Java Programming: Classes and Numeric Processes

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

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

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

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);

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

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

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 intnumStudents = 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 inttotal = 2 * cost; • The 2 is a literal because it is a literal representation of its value, (which can never be changed)

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

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

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)

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());

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

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;

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

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

Using floating point numbers • Print()andprintln()are overloaded as withints • 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 trailingf 3.14159f • To distinguish double literals that look like int literals, they must have a trailing d 98d

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

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 doesn’t work we will use the style showed in the text double num = Double.valueOf(br.readLine()).doubleValue();

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

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;

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 truck’s 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

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;

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;

Designing a class: Implementation Download Tollbooth.zip demo

Java Programming: Classes and Numeric Processes