CSE 1302

Lecture 7

Object Oriented Programming Review

Richard Gesick

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Topics• Identifying Classes & Objects• Static Members• Class Relationships• “this”• Parameters Revisited• Overloading

– Methods– Operators

• Testing

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Identifying Classes & Objects• Identify potential classes within the

specification• Nouns

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In Gaming…

• How do you identify the classes within a game?

• What have you seen so far in labs and assignments?

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What Goes in the Class

• How do you decide what should be in the class– Data– Methods

• Not easy, but must be done

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Static Members

• Static methods can be invoked via the class name

• Static variables are stored at the class level (one copy for all instances)

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The static Modifier• Remember that static methods (also called class methods) that

can be invoked through the class name rather than through a particular object

• For example, the methods of the Math class are static:

Math.sqrt (25)

• To write a static method, we apply the static modifier to the method definition

• The static modifier can be applied to variables as well

• It associates a variable or method with the class rather than with an object

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Static Variables• Static variables are also called class variables • Normally, each object has its own data space, but if a variable

is declared as static, only one copy of the variable exists

private static int count;

• Memory space for a static variable is created when the class in which it is declared is loaded

• All objects created from the class share static variables• Changing the value of a static variable in one object changes it

for all others• Local variables cannot be static

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Static Methods

public static int triple (int num){ int result; result = num * 3; return result;}

class Helper

Because it is static, the method can be invoked as:

value = Helper.triple (5);

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Static Methods

• Static methods cannot reference instance variables, because instance variables don't exist until an object exists

• However, a static method can reference static variables or local variables

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Class Relationships

Classes can have various relationships to one another. Most common are:

• Dependency (“uses”)• Aggregation (“has a”)• Inheritance (“is a”)

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Object Relationships

• Objects can have various types of relationships to each other

• A general association, as we've seen in UML diagrams, is sometimes referred to as a use relationship

• A general association indicates that one object (or class) uses or refers to another object (or class) in some way

• We could even annotate an association line in a UML diagram to indicate the nature of the relationship

Author Bookwrites

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Dependency• One class dependent (uses) another class

– Game uses ball, paddle– Ship uses bullet

• Sometimes, a class depends on another instance of itself– Is one date equal to another date?– Is one picture equal to another picture?

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Aggregation

• One class is “made up” of other classes• “has a” relationship

– Gameboard has a marble– Deck has a card

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Aggregation• An aggregate object is an object that contains

references to other objects

• For example, an Account object contains a reference to a String object (the owner's name)

• An aggregate object represents a has-a relationship

• A bank account has a name

• Likewise, a student may have one or more addresses

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Aggregation in UML• An aggregation association is shown in a UML class

diagram using an open diamond at the aggregate end

StudentBody

+ Main (args : String[]) : void

+ ToString() : String

1 2Student

- firstName : String- lastName : String- homeAddress : Address- schoolAddress : Address

+ ToString() : String

- streetAddress : String

- city : String- state : String

- zipCode : long

Address

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The this Reference• The this reference allows an object to refer to itself

• That is, the this reference, used inside a method, refers to the object through which the method is being executed

• Suppose the this reference is used in a method called tryMe

• If tryMe is invoked as follows, the this reference refers to obj1:

obj1.tryMe();

• But in this case, the this reference refers to obj2:

obj2.tryMe();

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The this reference• The this reference can also be used to

distinguish the parameters of a constructor from the corresponding instance variables with the same names

public Account (String name, long acctNumber, double balance){ this.name = name; this.acctNumber = acctNumber; this.balance = balance;}

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Assignment Revisited• The act of assignment takes a copy of a value and stores it in

a variable

• For primitive types:

num2 = num1;

Before

num1

5

num2

12

After

num1

5

num2

5

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Reference Assignment

• For object references, assignment copies the memory location:

bishop2 = bishop1;

Before

bishop1 bishop2

After

bishop1 bishop2

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Aliases• Two or more references that refer to the same

object are called aliases of each other

• One object (and its data) can be accessed using different reference variables

• Aliases can be useful, but should be managed carefully

• Changing the object’s state (its variables) through one reference changes it for all of its aliases

