BPJ444: Business Programming using JavaJava basics

Tim McKenna

Seneca@York

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.String empName;int empStartYear;

a method’s local variables and parameter names can be even shorter

Primitive Data Types

summary (The Java Tutorial) http://java.sun.com/docs/books/tutorial/java/nutsandbolts/datatypes.html

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 = 2200000000L; // a long integer literal

float r = 3.14f; // a float literal double s = 3.14; // default double literal

more Primitive Literals

boolean exit = true,done = false;

char c1 = 'a', // single character c2 = '\u0041', // Unicode escape seq.

// is 4 hex digitsc3 = '\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) http://java.sun.com/docs/books/tutorial/java/nutsandbolts/operators.html

types of operators: arithmetic, logical, relational, conditional, assignment

precedence of operators all similar to C language

Control Flow Statements

summary (The Java Tutorial) http://java.sun.com/docs/books/tutorial/java/nutsandbolts/flow.html

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 BigDecimaland Decimal Arithmetic FAQ

from an assignment that didn’t use BigDecimal…Type Vegan Survey Last Date RestaurantCode Y/N Amount Surveyed Name, LocationCF Y 3.59 2003-07-04 Blueberry Hill, YLM

Please enter survey amount (+ add, - subtract) > -3.59

Unable to subtract this amount -3.589999999999999857891452847979962825775146484375

because there is only 3.589999999999999857891452847979962825775146484375 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; // autoboxedInteger 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 int Integer.parseInt ( String s )

e.g. String s = "123456789";int i = Integer.parseInt ( s )

static String Integer.toString ( int i) e.g. int i = 123456789;

String s = Integer.toString ( i ) static Integer Integer.valueOf ( String s )

e.g. String s = "123456789";Integer intWrap = Integer.valueOf ( s )

or Integer intWrap = new Integer ( s )