Java Chapter 2 Creating Classes

Class Defines structure of an object or set of objects; Includes variables (data) and methods (actions) which determine how an object behaves

Construction a Data Declaration Section Variable classifications:  Instance – within a class but outside a method. Every object created receives a variable of this type. The keyword static may not be used.  Example:  public class Date  int year;  int month;  int day;Position: top of a class’s body. Values vary for each different instance of an object of that class.

Class variable  Class – located within a class’s body but outside any method. The keyword static must be used.  e.g. p public class Date  static double time;  public static final int YEAR = 1583; Note the modifier “final” – indicates that the variable YEAR is a constant. Note capitalization.

Local Variables  Local – within a method body. Neither an access specifier nor the keyword static may be used. public static void main (String[] args) { double payRate; int employeeNumber;...  parameter – within the () of a method’s header line. Neither an access specifier nor the keyword static may be used. public double area (double length; double width) {...

Declaration Statements 1. Naming a variable optionalAcccessSpecifier dataType variableName e. g. private double totalDebits  Use identifier rules 2. Initializing a variable’s value  If already declared: totalDebits = ;  If undeclared: private int age = 22;  Watch out for floats: float amtOwed = is incorrect

Task: Construct a program to calculate floor area In what terms could we define the floor? What datatype should we use?  private double width; (Reserved word private used almost universally for all instance variables)  private double length; Do I need to declare a variable area?

Data types Primitive vs. Reference See p. 61 in text Note that with floats – always add either f or F to the literal values float tax = f; Escape sequences – p. 67

Objectives You should be able to describe: Data Values and Arithmetic Operations Constructing a Data Declaration Section: Variables Completing the Class: Methods Assignment Operations

Objectives (continued) Program Design and Development: Object Identification Common Programming Errors

Data Values and Arithmetic Operations General data types:  Primitive  Reference Literal  Value that explicitly identifies itself  Example: 1 “xyz”

Data Values and Arithmetic Operations (continued) Figure 2.1: Primitive data types

Data Values and Arithmetic Operations (continued) Figure 2.2: Reference types

Integer Literals Zero or any positive or negative numerical value without decimal point Examples:  1  -406  Other data types:  Byte  Short integer

Integer Literals (continued) Table 2.1: Integer Data Types

Floating-Point Values Also called real numbers Numerical value with decimal point Examples:   0.0  Data types:  Double  Float

Floating-Point Values (continued) Table 2.2: Floating-Point Data Types

Character Values Letters of alphabet  Both uppercase and lowercase 10 digits 0 through 9 Special symbols such as 1 $., - ! Stored as 16-bit unsigned values using Unicode

Escape Sequences \ before characters Tells compiler to execute differently from normal behavior

Escape Sequences (continued) Table 2.4: Escape Sequences

Boolean Values Restricted to one of two values:  True  False

Arithmetic Operations Used for mathematical calculations Operators:  Addition: +  Subtraction: -  Multiplication: *  Division: /  Modulus: %

Arithmetic Operations (continued) Simple arithmetic expression Integer expression Real expression Mixed-mode expression Overloaded operator  Symbol represents more than one operation  Execution depends on types of operands encountered (e.x )

Integer Division Division of two integers c an produce strange results for unwary Integers cannot contain fractional part  It is dropped Example:  15/7 = 2

The Modulus Operator Retain remainder of division Operator symbol:  % Example:  9.2 % 4 = 1.2  15 % 4 = 3

Negation Unary operator  One that operates on single operand Same symbol as binary subtraction (-) Negates or reverses number

Negation (continued) Table 2.6: Summary of Arithmetic Operations

Operator Precedence and Associativity Rules for working with arithmetic operators:  Two binary arithmetic operator symbols must never be placed side by side  Parentheses may be used to form groupings Expressions in parentheses are evaluated first  Parentheses can be nested  Parentheses cannot be used to indicate multiplication

Operator Precedence and Associativity (continued) Order of operations:  All negations are done first  Multiplication, division, and modulus operations are computed first  Addition and subtraction are computed last

String Concatenation Joins two or more strings into single string Only operation that directly manipulates string data in similar fashion as numerical data Has same precedence as addition operator  Always use parentheses when performing arithmetic operations in any expression that contains a string e.g. “The sum is” produces The sum is 1020

Constructing a Data Declaration Section: Variables Values in computer program are stored and retrieved from computer’s memory unit Variable:  Name given by programmer  Refers to computer storage locations that store primitive data type value Selection of variable names is left to programmer

Constructing a Data Declaration Section Classifications of variables:  Instance  Class  Local  Parameter Dependent on:  Variable placement within class  Presence or absence of reserved word static

Constructing a Data Declaration Section (continued) Table 2.8: Determination of Variable Types

