march, 23 - 24, 2013 swu, blagoevgrad

Lesson 3Java and O-O Programming

/Methods, Classes, Inheritance, Polymorphism, Interfaces/

AUBG ICoSCIS TeamAssoc. Prof. Stoyan Bonev

March, 23 - 24, 2013 SWU, Blagoevgrad

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Lesson Contents:

Structured Programming – theory & practice OOP – theory & practiceJava methodsJava classesInheritancePolymorphismAbstract classes and Interfaces

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Structured Programming – theory

Structured programming: A technique for organizing and coding computer programs in which a hierarchy of modules is used, each having a single entry point and a single exit point, and in which control is passed downward through the structure without unconditional branches to higher levels of the structure.

Dividing a program into functions and modules .Thinking in Functions!Basic building construct – member function or

method

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OOP – theoryData Encapsulation & Data Hiding: Data and its

functions are said to be encapsulated into a single entity

Inheritance: The process of creating new classes or derived classes from existing classes or base classes. Inheritance as “is a kind of” relation btw classes.

Polymorphism: Ability to redefine methods for derived classes. Giving different meaning to the same thing.

Dividing a program into classes and objects .Thinking in Classes!Basic building construct – classes and objects

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StrProg – problem to solveTo develop – arithmetic operators processorA routine for each arithmetic operator is mustAddition: int add(int p1, int p2) { return p1+p2;}Expand it with routines for all arithmetic

operators: +, -, *, /, %, ^Solution looks like this:

public class StructProgArithCalc { public int add(int pa, int pb) { return pa + pb; } public static void main(String[] args) { System.out.println(" " + add(55, 66)); }}

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OOP – problem to solveTo develop – arithmetic operators processorA separate individual class with methods for each

arithmetic operator is mustJava, file OOPArithCalc.java

package ooparithcalcjava;

class ArithCalc {

public int add(int pa, int pb) { return pa + pb; }

}

public class OOPArithCalc {

public static void main(String[] args) {

ArithCalc myCalc = new ArithCalc();

System.out.println(" " + myCalc.add(55, 66));

}

}

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OOP – problem to solve

Build your own arithmetic operators processor.

Expand it with methods for all arithmetic operators: +, -, *, /, %, ^

Name file as OOPArithCalc.java

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More on Java syntax

Methods or member functions Predefined and User defined methods

9 9Java Programming: From Problem Analysis to Program Design, 4e

Predefined MethodsMethods already written and provided by JRE

Organized as a collection of classes (class libraries)

To use: import <package>;

Illustration: Math class, String class

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Predefined methods


Predefined Classes (continued)


Some Commonly Used String Methods


Some Commonly Used String Methods (continued)


User-Defined MethodsUser-defined methods in Java are classified in two

categories:Value-returning methods

Methods have a return data type Methods return a value of specific data type using return

statement Used in expressions, Calculate and return a value Can save value for later calculation or print value

Void methods Methods that do not have a return data type Methods do not use return statement to return a value

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Opening Problem

int sum = 0;for (int i = 1; i <= 10; i++) sum += i;System.out.println("Sum from 1 to 10 is " + sum);

sum = 0;for (int i = 20; i <= 30; i++) sum += i;System.out.println("Sum from 20 to 30 is " + sum);

sum = 0;for (int i = 35; i <= 45; i++) sum += i;System.out.println("Sum from 35 to 45 is " + sum);

Find the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively.

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Solution

public static int sum(int i1, int i2) { int sum = 0; for (int i = i1; i <= i2; i++) sum += i; return sum;}

public static void main(String[] args) { System.out.println("Sum from 1 to 10 is " + sum(1, 10)); System.out.println("Sum from 20 to 30 is " + sum(20, 30)); System.out.println("Sum from 35 to 45 is " + sum(35, 45));}

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Creating/Defining Methods

A method is a collection of statements that are grouped together to perform an operation.

public static int max(int num1, int num2) {

int result; if (num1 > num2) result = num1; else result = num2; return result;

}

modifier return value

type method name

formal parameters

return value

method body

method header

parameter list

Define a method Invoke a method

int z = max(x, y);

actual parameters (arguments)

method signature

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Calling Methods•In creating a method, you give a definition of what the method is expected to do.

•To use a method, you have to call it, or invoke it, or activate it in two ways depending on the method category:

•As a value, (i.e. operand of an expression)

Larger = max(20, 50);

System.out.println(“Result is=“, max(20,50));

•As a statement

System.out.println(“JAVA”);

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Calling Methods, cont.

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass the value of i pass the value of j

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void method example

public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); }

Suppose you invoke the method using nPrintln(“Welcome to Java”, 5);

What is the output?

Suppose you invoke the method using nPrintln(“Computer Science”, 15);

What is the output?

This type of method does not return a value. The method performs some actions.

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Passing parameters by Values

When calling a method, the argument value is passed /copied/ to the formal parameter.

The argument is not affected, regardless of the changes made to the parameter inside the method.

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Overloading MethodsOverloading the max Method

public static double max(int num1, int num2) { if (num1 > num2) return num1; else return num2;}

public static double max(double num1, double num2){ if (num1 > num2) return num1; else return num2;}

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Arrays and Methods

Passing Arrays to methods Through parameter passing mechanism by value

Returning an Array from a Method Through return stmt

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Arrays and Methods

public static int[] arrayProcess(int[] par) {

int j;

int[] b = new int[par.length];

for (j=1; j<par.length; j++) b[j] = par[j] *10;

return b;

}

Do you have any critical remark on the source text?

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Arrays and Methods


int[] ar = new int[10];

for (i=0; i < ar.length; i++) ar[i] = i;

int[] br;

br = arrayProcess(ar);

for (i=0; i < br.length; i++) System.out.print(br[i] + " ");

} // end of main

04/20/23 Assoc. Prof. Stoyan Bonev

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Pass By ValueJava uses pass by value to pass arguments to a method. There are important differences between passing a value of variables of primitive data types and passing arrays.

For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method.

For a parameter of an array type, the value of the parameter contains a reference to an array; this reference is passed to the method. Any changes to the array that occur inside the method body will affect the original array that was passed as the argument.

