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COS240 O-O Languages AUBG, COS dept

Lecture 12Title:

Java Classes and Objects

Reference: COS240 Syllabus

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Lecture Contents:

Defining Classes for Objects Constructing Objects using Constructors Accessing Objects via Reference Variables Visibility Modifiers, Accessors, Mutators Data Field Encapsulation Immutable Objects and Classes Static Variables, Constants and Methods The Scope of Variables The this keyword Class Abstraction and Encapsulation Inner Classes

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MotivationsAfter learning the preceding lectures, you are capable of solving problems using Java as conventional Prog Lang incl. selections, loops, methods, and arrays. However, these Java features are not sufficient for developing graphical user interfaces and large scale software systems. Suppose you want to develop a graphical user interface as shown below. How do you program it?

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OO Programming Concepts• O-OP involves programming using objects.

• An object represents an entity in the world that can be distinctly identified like a student, a desk, a circle, a button, and even a loan can all be viewed as objects.

• An object has a unique identity - state and behaviors.

• The state of an object consists of a set of data fields (known as properties) with their current values.

•The behavior of an object is defined by a set of methods. Invoking a method is like sending a message to ask an object to perform a task.

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Defining Classes for ObjectsAn object has both a state and behavior.•The state defines the object.•The behavior defines what the object does.

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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Objects

An object has both a state and behavior.

The state defines the object.

The behavior defines what the object does.

Class Name: Circle Data Fields:

radius is _______ Methods:

getArea

Circle Object 1 Data Fields:

radius is 10

Circle Object 2 Data Fields:

radius is 25

Circle Object 3 Data Fields:

radius is 125

A class template

Three objects of the Circle class

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Classes

Classes are constructs that define objects of the same type.

A Java class uses variables to define data fields and methods to define behaviors.

Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

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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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UML Class Diagram

Circle

radius: double Circle()

Circle(newRadius: double)

getArea(): double

circle1: Circle radius = 1.0

Class name

Data fields

Constructors and methods

circle2: Circle radius = 25

circle3: Circle radius = 125

UML Class Diagram

UML notation for objects

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Constructors

Circle() {}//----------------------------

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() {}//----------------------------

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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Creating Objects Using Constructors

Circle class differs from other classes already discussed:

It does not have a main() method and therefore cannot be run.

It is a definition used to declare and create Circle objects.

HOW? Using the new operator. See next slide

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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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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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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 Single Step

ClassName objectRefVar = new ClassName();

Example:

Circle myCircle = new Circle();

Create an objectAssign object reference

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Accessing Objects Referencing 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:

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

– See attached file Circle2.java

Version 2:– One .java source file, one class Circle with methods main(),

two Circle constructors and method getArea().– See attached file Circle1.java

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Hints When a program contains more than 1 class, how to

proceed? A program contains two classes. The first is TestCircle,

is the main class. Its sole purpose is to test the second class Circle. Every time you run the program, JRE invokes the main() method in the main class.

You can put the two or more classes into one file, but only one class in the file can be a public class. Furthermore, the public class must have the same name as the file name.

The main class contains the main() method that creates objects of the second class.

There are many ways to write Java programs. For instance, you can combine two classes into one

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CautionRecall that you use

Math.methodName(arguments) (e.g., Math.pow(3, 2.5))

to invoke a method in the Math class. Can you invoke getArea() using Circle.getArea()? The answer is no. All the methods used before this chapter are static methods, which are defined using the static keyword. However, getArea() is non-static. It must be invoked from an object using

objectRefVar.methodName(arguments) (e.g., myCircle.getArea()).

More explanations will be given in the section on “Static Variables, Constants, and Methods.”

