculateToll(Truck tr)
• public void receiptCollection()
• public void displayData()
• Instance Variables
• private int totalReceipts // could be double
• private int numTrucks

24
Designing a class Implementation

• Download Tollbooth.zip demo