04/20/23 Assoc. Prof. Stoyan Bonev

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public class Test { public static void main(String[] args) { int x = 1; // x represents an int value int[] y = new int[10]; // y is an array of int values m(x, y); // Invoke m with arguments x and y System.out.println("x is " + x); System.out.println("y[0] is " + y[0]); } // end of main public static void m(int number, int[] numbers) { number = 1001; // Assign a new value to number numbers[0] = 5555; //Assign new value to numbers[0] } // end of m} // end of class Test

Simple Example

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Exercise on methods

Expand the program Methods1.java adding more methods that reside in the same class with method main():

Iterative GCD – gcdi()Iterative/Recursive factorial – facti(), factr()Iterative/Recursive Fibonacci – fiboi(), fibor()Iterative/Recursive Sum(1..n) – sumi(), sumr()Iterative/Recursive Power operator simulator – powi(), powr()

Write a program to demonstrate the effect of passing by value. (method swap)

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ExercisesPalindrome: string that reads the same forwards

and backwards Write a value-returning method isPalindrome(), that returns true if a given string is palindrome, i.e. it reads the same way forwards and backwards.

The method isPalindrome() takes a string as a parameter and returns true if the string is a palindrome, false otherwise

Write a Java program to test the method


Solution: isPalindrome() Methodpublic static boolean isPalindrome(String str){ int len = str.length(); int i, j = len - 1;

for (i = 0; i <= (len - 1) / 2; i++) { if (str.charAt(i) != str.charAt(j)) return false; j--; } return true; }


Rewrite isPalindrome() Methodusing while loop stmt

public static boolean isPalindrome(String str){ int len = str.length(); int i, j; j = len - 1;

while (i <= (len - 1) / 2) { if (str.charAt(i) != str.charAt(j)) return false; i++;

j--; } return true; }

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Exercises

Write a value-returning method isVowel(), that returns true if a given character is vowel, and otherwise returns false.

Write a Java program to test the methodWrite a Java program to output the number of

vowels in a string.


The int variable num contains the desired sum to be rolled

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Moore on Java syntax

Classes and Objects

C# Programming: From Problem Analysis to Program Design 40C# Programming: From Problem Analysis to Program Design 40

Anatomy of a Class

Classes defined inside packages

Classes group: Data members

Member functions /methods/

All programs consist of at least one class that includes at least one method main()

Every class is named

C# Programming: From Problem Analysis to Program Design 41C# Programming: From Problem Analysis to Program Design 41

Class Definition• Building block or basic encapsulation constructs

of Java object-oriented program.

• Every executable statement must be placed inside a method and every method must be placed inside a class.

• Classes define a category, or type, of object.

• Classes define reference types that are the basic building blocks of Java programs, and they serve as ADT to create objects in OOP

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Defining Classes for Objects

An object has both a state and behavior.•The state defines the object.•The behavior defines what the object does.Data fields define state and methods define behaviors. Additionally, a class provides special type of methods, known as constructors, invoked to construct objects from the class.

Example: •A Circle object has a data field, i.e. radius which is the property to characterize a circle.

•One behavior of a circle is that its area can be computed using the method getArea()

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Classes class Circle {

/** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * 3.14159; }

}

Data field

Method

Constructors

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Constructors

Circle() { radius = 0.0;}//----------------------------

Circle(double newRadius) { radius = newRadius;}

Constructors are a special kind of methods that are invoked to construct objects.A constructor with no parameters is referred to as a no-arg constructor.

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Constructors

Circle() { radius = 0.0;

}//----------------------------

Circle(double radius) { this.radius = radius;}

Constructors are a special kind of methods that are invoked to construct objects.A constructor with no parameters is referred to as a no-arg constructor.

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Constructor-method with three differences

· Constructors must have the same name as the class itself.

· Constructors do not have a return type—not even void.

· Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

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Default Constructor

A class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

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Creating Objects Using Constructors

Syntax: new ClassName();

Example: To create anonymous objects

new Circle()

new Circle(5.0)

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Declaring Object Reference Variables

To reference an object, assign the object (being created using new operator) to a reference variable.

To declare a reference variable, use the syntax:

ClassName objectRefVar;

Example:Circle myCircle;

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Declaring/Creating Objectsin a two Steps

Circle myCircle;

myCircle = new Circle();

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Declaring/Creating Objectsin a Single Step

ClassName objectRefVar = new ClassName();

Example:

Circle myCircle = new Circle();

Create an objectAssign object reference

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Accessing ObjectsReferencing the object’s data:

objectRefVar.data

e.g., myCircle.radius

Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myCircle.getArea()

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Practical problem

Write a Java program that constructs an object with radius 5 and an object with radius 10 and display the radius and area of each of the two circles.

Change the radius of the second object to 100 and display the new radius and area.

Add a new method to return the circumference of the circle.

Hints: read next slide

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Hints Version 1, project Circle1:

One .java source file, one class Circle with methods:• two constructors,• method getArea()• method main().

See attached file Circle1.java Version 2, project Circle2:

One .java file, two classes:• Circle with methods: two constructors and method getArea()• TestCircle with method main() only

See attached file Circle2.java Version 3, project Circle3:

Two classes Circle and TestCircle, as in version 2 Each class saved in a separate file, but in the same package

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The null Value

If a data field of a reference type does not reference any object, the data field holds a special literal value, null.

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Default Value for a Data FieldThe default value of a data field is:null for a reference type, 0 for a numeric type, false for a boolean type, and '\u0000' for a char type.However, Java assigns no default value to a local variable inside a method (see next slide).

public class Test {


Student student = new Student();

System.out.println("name? " + student.name);

System.out.println("age? " + student.age);

System.out.println("isScienceMajor? " + student.isScienceMajor);

System.out.println("gender? " + student.gender);

}

}

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Example

public class Test { public static void main(String[] args) { int x; // x has no default value String y; // y has no default value System.out.println("x is " + x); System.out.println("y is " + y); }}

Compilation error: variables not initialized

Java assigns no default value to a local variable inside a method.

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Differences between Variables of Primitive Data Types and Object Types

1 Primitive type int i = 1 i

Object type Circle c c reference

Created using new Circle()

c: Circle

radius = 1

int i = 1;

Circle c = new Circle(1);

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Example ofUsing Instance and Class Variables and Method

Objective: Demonstrate the roles of instance and class variables and their uses.

Task: Modify the Circle program by adding a class variable numberOfObjects to track the number of Circle objects created.

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Visibility modifiers

private: private members are accessible only in the class itself

package: package members are accessible in classes in the same package and the class itself (by default)

protected: protected members are accessible in classes in the same package, in subclasses of the class, and in the class itself

public: public members are accessible anywhere the class is accessible

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public class Pencil {public String color = “red”;public int length;public float diameter;private float price;

public static long nextID = 0;

public void setPrice (float newPrice) {

price = newPrice;}

}public class CreatePencil {

public static void main (String args[]){

Pencil p1 = new Pencil();

p1.price = 0.5; // illegal

}}

Pencil

CreatePencil

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Visibility Modifiers and Accessor/Mutator MethodsBy default, the class, variable, or method can beaccessed by any class in the same package.

publicThe class, data, or method is visible to any class in any package.

private The data or methods can be accessed only by the declaring class.