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ParametersThere are three main types of parameters:1. Value - passes a copy (value) of the variable to the method. This

is the default. 2. Reference - passes a reference to the actual variable. Marked

with "ref", use this when you want to pass a value in and have any change to that value be persistent when the method is complete

3. Out - passes a reference to the actual variable. Marked with "out", use this when you want the method to generate a value and place it for later use in the actual variable (persists when the method is complete)

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Example 1 and the output is ?static void Main() {

int a = 42; Console.WriteLine (a); B (a); Console.WriteLine (a);

} static void B (int x) {

x += 9; Console.WriteLine (x);

}

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Example 2 and the output is ?static void Main() {

int a = 42; Console.WriteLine (a); B (ref a); Console.WriteLine (a);

} static void B (ref int x) {

x += 9; Console.WriteLine (x);

}

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Example 3 and the output is ?static void Main() {

int a; B (out a); Console.WriteLine (a);

} static void B (out int x) {

x = 9; Console.WriteLine (x);

}

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Example 4 and the output is ?class Z { public int y; } static void Main() {

Z myZ = new Z(); myZ.y = 42; Console.WriteLine (myZ.y); B (myZ); Console.WriteLine (myZ.y);

} static void B (Z x) {

x.y += 9; Console.WriteLine (x.y);

}

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Example 5 and the output is ?class Z { public int y; } static void Main() {

Z myZ = new Z(); myZ.y = 42; Console.WriteLine (myZ.y); B (ref myZ); Console.WriteLine (myZ.y);

} static void B (ref Z x) {

x = new Z(); x.y = 1; Console.WriteLine (x.y);

}

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• Be careful to note the difference between a pass-by-reference parameter and a parameter of a reference type.

• Use the activation stack to track local variables and how parameters of the above types affect the variables from one stack frame to the next.

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Overloading Methods• Method overloading is the process of using the same method

name for multiple methods

• The signature of each overloaded method must be unique

• The signature includes the number, type, and order of the parameters

• The compiler determines which version of the method is being invoked by analyzing the parameters

• The return type of the method is not part of the signature

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Overloading Methods

float tryMe (int x){ return x + .375;}

Version 1

float tryMe (int x, float y){ return x*y;}

Version 2

result = tryMe (25, 4.32f)

Invocation

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Overloading Operators

• In C#, not only can methods be overloaded, operators can be overloaded as well.

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• C# enables you to overload most operators to make them sensitive to the context in which they are used.

• Use operator overloading when it makes an application clearer than accomplishing the same operations with explicit method calls.

• Class ComplexNumber overloads the plus (+), minus (-) and multiplication (*) operators to enable programs to add, subtract and multiply instances of class ComplexNumber using common mathematical notation

•Operator overloading

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Operator Overloading• Keyword operator, followed by an operator

symbol, indicates that a method overloads the specified operator.

• Methods that overload binary operators must take two arguments—the first argument is the left operand, and the second argument is the right operand.

• Overloaded operator methods must be public and static.

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1 // Fig. 12.17: ComplexNumber.cs

2 // Class that overloads operators for adding, subtracting

3 // and multiplying complex numbers.

4 using System;

5

6 public class ComplexNumber

7 {

8 // read-only property that gets the real component

9 public double Real { get; private set; }

10

11 // read-only property that gets the imaginary component

12 public double Imaginary { get; private set; }

13

14 // constructor

15 public ComplexNumber( double a, double b )

16 {

17 Real = a;

18 Imaginary = b;

19 } // end constructor

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20

21 // return string representation of ComplexNumber

22 public override string ToString()

23 {

24 return string.Format( "({0} {1} {2}i)",

25 Real, ( Imaginary < 0 ? "-" : "+" ), Math.Abs( Imaginary ) );

26 } // end method ToString

27

28 // overload the addition operator

29 public static ComplexNumber operator +(

30 ComplexNumber x, ComplexNumber y )

31 {

32 return new ComplexNumber( x.Real + y.Real,

33 x.Imaginary + y.Imaginary );

34 } // end operator +

35

Overload the plus operator (+) to perform addition of ComplexNumbers