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Classes Methods and Properties

Introduction to Classes and Objects In object-oriented programming terminology, a class is defined as a kind of programmer-defined type From the natural language definition of the word “class”: Collection of members that share certain attributes and functionality Likewise classes in object-oriented programming In object oriented programming languages (like C#, Java) classes are used to combine everything for a concept (like date) Data ( state / attributes ) (e.g. date day , month, year ) Methods (behavior / tasks) (e.g. display date, increment date )

An Overview of Object Oriented (OO) Programming An example without OO programming - Calendar display program needs several utility functions/methods leap year check day of week function … day day of week month MonthName leap year year Data Functions . . . Is this structure complex? for some yes, for some no

An Overview of Object Oriented (OO) Programming OO version - Calendar display program Date concept is developed as a class data and methods combined together from the point of view of programmer Did you like this? for some yes, for some no OO approach is more suitable for a human being human cognition is mostly based on objects Data (day, month, year) Methods Day of the week Month name …

Introduction to Classes and Objects We define variables of types (like int, double). Similarly, we define objects of classes an object is a member of a class Why classes and objects? In other words, why object-oriented programming? It gives programmers the ability to write programs using off-the-shelf components without dealing with the complexity of those components Saves time and effort Objects are how real-world entities are represented. You may design and implement, and later use your own classes, but we will start with using other-programmers-defined classes Examples: we used the Console class this is what a programmer generally does

How to Use Classes? The behavior of a class is defined by its methods by which objects of that class are manipulated You should know about the methods and what they do name of the method parameters and parameter types return type functionality You don’t need to know how the method is implemented analogy: you can add two int variables using +, but you don’t need to know how computer really adds more analogy: you can drive cars, but you don’t need to know how the fuel injection works

The class Dice Computer simulated dice not real dice, but have the same functionality random number between 1 and “number of sides” in this class, we can have dice objects with any number of sides State number of sides roll count Methods Dice(int sides) // constructor – constructs a die with given number of sides int Roll() // return the random roll int NumSides() // how many sides int NumRolls() // # of times this die rolled Dice objects will work as pseudo-random number generator Random class from .NET library

Using the class Dice Console.WriteLine("Rolling {0} sided die.", cube.NumSides()); Console.WriteLine(cube.Roll()); Console.WriteLine(cube.Roll()); Console.WriteLine("Rolled {0} times.", cube.NumRolls()); Dice cube = new Dice(6); // construct six-sided die Dice dodeca = new Dice( 12 ); // construct twelve-sided die See UseDice.cs for full program methods constructor

State and Behavior Behavior of a class is what a class does described in verbs babies eat, cry dice are rolled In OO programming terminology, behavior is defined by public methods for Dice class, methods are the Dice constructor , NumRolls() , NumSides() and Roll() State of a class depends on physical properties cars have four wheels, different colors dice have a number of sides In OO programming, State is defined by private data also called member data , instance variables , or data fields for Dice class, mySides and myRollCount (see Dice.cs )

Objects An object is an instance of a class When created, in memory a set of private data members are allocated and initialized according to the constructor method In other words, each object has a different state However, objects share method implementations The same function name is used on all objects of the same class When a me thod is called on an object, that object’s private data members are accessed and/or modified

Anatomy of the Dice class The class Dice Objects: 6-sided dice, 32-sided dice, one-sided dice Methods: Roll(), NumSides(), NumRolls() A Dice object has state and behavior Each object has its own state, just like each int has its own value Number of times rolled, number of sides All objects in a class share method implementations, but access their own state How to respond to NumRolls() ? Return my own # of rolls

What to know? Client programmer (programmer who uses the classes) needs to know the interface public methods and constructors p arameters, how they behave does not need to know private data does not need to know how the methods are implemented

From interface to use, the class Dice static void Main(string[] args) { Dice cube = new Dice(6); Dice dodeca = new Dice(12); Console.WriteLine(cube.Roll()); Objects constructed 0 myRollCount my Sides 6 cube 0 myRollCount my Sides 12 dodeca Method invoked 1 myRollCount my Sides 6 cube

Let’s look at the Dice.cs Definition and implementation of the Dice class

Understanding Class Implementations Private data members are global such that they are accessible by all class member function s e.g. in the implementation of Roll function, mySides and myRollCount are not defined , but used