The get and set methods are used to read and modify private properties.

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Accessor/Mutator MethodsColoquially,

a get method is referred to as a getter (or accessor) a set method is referred to as a setter (or mutator)

A get method has following signature: public returntype getPropertyName()

If return type is boolean, get method is like: public boolean isPropertyName()

A set method has following signature: public void setPropertyName(dataType propertyValue)

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More on OOP syntax - Java class Book {

private String m_Author;public Book() { m_Author = " ";}public Book(String pb) {

m_Author = pb; }

// method accessorpublic String getAuthor() { return m_Author; }

// methods mutatorspublic void setAuthor(String pb) { m_Author = pb; }

} // end of class Book

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Accessor/Mutator Methods

See file ModernObjectProg1.java

Modify the Circle class with get /accessor/ method and set /mutator or mutators/ method(s) associated to the rad data field for the radius of the circle.

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Passing Objects to Methods

Passing by value for primitive type value (the value is passed to the parameter)

Passing by value for reference type value (the value is the reference to the object)

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The this Reference Need to reference a class’s hidden variable in a method. A property

name is used as a parameter in a set method for the property Hidden static variable is accessed using class name reference Hidden instance variable is accessed using keyword this

class Foo {int i= 5; static double k=0;void setI(int i) {

this.i = i; }

void setK(double k) {Foo.k = k; }

}

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The this Reference Keyword this can also use inside a constructor to invoke another constructor of the

same class. public class Circle { private double radius;

public Circle (double radius) {this.radius = radius;

}

public Circle() {this(1.0);}

public getArea() { return this.radius * this.radius * Math.PI; }}// this(1.0) has to appear first in the constructor before any other statements.

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Array of Objects Circle[] circleArray = new Circle[10];

An array of objects is actually an array of reference variables. So invoking circleArray[1].getArea() involves two levels of referencing as shown in the figure. circleArray references to the entire array. circleArray[1] references to a Circle object.

reference

Circle object 0 circleArray[0]

…

circleArray circleArray[1]

circleArray[9]

Circle object 9

Circle object 1

C# Programming: From Problem Analysis to Program Design 70

Practical session

Reminder: Program Methods1.java includes class Methods1 with methods main() and gcdr()

public class Methods1 { public static void main(String[] args) {

… gcdr(a, b)); } static int gcdr(int m, int n) {

… } } // end of class Methods1



Practical session

Reorganize Methods1.java program to Methods2.java like this: the same method gcdr() appears in three classes

public class Methods2 { public static void main(String[] args) { … } static int gcdr(int m, int n) { … } }

class Arithmetic1 {public static int gcdr() { … }

} class Arithmetic2 {

public int gcdr() { … } }



Practical sessionAnalyze the syntax correctness of the main() method public

class Methods2 { public static void main(String[] args) { int a, b; Arithmetic2 ar = new Arithmetic2();

Methods2 pr = new Methods2(); a = 24; b = 15;

System.out.print(" " + gcdr(a, b)); System.out.print(" " + Methods2.gcdr(a, b)); System.out.print(" " + pr.gcdr(a, b));

a = 20; b = 15; System.out.println(" " + Arithmetic1.gcdr(a, b));

a =17; b = 15; System.out.println(" " + ar.gcdr(a, b)); }

static int gcdr(int m, int n) { if (n == 0) return m; else return gcdr(n, m % n); }} // end of class Methods2



Practical sessionAnalyze the syntax correctness of the main() method

class Arithmetic1 {public static int gcdr(int m, int n) {if (n == 0) return m;

else return gcdr(n, m % n); }} // end of class Arithmetic1

class Arithmetic2 {public static int gcdr(int m, int n) {if (n == 0) return m;

else return gcdr(n, m % n); }} // end of class Arithmetic2


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Recap: a static data variable is one whose value is shared by all class instances – true for Java and C#.

There is only one occurrence of it. The static data member is associated with the class, rather than with each instance.

A static method allows us to use that method from a class without first instantiating an object of the class - true for Java and C#.

The main()/Main() methods of Java and C# are static methods – the main()/Main() methods can be used without first instantiating any objects of the class.

C# may have a static constructor. Java uses a static initialization block (initializer).


Practical session – digression on C#Program Methods2.cs: same method gcdr() appears in three

classes namespace Methods2 {

class Program {

Method static void Main() { … }Method static int gcdr() { … }

} class Arithmetic1 {

Method public static int gcdr() { … } } class Arithmetic2 {

Method public int gcdr() { … } }

}C# Programming: From Problem Analysis to Program Design 75


Practical session - digression on C#Analyze the syntax correctness of the Main() method static

void Main(string[] args) { int a, b; Arithmetic2 ar = new Arithmetic2(); Program pr = new Program();

a = 24; b = 15; Console.WriteLine("\n“+Program.gcdr(a, b)+" "+gcdr(a, b));

Console.WriteLine("\n“+pr.gcdr(a, b));

a = 20; b = 15; Console.WriteLine("\n\n" + Arithmetic1.gcdr(a, b));

a = 17; b = 15; Console.WriteLine("\n\n" + ar.gcdr(a, b)); }



Practical session

Expand the reorganized program Methods2.java adding more methods that reside in the same class with method main() and in classes Arithmetic1 and Arithmetic2:

Iterative GCD – gcdi()Iterative/Recursive factorial – facti(), factr()Iterative/Recursive Fibonacci – fiboi(), fibor()Iterative/Recursive Sum(1..n) – sumi(), sumr()Iterative/Recursive Power operator simulator –

powi(), powr()C# Programming: From Problem Analysis to Program Design 77


Practical Tasks

Develop class CounterData field: countConstructorsGetter methodSetter methodMethod: incCount

79

Practical sessionProblem: Write a program to develop ADT Counter

class:Data field: int m_countMethods:

no-arg constructor1-arg constructorvoid incCount()get method /accessor/set method /mutator/

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Practical session

IDE NetBeans:

Project name: Counter1 – user specified

IDE generates: package name counter1

class name Counter1

Class includes ADT functionality

+

tester method main(…)

81

Practical session

IDE NetBeans:

Project name: Counter2 – user specified

IDE generates: package name counter2

class name Counter2 (main class)

To add new class:

File > New File > Choose category: Java,

File type : Java class

Name: Counter


Practical Tasks

Develop class DistanceData fields: feet, inchesConstructorsGetter methodsSetter methods

83

More on Java syntax

Inheritance

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More on OOP syntax - Java class Counter // file OOPInheritanceCounter.java { protected int count; public Counter() { count = 0; } public Counter(int pa) { count = pa; } public void IncCount() { count = count + 1; } public void setCount(int pa) { count = pa; } public int getCount() { return count; } }

class CountDn extends Counter { public void DecCount() { count = count - 1; } }

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MotivationsSuppose you will define classes to model:

students, instructors, professors.

