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Algorithms

1. Objects and Classes

Objectives

oReferences and Aliases oArguments and Parameters oPassing Arguments oConstructors oThe toStringMethod oStatic Fields and Methods

Algorithms -1. Objects and Classes

Objectives

oAdapters oInvoking constructors from within constructors oPrivate fields and methods of the base class oOverriding, overloading methods oProtected access oMultiple inheritance oType compatibility and base classes oThe class Object oAbstract classes and methods

Objectives

oWriting an Interface oImplementing an Interface oAn Interface as a Data Type oType Casts Within an Interface Implementation oExtending an Interface oNamed Constants Within an Interface oInterfaces Versus Abstract Classes

Objects

oContains data oPerforms certain actions a given problem

Classes

oThe same kinds of data oThe same methods of oWhat the object is oWhat it can do

Classes

Fig. 1-1 An outline

of a class

Class Instantiation

Fig. 1-2 Three instances of the class automobile

Methods in Java

the class oInvokes the constructor method

Methods in a Java Class

Fig. 1-3 A variable that references an object.

References and Aliases

byte, short, int, long float, double, char, boolean

String greeting = "Howdy";

ogreetingis a reference variable instance, they are considered aliases

References and Aliases

Fig. 1-4 Aliases of an object

Arguments and Parameters

Name joe = new Name();

joe.setFirst ("Joseph"); joe.setLast ("Brown"); the methods arguments as there are formal parameters in the declaration

Defining a Class

oThey will require accessor and mutator methods public class Name { private String first; // first name private String last; // last name < Definitions of methods are here. > } // end Name

Methods

oThis is a valuedmethod oReturns a String public void setLast(String lastName) { last = lastName; } // end setLast public String getLast() { return last; } // end getLast

Naming Convention

oUse verb or action phrase e.g. getLast oUse noun or descriptive phrase e.g public class Name oA variable declared within a method

Passing Arguments

oFor primitive type, parameter initialisedto value of argument in call o"Java manipulates objects 'by reference,' but it passes object references to methods 'by value.'"

Passing Arguments

Fig.1-5 a & b The method giveLastNameTomodifies the object passed to it as an argument.

Name jamie

jamie.giveLastNameTo(jane);

Public class Name{

public void giveLastNameTo(Name child) { child.setLast(last); } // end giveLastNameTo

Example In Class

Static Fields & Methods

one object shared by allthe instances of the class static field, static variable, class variable

Data Structures -1. Objects and

Classes

Static Fields & Methods

Data Structures -1. Objects and

Classes

Fig. 1-8 A static PIversus a non static field

Packages

grouped into a package package package myStuff; oGive folder same name as the package import myStuff.*;

Data Structures -1. Objects and

Classes

Composition

of another class

Data Structures -1. Objects and

Classes

Fig. 1-9 A Studentobject composed of other objects

A "has a"

relationship

Inheritance

oAdding to details of the base class oRevising details of the more general class oSaves work oCommon properties and behaviors are define only once for all classes involved

Algorithms -1. Objects and Classes

Inheritance

Algorithms -1. Objects and Classes

Fig. 1-10 A hierarchy of classes.

An "is a"

relationship public class Car extendsAutomobile } // end Car

Car toyota = new Car();

Base Class Constructor

oThe constructor must call the base class constructor the constructor of the base class oWhen superis used, it must be the first action in the derived constructor definition oMust notuse the name of the constructor

Data Structures -1. Objects and

Classes

Base Class Constructor

Data Structures -1. Objects and

Classes

public class Automobile extends Vehicle{ privateint year; // year of manufacture privateString colour; publicAutomobile(int iYear, String sColour){ year = iYear; colour = sColour; } // end Automobile public class Car extends Automobile{ private int engineSize; privateString modelType; // saloon, hatchback public Car (int iYear, String sColour, int iEngine,

String sModel){

super (iYear, sColour); engineSize = iEngine; modelType = sModel; } // end Car

Accessing Inherited Data Fields

oNot accessible by name within definition of a method from another class including a derived class oStill they are inheritedby the derived class class e.g. getModel(); unavailable to derived classes oBut usually not a problem private methods are used only for utility duties within their class

Data Structures -1. Objects and

Classes

Overriding Methods

signature as in base class oSame name oSame return type oSame number, types of parameters use the definition from thederived class overridden method of the base class

Data Structures -1. Objects and

Classes

Overriding Methods

Data Structures -1. Objects and

Classes

public class Automobile extends Vehicle{ privateint year; // year of manufacture privateString colour; publicString toString(){ return } // end Automobile public class Car extends Automobile{ private int engineSize; privateString modelType; // saloon, hatchback public String toString(){ return super modelType; } // end Car

Overriding Methods

o derived from a base class oAll three classes have a method with the same signature class cannotinvoke the method in the base class's base class oThe construct super.superis illegal

Data Structures -1. Objects and

Classes

Overloading Methods

oThe same name o oDifferentnumber or type of parameters oand oThe base class method with the same name due to the different parameters

Data Structures -1. Objects and

Classes

Overloading Methods

definition cannotbe overridden oSo that the behavior of the constructor will not be changed

Data Structures -1. Objects and

Classes

public final void whatever()

Multiple Inheritence

derive class Cfrom classes Aand B oA derived class can have only onebase class oA derived class can have multiple interfaces

Data Structures -1. Objects and

Classes

Object Types of Derived Classes

oClass Car, oDerived from class Automobile object

An object of a derived class is alsoan

object of the base class

Data Structures -1. Objects and

Classes

Polymorphism

cause different actions oHappens according to the kinds of objects that invoke the methods

Data Structures -1. Objects and

Classes

UndergradStudent ug = new UndergradStudent(. . .);

Student s = ug; // s and ug are aliases

s.displayAt(2); ug.displayAt(4);

The object still remembers it is of

type UndergradStudent public void displayAt(int numLines){ for (int count=0;count