Understanding Class Implementations Constructors should assign values to each instance variable this is what construction is not a rule, but a general programming style All data should be private Provide propertied or methods as needed

Random class Objects of class Random can produce random byte, int and double values. Method Next of class Random generates a random int value. The values returned by Next are actually pseudorandom numbers —a sequence of values produced by a complex mathematical calculation. The calculation uses the current time of day to seed the random-number generator. If you provide Next with two int arguments, it returns a value from the first argument’s value up to, but not including, the second argument’s value. The calculation that produces the pseudorandom numbers uses the time of day as a seed value to change the sequence’s starting point. You can pass a seed value to the Random object’s constructor. Given the same seed value, the Random object will produce the same sequence of random numbers.

Access Modifiers class Dice { private int myRollCount; // # times die rolled private int mySides; // # sides on die public Dice(int sides) { } public int Roll() { } } Default is private . (if there is no access modifier)

Access modifiers public Methods and Constructors as seen by programmer Programmer can use the methods and properties defined in the public section only private Mostly the data part of the class Necessary for internal implementation of class Not accessible by programmer protected we will see this in inheritance internal Accessible only by methods in the defining assembly protected internal we will see this in inheritance

Member Data (instance variables) class Dice { private int myRollCount; private int mySides; } Will the following code compile? Dice cube = new Dice(6); Console.WriteLine("Number of sides: {0}", cube. mySides ); How to fix this? Console.WriteLine("Number of sides: {0}", cube. NumSides ()); Hiding data (encapsulation): why? you can drive cars, but you don’t need to know how the fuel injection works when the car’s fuel injection changes, you can still drive that new car

Properties private int myRollCount ; // # times die rolled // property to get and set the number of sides public int NumRolls { get { return myRollCount; } // end get set { myRollCount = value; } // end set } // end property NumRolls Does get makes sense? How about set ?

Properties private int mySides; // # sides on die // property to get and set the number of sides public int NumSides { get { return mySides; } // end get set { mySides = value; } // end set } // end property NumSides Console.WriteLine("Number of sides: {0}", cube. NumSides ); Does get makes sense? How about set ?

Autoimplemented Properties // property to get and set the roll count public int NumRolls { get; private set; } // property to get and set the number of sides public int NumSides { get; set; } public Dice(int sides) { NumRolls = 0; NumSides = sides; }

Methods The best way to develop and maintain a large application is to construct it from small, simple pieces  divide and conquer Methods allow you to modularize an application by separating its tasks into reusable units. Reuse the Framework Library, do not reinvent the wheel Divide your application into meaningful methods such that it is easier to debug and maintain. Methods == Worker analogy:

Methods syntax access_modifier return_type func_name ( parameter list ) { statement_1; … statement_n; return return_type; } (type param 1 , type2 param 2 , …, type param n ) public, private Examples: public int Roll () public Dice ( int sides ) public static void Main ( string[] args )

Methods syntax (cont’d) There are three ways to return control to the statement that calls a method . Reaching the end of the method. A return statement without a value. A return statement with a value. There could be more than one return in a method. At least one return is required in a non-void method.

Method Overloading void DoSomething( int num1, int num2 ); void DoSomething( int num1, int num2, int num3 ); void DoSomething( float num1, float num2 ); void DoSomething( double num1, double num2 ); The compiler distinguishes overloaded methods by their signature —a combination of the method’s name and the number, types and order of its parameters. Method calls cannot be distinguished by return type  compile error int SquareRoot( int num ); double SquareRoot( int num ); Constructor can be overloaded too.

Scope of Variables The basic scope rules are as follows: The scope of a parameter declaration is the body of the method in which the declaration appears. The scope of a local-variable declaration is from the point at which the declaration appears to the end of the block containing the declaration. The scope of a non-static method, property or field of a class is the entire body of the class. If a local variable or parameter in a method has the same name as a field, the field is hidden until the block terminates Let’s see an example: scope.cs

Static Methods public class ScopeTest { public static void Main(string[] args) … } public static class Math { … public static int Max(int val1, int val2); public static int Min(int val1, int val2); … } You do not need to create an object in memory to use the method, you simply use the class’s name to call the method Other methods of the class cannot be called from a static method, only static methods can be called from other static methods

Math class example ( maximum3.cs ) Let’s write a program that finds the maximum of 3 numbers Randomly generate the numbers Then let’s use the Math class to find the maximum

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