You can build a separate independent class for each one of the listed categories.

BUT: These classes have many common features. What is the best way to design these classes so to avoid

redundancy? The answer is to use inheritance.

86

MotivationsSuppose you will define classes to model

circles, rectangles, triangles.

You can build a separate independent class for each one of the listed categories (2D geometric figures).

BUT: These classes have many common features. What is the best way to design these classes so to avoid

redundancy? The answer is to use inheritance.

87

Brief presentation

88

Inheritance

A class can inherit properties and methods from another class, called its parent.

In Java, this is called extending a class.

The class doing the extending is the child- or sub-class.

The class getting extended is the parent or super-class.

89

Inheritance

In Java, a class can only extend one parent. You can also:

Add new properties/attributes/data fields Add new methods Override the methods of the super-class

Classes are arranged in a treelike hierarchy. There is one class at the top, or root, of the

hierarchy, named Object Every class except Object has one “parent”

class, or super-class single inheritance.

90

Inheritance Example

public class Person { protected String name; public void setName(String name) { this.name = name; } public String getName() { return name; }}

public class Student extends Person { private double accountBalance; public void increaseBalance(double amount) { this.accountBalance += amount; } public double getAccountBalance() { return accountBalance; }}

Here is theextends keyword,signaling inheritance

Parent Child

91

Constructors in extended classes

• A constructor of the extended class can invoke one of the superclass’s constructors by using the super method in Java.

• If no superclass constructor is invoked explicitly, then the superclass’s default constructor is invoked automatically by the runtime system as the first statement of the extended class’s constructor.

• FYI: in C#, the base method is used, not super.

• In both C# and Java, may use base/super to invoke any overridden method in the parent class.

E.g. super.ParentMethod();

92

Superclass

public class Person{protected String name;

public Person() {name = “no_name_yet”;

}

public Person(String initialName) {

this.name = initialName;}

public String getName() {return name;

}

public void setName(String newName){name = newName;

}

Subclass

public class Student extends Person {private int studentNumber;

public Student() {super(); // superclassstudentNumber = 0;

}

public Student(String initialName,int initialStudentNumber) {super(initialName);

studentNumber = initialStudentNumber;

}

public int getStudentNumber() {return studentNumber;

}

public void setStudentNumber(int newStudentNumber ) {

studentNumber = newStudentNumber;

}

93

Comprehensive presentation

94

Superclasses and Subclasses

Geometric objects: circles and rectangles Common properties:

Color, filled/nofilled, dateCreated

Circle – specific properties: radius

Rectangle – specific properties: width, height

95

Superclasses and Subclasses

GeometricObject -color: String

-filled: boolean

-dateCreated: java.util.Date

+GeometricObject()

+GeometricObject(color: String, filled: boolean)

+getColor(): String

+setColor(color: String): void

+isFilled(): boolean

+setFilled(filled: boolean): void

+getDateCreated(): java.util.Date

+toString(): String

The color of the object (default: white).

Indicates whether the object is filled with a color (default: false).

The date when the object was created.

Creates a GeometricObject.

Creates a GeometricObject with the specified color and filled values.

Returns the color.

Sets a new color.

Returns the filled property.

Sets a new filled property.

Returns the dateCreated.

Returns a string representation of this object.

Circle -radius: double

+Circle()

+Circle(radius: double)

+Circle(radius: double, color: String, filled: boolean)

+getRadius(): double

+setRadius(radius: double): void

+getArea(): double

+getPerimeter(): double

+getDiameter(): double

+printCircle(): void

Rectangle -width: double

-height: double

+Rectangle()

+Rectangle(width: double, height: double)

+Rectangle(width: double, height: double color: String, filled: boolean)

+getWidth(): double

+setWidth(width: double): void

+getHeight(): double

+setHeight(height: double): void

+getArea(): double

+getPerimeter(): double

96

Superclasses and Subclasses Inheritance illustrated as skeletal Java source

class GeometricObject {… } class Circle extends GeometricObject {…} class Rectangle extends GeometricObject {…} public class TestProgram {

public static void main(…) { …

}…}

97

class GeometricObject

Inheritance illustrated as skeletal Java source

See detailed source text in file

ProgGeometricObject.java

98

Task 1

Write a Java program to illustrate the inheritance relation among classes Circle and Cylinder

Super class: Circle

Sub class: Cylinder

“is-a” relation: Cylinder is-a Circle

Hint: Follow the style and ideology of Java program

ProgGeometricObject.java

99

Are superclass’s Constructor Inherited?

No. They are not inherited. They can not be invoked by name.

They are invoked implicitly or may be invoked explicitly using the super keyword.

A constructor is used to construct an instance of a class. Unlike properties and methods, superclass's constructors are not inherited in the subclass. They can only be invoked from the subclasses' constructors, using the keyword super. If the keyword super is not explicitly used, the superclass's no-arg constructor is automatically implicitly invoked.

100

Superclass’s Constructor Is Always Invoked

A constructor may invoke an overloaded constructor (using this) or its superclass’s constructor (using super). If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example,

public A(double d) { // some statements

}

is equivalent to

public A(double d) { super(); // some statements

}

public A() {

}

is equivalent to

public A() { super();

}

101

Using the Keyword super

To call a superclass constructor

super();

super(arguments);

To call a superclass method super.method(arguments);

• Keyword this used to refer to the calling object.

• Keyword super refers to the superclass of the class in which super appears. This keyword can be used in two ways:

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Example 3-arg constructor of Circle class:public Circle(double radius, String color, boolean filled) { setColor(color); setFilled(filled);

this.radius = radius; }

Can be replaced with:public Circle(double radius, String color, boolean filled) { super(color, filled);

this.radius = radius; }

103

Constructor chainingFile SonFatherGrandFather.javaGiven the class hierarchy:Class GrandGrandFather – super class

↑Class GrandFather – sub class and super class

↑Class Father – sub class and super class

↑Class Son – sub sub sub class

104

Constructor chaining

File Faculty.java. Given the class hierarchy:

Class Person – super class

↑Class Employee – sub class and super class

↑Class Faculty – sub sub class

105

Overriding Members

You can override (most) methods in a parent class by defining a method with the same name and the same parameter list in the child class.