Creating Objects Objects  Only created from instance variables declared in data declaration section or from other object types Methods  Provide operations that can be applied to created objects or create general-purpose functions Independent of any one object

Creating Objects (continued) Figure 2.6: The RoomType class

Creating Objects (continued) Using dynamic memory allocation operator also called:  Creating an instance  Instantiating an object Reference variable  Reference location in memory where actual object’s values located

Creating Objects (continued) Private  Once object is created, its private variables can only be accessed by its class methods  Safety feature provided by object-oriented languages Cleansing Memory  Memory leak problem  Objects keep track of who references them  JVM cleans unused memory

Creating Objects (continued) Figure 2.8: Instantiating an object

Specifying Storage Allocation Declaring variables protects against typos  Compiler catches errors Each data type has its own storage requirements  Compiler pre-allocates memory based on data type Definition statements  Statements that cause variables to be created

Completing the Class: Methods Classes provide methods to:  Initialize values stored in each instance variable to user-selected values  Display values  Modify values Format of method header:  public returnType methodName(parameter list) Public  Means method can be used in other classes

Constructor Methods Method that has same name as class Automatically called each time object created Purpose:  Initialize new object’s instance variables Format: public ClassName(parameter list) { method body }

Accessor Methods Provide means for reading values stored in object’s variables Referred to as get() method

Mutator Methods Provide means for changing object’s values  After object created Also known as set() methods

Convenience Can be convenient to place two classes in one file  Second class’s main() method is placed within first class’s methods section  Both classes are stored in same file public reserved word removed from class that does not contain main() method

Assignment Operations Most basic statements for initializing variables General syntax:  variable = expression; Example:  length = 25; Expression  Any combination of constants and variables that can be evaluated to yield a result

Assignment Operations (continued) All variables used in expression must previously have been given valid values Must be a variable listed immediately to left of equal sign Has lower precedence than any other arithmetic operator

Multiple Declarations Variables with same data type can be grouped  Declared using single declaration statement Frequently used in declaring method’s internal variables Coercion  Forced conversion based on data types

Assignment Variations In an assignment expression:  Variable to left of equal sign can also be used to right of equal sign Shortcut assignment operators:  +=  -=  *=  /=  %=

Accumulating Values added to program variable as program progresses Uses assignment operator:  sum = sum + 10,

Counting Counting statement form:  variable = variable + fixedNumber; Example:  i = i + 1; Increment/decrement operators: ++--  Example : i++

Procedural versus Object Orientations Procedure-oriented programming  Emphasis on operations to be performed Object-oriented programming  Emphasis on attributes and behavior of objects

Common Programming Errors Forgetting to declare all variables used in class Forgetting to assign or initialize values for all variables before variables are used in expression Mixing data types in same expression without clearly understanding effect produced Defining more than one default constructor for class

Summary Primitive data types:  Numerical  Boolean Every variable must be declared as to type of value it can store Class  Programmer-defined data type

Summary (continued) Classes contain methods:  Constructor  Accessor  Mutator Expressions evaluated according to precedence and associativity of operators used Assignment symbol operator:  =

Creating Objects public class UseRoomType { public static void main(String[] args) { RoomType roomOne; // declare a variable of type RoomType roomOne = new RoomType(); // create and // initialize an object of type RoomType } }

RoomType roomOne; roomOne declared as a variable of type RoomType. Notice the capitalizations– what does that indicate? roomOne is a programmer devised variable RoomType is a class Why can’t we declare private RoomType roomOne; ? Ans: placed within a method and all variables declared in a method are local and therefore private The contents of the memory storage of roomOne is null because it does not yet refer to a valid object. It is a reference variable.

The RoomType class public class RoomType { // data declarations section private double length; // declare length as a double variable private double width; // declare width as a double variable }

The process of creating a new object using the dynamic memory allocation operator is referred to as both creating an instance and instantiating an object. Length and width – instance variables: an instance of them only comes into existence when an object is created. Each created object will contain one instance of each instance variable The variable is referenced using the object’s name – e.g. roomOne.length or roomTwo.length

// File:DemoOperators.java // Description: Test program // This program will test the combination of integers and real numbers // and will explore the modulus operator // Programmer: M. Markel // Date: 2/07 public class DemoOperators { public static void main(String[] args) { System.out.println("9/2 = "+ 9/2); System.out.println("9%2 = "+ 9%2); System.out.println("9.0/2 = "+ 9.0/2); System.out.println("9/2.0 = "+ 9/2.0); System.out.println("9.0%2 = "+ 9%2); System.out.println("9.0f/2 = "+ 9.0f/2); System.out.println("9.0/1.2 = "+ 9.0/1.2); System.out.println("9.0%1.2 = "+ 9.0%1.2);