BPJ444: Business Programming using Java Java basics Tim McKenna

Outline Java classes and members primitive and reference fields operators control flow statements the BigDecimal class the wrapper classes

Java Class source code file name: Employee.java instance and static fields (variables)  state or attributes instance and static methods  behaviour constructors create objects setters and getters control access to fields the keyword: this nOTe: Java cares deeply about case

ClassName noun or noun phrase use Title Case: CapitalizeEachWord class Employee class EmployeeSalary extends Employee name.java file must be exactly the same as class name within the file

nameMethod verb or verb phrase start in lowercase capitalizeEachSubsequentWord e.g. void giveRaise(int increase) public String getAddress()

fields or variables noun or noun phrase start in lowercase capitalizeEachSubsequentWord try for short and memorable e.g. StringempName; intempStartYear; a method’s local variables and parameter names can be even shorter

Primitive Data Types summary (The Java Tutorial)  numeric types: int, double  boolean logic type: boolean  character type: char primitives contain a simple numeric value for language efficiency primitives are not objects

Primitive Literals byte b = 2;// an int literal is used short x = 23;// an int literal is used int y = 23;// default int literal int z = 0123;// octal value (decimal value=83) int v = 0x1111; // hex value (decimal value=4361) long t = L; // a long integer literal float r = 3.14f;// a float literal double s = 3.14;// default double literal

more Primitive Literals booleanexit = true, done = false; char c1 = 'a', // single character c2 = '\u0041', // Unicode escape seq. // is 4 hex digits c3 = '\n'; // ASCII escape seq. \n is new line (works in some GUI widgets) \t is tab(does not work in any GUI widgets)

Object Reference Types field containing a single value which refers to an object  i.e. a memory address, a pointer  not modifiable by programmer  initial value is null until assigned  examples: object names, array names, class names Example: References.java

Using Classes examples: Employee.java a template for an employee Employees.java creates a company of employees

Data Type Conversion automatic widening conversion  if it fits, no casting needed  int auto cast to long l = i;  int auto cast to double d = i;  char auto cast to int i = 'a'; casting: explicit narrowing conversion Example: Casting.java …be careful about overflow. see e.g.

Operators in Java summary of operators (Java Tutorial) types of operators: arithmetic, logical, relational, conditional, assignment precedence of operators all similar to C language

Control Flow Statements summary (The Java Tutorial) branching statements  break (same as C )  break with a label (new in Java)  continue (same as C )  continue with a label (new in Java) Examples: BreakDemo.java, BreakWithLabelDemo.java, ContinueDemo.java, ContinueWithLabelDemo.java

The BigDecimal Class 98.7% of numeric data in business applications has a predetermined number of decimals:  currency, exchange rates, unit costs, discounts, taxes  precision: digits in number including decimals  scale: number of decimal places JDBC maps DB decimal fields to BigDecimal BigD class provides control of rounding behaviour Note: double primitive is only an approximation! Examples: PaySchedule.java, PaySchedule2.java, DontUseDouble.java

Why use BigDecimal instead of double? from the IBM computer scientist who wrote the new, improved BigDecimal class: Using Java 5.0 BigDecimal and Decimal Arithmetic FAQUsing Java 5.0 BigDecimalDecimal Arithmetic FAQ from an assignment that didn’t use BigDecimal… Type Vegan Survey Last Date Restaurant Code Y/N Amount Surveyed Name, Location CF Y Blueberry Hill, YLM Please enter survey amount (+ add, - subtract) > Unable to subtract this amount because there is only left!

BigDecimal arithmetic BigDecimal sum, difference, …; // note: immutable class sum = addend.add(augend); // a = b + c sum = sum.add(anotherBigDecimal); // a += d difference = minuend.subtract(subtrahend); // a = b - c

BigDecimal arithmetic import static java.math.RoundingMode.HALF_UP; // standard rounding, import the constant BigDecimal product, quotient; // immutable product = multiplicand.multiply(factor); product = // round result to 2 decimals product.setScale(2, HALF_UP); product = // multiply and round multiplicand.multiply(factor).setScale(2, HALF_UP); quotient = // round result to 2 decimals dividend.divide(divisor, 2, HALF_UP);

BigDecimal comparisons import static java.math.BigDecimal.ZERO; payment.compareTo(interest) > 0 "if payment is greater than interest " principal.compareTo(payment) <= 0 "if principal is less than/equal to payment" principal.compareTo(ZERO) == 0 "if principal is equal to zero" principal.equals(ZERO) …may not be what you mean. see API.

The Wrapper Classes classes representing primitive data types: Byte, Short, Integer, Long, Float, Double, Character primitive data type may need to be wrapped in an object, e.g. to be stored in a List which accepts only objects. J2SE 1.5 now does autoboxing to/from primitives and object wrappers Example: WrapperDemo.java

Integer Class constructors  wrap primitive integer value into an object: Integer intWrap = 123; // autoboxed Integer intFromString = new Integer(“123”) extraction of a value from the object  public int intValue ( ) e.g. int i = intWrap.intValue(); // old way int I = intWrap; // new way autoboxed  public String toString() e.g. String s = intWrap.toString();

Integer Class three static methods (class or utility methods)  static intInteger.parseInt ( String s ) e.g. String s = " "; int i = Integer.parseInt ( s )  static StringInteger.toString ( int i) e.g. int i = ; String s = Integer.toString ( i )  static Integer Integer.valueOf ( String s ) e.g. String s = " "; Integer intWrap = Integer.valueOf ( s ) orInteger intWrap = new Integer ( s )