This is useful for changing the behavior of a class without changing its interface.

106

Example of Overriding

Here we override to change method getIdentifier()

public class Person { private String name; private String ssn; public void setName(String name) { this.name = name; } public String getName() { return name; } public String getIdentifier() { return ssn; }}

public class Student extends Person { private String studentId; private double accountBalance; public void increaseBalance(double amount) { this.accountBalance += amount; } public double getAccountBalance() { return accountBalance; } public String getIdentifier() { // override return studentId; // use student id instead of ssn }}

107

Overriding vs. Overloading public class Test { public static void main(String[] args) { A a = new A(); a.p(10); a.p(10.0); } } class B { public void p(double i) { System.out.println(i * 2); } } class A extends B { // This method overrides the method in B public void p(double i) { System.out.println(i); } }

public class Test { public static void main(String[] args) { A a = new A(); a.p(10); a.p(10.0); } } class B { public void p(double i) { System.out.println(i * 2); } } class A extends B { // This method overloads the method in B public void p(int i) { System.out.println(i); } }

108

Overriding vs. Overloading

Overloading methods in base class and derived class

Base class and derived class have method with Same name Different type and/or number of parameters

Test Java Program TestOverLoad.java

109


// file TestOverload.java

class B { public void p(int par) { System.out.println(" Base class method " + par*2 ); }} class A extends B { public void p(double par) { // this method overloads base class method p() System.out.println(" Child class method " + par); }}

public class TestOverLoad { public static void main(String[] args) { A a = new A(); a.p(10); a.p(10.0); }}

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Overriding methods in base class and derived class

Base class and derived class have method with Same name Same type and/or number of parameters

Test Java Program TestOverRide.java

111


// file TestOverRide.java

class B { public void p(double par) { System.out.println(" Base class method " + par*2 ); }} class A extends B { public void p(double par) { // this method overrides base class method p() System.out.println(" Child class method " + par); //super.p(10); //super.p(10.0); }}

public class TestOverRide { public static void main(String[] args) { A a = new A(); a.p(10); a.p(10.0); }}

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The Object Class and Its Methods

Every class in Java is descended from the java.lang.Object class. If no inheritance is specified when a class is defined, the superclass of the class is Object.

public class Circle { ... }

Equivalent public class Circle extends Object { ... }

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The Object class

It is important to be familiar with methods provided by the Object class so that you can use them in your classes.

Some of these methods are: public boolean equals(Object object) public int hashCode() public String toString()

114

The toString() method in ObjectThe toString() method returns a string representation of the object. The default implementation returns a string consisting of a class name of which the object is an instance, the at sign (@), and a number representing this object.

Circle circle = new Circle();

System.out.println(circle.toString());

The code displays something like Circle@15037e5 . This message is not very helpful or informative. Usually you should override the toString method so that it returns a digestible string representation of the object.

115

More on Java syntax

Polymorphism

116

More on OOP syntax - Java // file OOPPolymorphism.javaclass Base { public void Show() { System.out.println( "\n Base:");}};

class Derived1 extends Base { public void Show() { System.out.println( "\n Derived1:");}};

class Derived2 extends Base { public void Show() { System.out.println( "\n Derived2:");}};public class OOPPolymorhismJava { public static void main(String[] args) { Base ptr = new Base(); ptr.Show(); ptr = new Derived1(); ptr.Show(); ptr = new Derived2(); ptr.Show(); }}

117

MotivationsIt is a challenge from one side but gives power and flexibility if you are able to perform different tasks using the same calling statement. What is the way to achieve that effect? The answer is to apply POLYMORPHISM.

How to interpret the term POLYMORPHISM?

Polymorphism (from Greek meaning “many forms” )== Giving different meaning to the same thing= Variable of a super type can refer to a subtype object

In assignment statement or Through parameter passing mechanism

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Polymorphism Inheritance is a relation that enables a sub class to inherit

features from its superclass with additional new features.A subclass is a specialized form of its superclass.Every instance of a sub class is an instance of a superclass

BUT not vice versa.Example: Every circle is a geometric object, BUT not every

geometric object is a circle. !!! You can always assign an instance of a subclass to a

reference variable of its superclass type. !!! You can always pass an actual argument /instance of a

subclass type/ to meet corresponding formal parameter /instance of its superclass type/.

119

Consider code in Java // source text file: ProgBaseDerv1Derv2.java

given inheritance hierarchy

Object|

Base/ \

Derived1 Derived2

Classes Base, Derived1, Derived2 provide method toString() and method show()

120

Consider code in Java class Base {

public void show() { System.out.println("Base " );} public String toString() { return "\n\nBase"; }

} // end of class Base

class Derived1 extends Base { public void show() { System.out.println("Derived1" );} public String toString() { return "\n\nDerived1"; }} // end of class Derived1

class Derived2 extends Base { public void show() { System.out.println("Derived2" );}

public String toString() { return "\n\nDerived2"; }} // end of class Derived2

121

Consider code in Java // source text file: ProgBaseDerv1Derv2.javapublic class ProgBaseDerv1Derv2 {

public static void main(String[] args) { Object o = new Object(); System.out.println(" " + o.toString());

Base a = new Base(); System.out.println(" " + a.toString());

a.show();

Derived1 b = new Derived1(), cir1 = new Derived1(); System.out.println(" " + b.toString()); b.show();

Derived2 c = new Derived2(), rect1 = new Derived2(); System.out.println(" " + c.toString()); c.show();

Object[ ] arr = new Object[4]; arr[0] = o;

arr[1] = a;arr[2] = b;arr[3] = c;for(int i=0; i<4; i++) System.out.println( arr[i].toString());

// o is named polymorphic variableo=a; o=b; o=c;a=b; a=c;

} // end of method main()} // end of class ProgBaseDerv1Derv2

122

Comments

Ref variable o is Object type.Array ref variable arr is Object type.We can assign any instance of Object ((eg. New Base, new

Derived1, or new Derived2 to o or to arr array elementGEN RULE: An object of a subtype can be used wherever

its supertype value is required or in other words a ref var of a super class type can point to an object of its sub class type.

This feature is known as polymorphism.

