classes, types and objects

Classes, Types and ObjectsClasses, Types and Objects

Data StructuresData StructuresChapter 1Chapter 1

Prof. Vidya ManianDept. of Electrical and Comptuer Engineering

ICOM 4035 Java Programming BasicsECE, UPRM

Overview

• Objects, classes• expressions• Control flow• Arrays

ICOM 4035 ECE, UPRM 2

Objects

• Objects –store data and provide methods for accessing and modifying data

• An object is an instance (member) of a class• Class – specifies type of objects, operations

that it performs• Data of objects are stored as instance

variables (fields) (of types e.g. integers, floating point numbers, Booleans)


Operations

• Methods :constructors, procedures, and functions

• Example: class declaration


• Counter class is public-other class can create and use a counter object

• Instance variable –count• Constructor method initializes it to 0• Accessor method, get Count• Update methods, incrementCount,

decrementCount


Class modifiers

• Abstract-empty methods with abstract keyword• Final – cannot have subclasses• Public – can be instantiated by anything that

imports the class• Base types (primitve types (boolean, char, byte,

short, int, long, float, double)• Int x, x=14 or x=195• Float, y=3.1415


Example of base types


• Comments: /** **/ or //• ‘new’ creates a new object and returns

reference to that object


new operator

• A new object is dynamically allocated in memory, initialized to default values ‘null’

• Constructor is called• The new operator returns a reference (a

memory address) to the newly created object


Number classes

• Byte n.byteValue()• Shortn.shortValue()• Integer n.intValue()• Long n.longValue()• Float n.floatValue()• Double n.doubleValue()


String objects

• String is a sequence of characters that comes form some alphabet (set of possible characters)

• P = “barney the bear”• Length 15• Concatenation• String s = “kilo” + “meters”


Object reference

• Reference variable is a pointer to some object


Dot operator• Can have many references to the same object• Dot operator”.”-access methods and instance variables

associated with an object• Examples of signatures (methods name with parameters)• oven.cookDinner();• oven.cookDinner(food);• oven.cookDinner(food,seasoning);• Note: Java doesn not allow 2 methods with the same

signature to return different types


Instance Variables• Instance variables must have a type (int, float,

double) or a reference type : class such as String, an array

• A referrence variable v, points to object o, we can access any of the instance variables fo o that the access rules allow

• Public instance variables are accessible by everyone• Dot operator can get and set the value of any

instance variable (v.i)


• gnome.name =“professor smith”;• gnome.age=35;• gnome –reference to a Gnome object, with public

instance variables name and age• Variable modifiers• Public: anyone can access• Protected: only methods of the same package or of

its subclasses can access protected instance variables• Private: only methods of the same class can access it


• Static: variables are used to store “global” information about a class

• Final: a final instance variable is one that must be assigned an initial value, and can never be assigned a new value after that (MAX_HEIGHT)


• What are the base types ?• What is variable gnomeBuddy ?• What is variable name ?• Note: Constant values associated with a class

should be declared to be both static and final


Enum Types

• Take on values from a specified set of names• Modifier enum name {value_name0,

value_name1,….};• Modifer – black, public, protected or private• Name – any legal Java identifier• public enum Day

{MON,TUE,WERD,THU,FRI,SAT,SUN}


• Output:• Initially d is MON• Then it is WED• I say d and t are the same: true


Methods

• Similar to functions and procedures• Method has 2 parts: body and signaturemodifiers type name(type0 parameter0, …typen-1 parametern-1) {// method body…}

• type is the return type of the method, name – name of the method

• When a method of a class is called, it is invoked on a specific instance of the class and can change the state of the object


• Similar to instance variables, Method modifiers can be public, protected, private or abstract

• Note: All parameters are passed by value, changing the internal reference inside a method will not change the reference that was passed in


Constructors• A constructor is a special kind of method that

is used to initialize newly created objects• Note: name of constructor, name, must be

same as the name of the class it constructs• Return type not specified for constructor


• Constructor definition and invocation• Return statements are not allowed in a constructor

body• A new instance of this class is created and its

constructor is used to initialize its isntance variables• Must be called using new operator• Fish myFish = new Fish(y, “Wally”);• Classes that define Stand – alone Java program has

the main method


• Java Aquarium //system looks for compiled version of Aquarium class and invokes the special main method in that class


Statement blocks and local variables

• Two ways of declaring local variables• type name;• type name = initial_value;


Expressions• Expressions involve the use of literals, variables and

operators• Variables and constants are used expressions to define

new values/modify variables• Literal: is any “constant” value

– Null object reference– Boolean: true and false– Integer: type int, (32-bit integer), long iteger: 176L or -52l (64-bit

integer)– Floating point: default for floating point numbers is double,

3.14E2 or .19e10


– Character, ‘a’, ‘?’ are character constants– Special characters

– String literal: sequence of characters “have a nice day”


Operators• Assignment operator : variable=expression• i = j = 25;• Arithmetic operators: +, -, *, /, %• Increment and decrement operators

– int i=8; – int j=i++; – int k=++i; – int m=i--;– int n = 9+i++;


Logical operators• < , <=, ==, !=, >=, >• Boolean: ! Not• && conditional and, || conditional or• Bitwise operators: ~ bitwise complement, &

bitwise and, | bitwise or, ^ bitwise exclusive or, <<, >>, >>>

• Operational assignment operators: variable op = expression

• Variable = variable op expression


Arrays


Declaring arrays• Element_type[] array_name = {init_val_0,

init_value_1,.., init_val_N-1}• int[] primes = {2,3,5,7,11}• element_type[] array_name;• new element_type[length]


Arrays are objects


Cloning an array


// Example 1 // --------- // First declare//reference and then construct it. int[] ExampleArray1; ExampleArray1 = new int[24];

// Example 2 // --------- // This can be considered//the short form for declaring and construction.

int[] ExampleArray2 = new int[24]; ICOM 4035 ECE, UPRM 36

// Example 3 // --------- // Construct and assign//an array using a single command. String[] ExampleArray3 = { "Sun Solaris" , "HP-

UX" , "Linux" , "MS Windows" , "Macintosh" }; int[] array = null;int[] arr = new int[] {1,2,3}; int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} }; int[][] myArray = new int[5][]; ICOM 4035 ECE, UPRM 37

Testing 2D array


class TestTwoDimArrays { // initialize # of rows static int [][] myArray = new int[3][]; public static void main(String[] args) { myArray[0] = new int[3]; // initialize # of cols myArray[1] = new int[4]; // in each row myArray[2] = new int[5]; for(int i=0; i<3; i++) // fill and print the array fillArray(i, i+3); System.out.println(); } // end main() private static void fillArray(int row, int col) { for( int i=0; i<col; i++) myArray[row][i] = i; for( int i=0; i<col; i++) System.out.print(myArray[row][i]); System.out.println(); } }

Exercises in group

• Suppose that we create an array A of GameEntry objects, with integer scores field, we cloe A and store the result in an array B. If we set A[4].score =550, what is the score value of GameEntry object referenced by B[4]?

• Write a Java method that takes an array of int values and determines if all the numbers are different from each other.


classes, types and objects

Documents

access methods

instance variable v

instance member

instance variables fo

countupdate methods

final instance va

object o

reference type