123

Consider code in Java // source text file: ProgPolymorphismDemo.java

given inheritance hierarchy

Object

|

GeometricObject

/ \

Circle Rectangle

124

Consider code in Java // source text file: ProgpolymorphismDemo.java

public static void main(String args[]) { displayObject(new Circle(1, "red", false)); displayObject(new Rectangle(1, 1, "blue", true)); Circle aa = new Circle(1, "red", false); displayObject(aa); Rectangle bb = new Rectangle(1, 1, "blue", true); displayObject(bb); } // end of main public static void displayObject( GeometricObject o) { System.out.println("Created:"+o.getDateCreated()+" color:"+o.getColor()+" Filled:"+o.isFilled()); // or System.out.println("Created:"+o.dateCreated+" color:"+o.color+" Filled:"+o.filled); }

125

Comments The formal param is GeometricObject o.The actual argument may be any instance of GeometricObject

(eg. New Circle or new Rectangle)GEN RULE: An object of a subtype can be used wherever its

supertype value is required or in other words a ref var of a super class type (formal param) can point to an object of its sub class type (actual arg).

This feature is known as polymorphism.

126

Dynamic Binding // file: ProgGeometricObject3.java Object o = new GeometricObject(); System.out.println(o.toString()); Object aa = new Circle(1); System.out.println(aa.toString());

Q. Which toString() method is invoked by o and by aa?Before answer, let introduce two terms:

declared typeactual type

127

Dynamic Binding Object o = new GeometricObject();

A variable must be declared a type. The type of a variable is called its declared type.

o’s declared type is Object.The actual type is the actual class for the object referenced by

the variableo’s actual type is GeometricObject

Which toString() method is invoked by o is determined by o’s actual type.

This is known as dynamic binding

128

Dynamic Binding

Given a hierarchy of classes: C1, C2, ..., Cn-1, Cn, where C1 is a subclass of C2, C2 is a subclass of C3, ..., Cn-1 is a subclass of Cn. That is, Cn is the most general class, and C1 is the most specific class. In Java, Cn is the Object class.Dynamic binding works as follows: Suppose an object o is an instance of class C1. If o invokes a method p, the JVM searches the implementation for the method p in C1, C2, ..., Cn-1 and Cn, in this order, until it is found. Once an implementation is found, the search stops and the first-found implementation is invoked.

Cn Cn-1 . . . . . C2 C1

Object Since o is an instance of C1, o is also an

instance of C2, C3, …, Cn-1, and Cn

129

class Person extends Object { public String toString() { return "Person"; }} class Student extends Person { public String toString() { return "Student"; }}

class GraduateStudent extends Student {}

public class PolymorphismDemo { public static void main(String[] args) { m(new GraduateStudent()); m(new Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System.out.println(x.toString()); }}

Program output is:

Can you analyze?

What is the expected result?

130

class Person extends Object { public String toString() { return "Person"; }} class Student extends Person { public String toString() { return "Student"; }}

class GraduateStudent extends Student {}

public class PolymorphismDemo { public static void main(String[] args) { m(new GraduateStudent()); m(new Student()); m(new Person()); m(new Object()); } public static void m(Object x) { System.out.println(x.toString()); }}

Program output is:

StudentStudentPersonJava.lang.Object@16f0472

131

Method Matching vs. Binding

Matching a method signature and binding a method implementation are two separate issues.

The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compile time (early, static binding).

A method may be implemented in several subclasses. The JVMe dynamically binds the implementation of the method at run time, decided by the actual type of the variable (late, dynamic binding).

132

Casting ObjectsYou have already used the casting operator to convert variables of one primitive type to another.

double d1=3.156, d2; int a1, a2=55;// casting – convert from int to doubled2 = a2; // allowedd2 = (double) a2; // allowed// casting – convert from double to int

a1 = d1; // compiler errora1 = (int) d1; // allowed

Same approach applied when casting objects

133

Casting Objects Casting can also be used to convert an object of one class type to another within an inheritance hierarchy. In the preceding section, the statement

m(new Student());

assigns the object new Student() to a formal parameter of the Object type. This statement is equivalent to:

Object o = new Student(); // Implicit castingm(o);

The statement Object o = new Student(), known as implicit casting, is legal because an instance of Student is automatically an instance of Object.

134

Why Casting Is Necessary?Suppose you want to assign the object reference o to a variable of the Student type using the following statement:

Student b = o; A compilation error would occur. Why does the statement

Object o = new Student();work and the statement

Student b = o;doesn’t? This is because a Student object is always an instance of Object, but an Object is not necessarily an instance of Student. Even though you can see that o is really a Student object, the compiler is not so clever to know it. To tell the compiler that o is a Student object, use an explicit casting. The syntax is similar to the one used for casting among primitive data types. Enclose the target object type in parentheses and place it before the object to be cast, as follows:

Student b = (Student)o; // Explicit casting

135

TIP

To help understand casting, you may also consider the analogy of fruit, apple, and orange with the Fruit class as the superclass for Apple and Orange. An apple is a fruit, so you can always safely assign an instance of Apple to a variable for Fruit. However, a fruit is not necessarily an apple, so you have to use explicit casting to assign an instance of Fruit to a variable of Apple.

136

Casting fromSuperclass to Subclass

Explicit casting must be used when casting an object from a superclass to a subclass. This type of casting may not always succeed.

Apple x = (Apple)fruit; // explicit casting only allow

Orange x = (Orange)fruit;

Apple x = fruit; // implicit casting not allowed

Orange x = fruit; // implicit casting not allowed

137

Casting from Subclass to Superclass

Explicit or implicit casting must be used when casting an object from a subclass to a superclass. This type of casting may always succeed.

Fruit f1 , f2;

Apple apl = new Apple();

f1 = apl; // implicit casting allowed

f2 = (Fruit)apl; // explicit casting allowed

138

Casting fromSuperclass to Subclass

Explicit casting must be used when casting an object from a superclass to a subclass. This type of casting may not always succeed.

Object o = new GeometricObject(); Circle ac = new Circle(); ac = o; // syntax error, not every geometric //object is necessarily a circle.

ac = (Circle) o; // OK, but casting may not // always succeed

139

Casting from Subclass to Superclass

Explicit or implicit casting must be used when casting an object from a subclass to a superclass. This type of casting may always succeed.

Object o = new GeometricObject(); Circle ac = new Circle();

o=ac; // OK

o = (GeometricObject)ac; // OK

o = (Circle)ac; // OK

140

The instanceof OperatorUse the instanceof operator to test whether an object is an instance of a class:

Object myObject = new Circle();... // Some lines of code/** Perform casting if myObject is an instance of Circle */

if (myObject instanceof Circle) { System.out.println("The circle diameter is " + ((Circle)myObject).getDiameter()); ...}

141

Example (8e): Demonstrating Polymorphism and Casting

// source text file: ProgCastingDemo.javaGiven: inheritance relation Object - GeometricObject – Circle,RectanglegeometricObject (base class) – Circle, Rectangle (sub classes)

Problem: write a program that creates two Object instances initialized as Circle(1.0) object and as a Rectangle(1.,1.0) object, and invokes the displayObject() method to display the objects. The displayObject() displays the area and diameter if the object is a circle, and displays area if the object is a rectangle.

142

Example (8e): Demonstrating Polymorphism and CastingExamine the source text:Q. which is the declared type and which is the

actual type of o1 and o2?

Object o1 = new Circle(1, "red", false);

Object o2 = new Rectangle(1, 1, "blue", true);

143


Examine the source text:Q. what output is to display after following stmts?

System.out.println(" "+ (o1 instanceof Object ) ); System.out.println(" "+ (o1 instanceof GeometricObject) ); System.out.println(" "+ (o1 instanceof Circle ) ); System.out.println(" "+ (o1 instanceof Rectangle ) );

144


Examine the source text:Q. what output is to display after following stmts?

System.out.println(" "+ (o2 instanceof Object ) ); System.out.println(" "+ (o2 instanceof GeometricObject) ); System.out.println(" "+ (o2 instanceof Circle ) ); System.out.println(" "+ (o2 instanceof Rectangle ) );

145


Examine the source text:A. what output is to display after following stmts?

o1 o2Object true trueGeometricObject true trueCircle true falseRectangle false true

146

The final Modifier The final class cannot be extended: final class Math { ... }

The final variable is a constant: final static double PI = 3.14159;

The final method cannot beoverridden by its subclasses.

147

More on Java syntax

Abstract Classes and Interfaces

148

Interfaces & Abstract Classes

The Interface concept

149

Interface

public interface Driver { void turnWheel(double angle); void pressAccelerator(double amount); void pressBrake(double amount);}

public class BusDriver implements Driver { // must include implementation for each of the

three methods from Driver}

No method bodies – just method header.

Must provide implementation when used.

public interface Driver { void turnWheel(double angle); void pressAccelerator(double amount); void pressBrake(double amount);}

150

public class BusDriver extends Person implements Driver { // must include implementation for each of the

three methods from Driver}

May also have

This is a “back door” approach to multiple inheritancefor Java via extends and implements.

Single inheritance via extendsand extra inheritance via implements.

151

Interfaces & Abstract Classes

The Abstract Class concept

152

Abstract Classes

A class where some methods are unspecified (like in an interface). The unspecified methods are declared abstract. The class also is declared abstract.

An abstract class must be sub-classed (i.e. extended) - you cannot instantiate objects of an abstract type.

Useful if you want to specify an “interface” along with some default behaviors.

Similar for C#

153

Abstract Class Example

abstract class CacheFile { String filename; byte[] contents;

void flush() { // Write file to disk } void refresh() { // Load file from disk } abstract String deserialize(); abstract byte[] serialize(String s);}

These methods are defined

These methods are abstractbecause how you want

to store data into the file isapplication-dependent

May have defined methods and abstract methods.

154

Abstract Classes & Interfaces Comprehensive presentation

155

Inheritance hierarchy If you move from super class down to sub class, classes

become more specific and more concrete. If you move from sub class up to super class, classes

become more general and less specificSometimes super class is so abstract that it cannot have any

specific instances.Such a class is referred to as an ABSTRACT CLASS

Cn Cn-1 . . . . . C2 C1

Object Since o is an instance of C1, o is also an

instance of C2, C3, …, Cn-1, and Cn

156

GeometricObject – Circle, Rectangle

Object

↑

GeometricObject

↑ ↑

Circle RectangleCommon properties:


Circle – specific properties: radiusRectangle – specific properties: width, height

157


GeometricObject

↑ ↑



Circle – specific behavior: getArea(), getPerimeter()

Rectangle – specific behavior: getArea(), getPerimeter()

158


GeometricObject

↑ ↑



Circle – specific properties and behavior: Radius, getArea(), getPerimeter()

Rectangle – specific properties and behavior: width, height, getArea(), getPerimeter()

159

Inheritance hierarchy Is it reasonable to generalize getArea(), getPerimeter() methods?From one side: Since we can compute area and perimeter for all

geometric objects, it is better to define getArea(), getPerimeter() as methods in super class GeometricObject.

From other side: Both methods cannot be implemented in the base class because their implementation depends on specific properties (radius or height/width) of the geometric object defined in the sub classes.

Such methods are referred to as abstract methods and are denoted in the super class using modifier abstract.

Class with abstract methods becomes an abstract class and is must to be also denoted abstract. See next slide.

160

Open file ProgGeometricObjectAbstractClass.java

abstract class GeometricObject {… public abstract double getArea();public abstract double getPerimeter();}

You cannot create instances of abstract classes using new operator.Constructors in abstract classes are protected because they are used

only by subclasses.Super class defines common features (incl. methods getArea() and

getPerimeter()). Because you don’t know how to compute area and perimeter of geometric objects, both methods are defined as abstract methods. These methods are implemented/overridden in the subclasses.

161


class Circle extends GeometricObject { …

public double getArea() { return Math.PI * radius * radius; }

public double getPerimeter() { return 2 * Math.PI * radius; } public String toString()//overridden m.

{return "Circle:\n"+super.toString();}

} // end of class

162


class Rectangle extends GeometricObject {…

public double getArea()

{ return width * height; }

public double getPerimeter()

{ return 2 * (width + height);}

public String toString() //overridden m.

{return "RecRec:\n"+super.toString();}

} // end of class

163


Why do we need abstract methods? What benefits if any?

Browse the main() method: It creates two geometric objects – circle, rectangle Invokes equalArea() method – have a careful look at the

formal parameters type Invokes displayGeometricObject() method – have

a careful look at the formal parameter type

164

Open file ProgGeometricObjectAbstractClass.javapublic class ProgGeometricObjectAbstractClass {

public static void main(String args[]) { GeometricObject geo1 = new Circle(5.); GeometricObject geo2 = new Rectangle(5., 3.);

System.out.println("The two object have same area? "+ equalArea(geo1, geo2) ); displayGeometricObject(geo1); displayGeometricObject(geo2); } // end of main public static boolean equalArea( GeometricObject o1, GeometricObject o2) { return o1.getArea() == o2.getArea(); } public static void displayGeometricObject(GeometricObject o) { System.out.println("The area is " + o.getArea() ); System.out.println("The perimeter is " + o.getPerimeter() ); } }

165


Thanks to declaring abstract methods in super class, it is valid to create a connection btw super class and sub classes through getArea()/getPerimeter() methods and apply the polymorphic approach or in other words:

Reference variable of a super class type (formal parameter) can point to an object of its sub class type (actual argument – look at its actual type).

166


Equalarea() method to have two parameters GeometricObject and to compare to equality the area of a circle and the area of a rectangle.

public static boolean equalArea(GeometricObject o1, GeometricObject o2) {

return o1.getArea() == o2.getArea(); }

167

Open file ProgGeometricObjectAbstractClass.java displayGeometricObject() method to have a

parameter GeometricObject and to display: the area and perimeter of a circle or the area and perimeter of a rectangle

(depends on actual argument) when method called.

public static voiddisplayGeometricObject(GeometricObject o) {

System.out.println("The area is " + o.getArea() ); System.out.println(“Perimeter is "+o.getPerimeter()); } // end of method

168

Inheritance, polymorphism and abstract classes

Given the source:

GeometricObject geo1 = new Circle(5.);GeometricObject geo2 = new Rectangle(5., 3.);

GeometricObject[] o = { new Circle(2.), new Circle(3.), new Rectangle(5., 3.),

new Rectangle(4.,6.) };

for (int i=0; i< o.length; i++)

System.out.println("The two objects have same area?"+ equalArea(geo1,o[i]));

Q.: Can you guess/predict the expected output?

169


Given the source:

GeometricObject geo1 = new Circle(5.);GeometricObject geo2 = new Rectangle(5., 3.);

GeometricObject[] o = { new Circle(2.), new Circle(3.), new Rectangle(5., 3.),

new Rectangle(4.,6.) };

for (int i=0; i< o.length; i++)

System.out.println("The two objects have same area?"+ equalArea(geo2,o[i]));

Q.: Can you guess/predict the expected output?

170


Expected output:

Geo1 Geo2 ======================= false false false false false true false false

171

Digression on equalArea() problem

The task to compare the area of a circle and the area of a rectangle could be solved without specifying abstract class GeometricObject and erasing the getArea()/getPerimeter() methods from the super class.

BUT: such a solution should lose the advantages provided when using polymorphism

172

Java predefined abstract classes

173

Java library: Calendar - GregorianCalendar

java.util.Calendar is an abstract base class for extracting detailed calendar info: y, m, d, h, m, s

Subclasses of Calendar can implement specific calendar systems: Gregorian calendar, Lunar calendar, Jewish calendar

Java supports subclass java.util.GregorianCalendar

174

Open file ProgCalendar.java

Comments:Calendar cl1 = new GregorianCalendar();

cl1 declared type is Calendar cl1 actual type is GregorianCalendar Polymorphism in action: in other words a ref var of a super class type

can point to an object of its sub class type.Task: Modify the ProgCalendar.java source file with String dayNameOfWeek(int dayOfWeek) method to return strings “Sunday”, “Monday”, … “Saturday” depending on calling integer argument value 1, 2, ...7

175

Interfaces

176

InterfacesInterface is class-like construct that contains only

constants and abstract methods.Interface is similar to abstract class, but its intent is to

specify common behavior for objects that belong even to different inheritance hierarchies.

Reminder: Abstract class contain regular methods and abstract methods and polymorphic approach is within one only specific inheritance hierarchy.

177

InterfacesTo distinguish an interface from a class, Java uses interface

reserved word:

public interface Edible { public abstract String howToEat();}

Each interface to save in separate file like Edible.java.You cannot create an instance of interface with newYou can use interface as data type for ref var, or as a result of

casting

178

Interface exampleGiven: Edible interface and two separate independent

inheritance hierarchies:===================================================================================================

Animal Fruit(Edible) ↑ ↑ ↑ ↑

Tiger Chicken(Edible) Apple Orange===================================================================================================

Animal – regular base classFruit – base class implements Edible interfaceChicken – child class implements Edible interfaceTiger – regular child classApple, Orange – regular child classes

179

Open files Edible.java & ProgInterfaceEdible.java

Task: using Edible interface to specify which object is edible, in other words has implemented method howToEat().

Browse the source text

180


Chicken extends Animal, implements Edible to specify that chickens are edible, implements method howToEat()

Tiger extends Animal, does not implement Edible i.e. tigers are not edible, no need to implement method howToEat()

181


Fruit implements Edible, and does not implement howToEat() method.

Therefore Fruit is to be modified as abstract class and

concrete subclasses of Fruit must implement howToEat() method, as it is done with Apple class and Orange class

182


Run the application

183

Java supported interfaces

184

Java supported interfaces: Comparable

Problem: we need a method to return the larger of two objects of the same type:

Two students, Two dates, Two circles, Two rectangles It is must the two objects to be comparableComparability is considered a common behavior Java provides Comparable interface for this purpose package java.lang; public interface Comparable { public int compareTo(Object o); {

185


Many Java predefined classes (String, Date) implement Comparable interface to define an order for the objects. String, Date classes provide compareTo() method. Their definition is like this:

//-------------------------------------public class String extends Object implements Comparable {

// class body }//-------------------------------------Public class Date extends Object

implements Comparable { // class body }

186


Given String s; Date d;//-------------------------------(s instanceof String) is true(s instanceof Object) is true(s instanceof Comparable) is true//-------------------------------(d instanceof Date) is true(d instanceof Object) is true(d instanceof Comparable) is true

187

User Defined class implement interfaces: Comparable

Reminder : class RectangleWe cannot compare instances of Rectangle, because

Rectangle does not implement Comparable interfaceHow to proceed?

Re edit Rectangle class to implement ComparableOR Create new class ComparableRectangle to extend Rectangle

and to implement Comparable

188


Solution scheme for the second option:

GeometricObject ↑

Rectangle Comparable ↑ ↑

ComparableRectangle - - - - -

189


Open source file ProgComparableRectangle.javaSource text skeleton for ComparableRectangle

public class ComparableRectangle

extends Rectangle

implements Comparable {

public ComparableRectangle(…) { …}

public int compareTo(Object o) {…}

}

190


How to use the compareTo() method

ComparableRectangle r1 = new ComparableRectangle(4.,5.);



System.out.println(r1.compareTo(r2));



191

Questions? And/Or

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Your Attention!

march, 23 - 24, 2013 swu, blagoevgrad

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