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Reference Data FieldsThe data fields can be of reference types. For example, the following Student class contains a data field name of the String type.

public class Student {

String name; // name has default value null

int age; // age has default value 0

boolean isScienceMajor; // isScienceMajor has default value false

char gender; // c has default value '\u0000'

}

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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 {

public static void main(String[] args) {

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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Copying Variables of Primitive Data Types and Object Types

i

Primitive type assignment i = j

Before:

1

j

2

i

After:

2

j

2

c1

Object type assignment c1 = c2

Before:

c2

c1

After:

c2

c1: Circle

radius = 5

C2: Circle

radius = 9

c1: Circle

radius = 5

C2: Circle

radius = 9

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Garbage Collection

As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM.

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Garbage Collection, cont

TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable.

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Instance Variables andInstance Methods

Instance variables belong to a specific instance.

Instance methods are invoked by an instance of the class.

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Static Variables, Constants, and Methods

Static variables are shared by all the instances of the class.

Static methods are not tied to a specific object.

Static constants are final variables shared by all the instances of the class.

To declare static variables, constants, and methods, use the static modifier.

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Static Variables, Constants, and Methods, cont.

Circle radius: double numberOfObjects: int getNumberOfObjects(): int +getArea(): double

1 radius

circle1 radius = 1 numberOfObjects = 2

instantiate

instantiate

Memory

2

5 radius

numberOfObjects

UML Notation: +: public variables or methods underline: static variables or methods

circle2 radius = 5 numberOfObjects = 2

After two Circle objects were created, numberOfObjects is 2.

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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 Methods

By 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 Methods Coloquially,

– 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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Accessor/Mutator Methods

See file ModernObjectProg1.java

Modify the Circle class with get /accessor/ method and set /mutator or mutators/ method(s)

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The private modifier restricts access to within a class, the default modifier restricts access to within a package, and the public modifier enables unrestricted access.

public class C1 { public int x; int y; private int z; public void m1() { } void m2() { } private void m3() { } }

public class C2 { void aMethod() { C1 o = new C1(); can access o.x; can access o.y; cannot access o.z; can invoke o.m1(); can invoke o.m2();

cannot invoke o.m3(); } }

package p1; package p2;

public class C3 { void aMethod() { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; can invoke o.m1(); cannot invoke o.m2(); cannot invoke o.m3(); } }

class C1 { ... }

public class C2 { can access C1 }

package p1; package p2;

public class C3 { cannot access C1; can access C2; }

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NOTE

An object cannot access its private members, as shown in (b). It is

OK, if the object is declared in its own class, as shown in (a). public class Foo { private boolean x; public static void main(String[] args) {

Foo foo = new Foo(); System.out.println(foo.x);

System.out.println(foo.convert()); } private int convert(boolean b) { return x ? 1 : -1; } }

(a) This is OK because object foo is used inside the Foo class

public class Test { public static void main(String[] args) { Foo foo = new Foo(); System.out.println(foo.x); System.out.println(foo.convert(foo.x)); } }

(b) This is wrong because x and convert are private in Foo.

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Why Data Fields Should Be private?

To protect data.

To make class easy to maintain.

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Example ofData Field Encapsulation

Circle

-radius: double

-numberOfObjects: int

+Circle()

+Circle(radius: double)

+getRadius(): double

+setRadius(radius: double): void

+getNumberOfObject(): int

+getArea(): double

The radius of this circle (default: 1.0).

The number of circle objects created.

Constructs a default circle object.

Constructs a circle object with the specified radius.

Returns the radius of this circle.

Sets a new radius for this circle.

Returns the number of circle objects created.

Returns the area of this circle.

The - sign indicates private modifier

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Immutable Objects and Classes

If the contents of an object cannot be changed once the object is created, the object s called an immutable object and the class is called an immutable class.

If you delete the set method(s) of the Circle class and the radius data field is qualified private, the class would be immutable.

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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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Passing Objects to Methods, cont.

Space required for the main method int n: 5 myCircle:

Stack Space required for the printAreas method int times: 5 Circle c:

reference A circle object

Heap

reference

Pass by value (here the value is the reference for the object)

Pass by value (here the value is 5)

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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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Class Abstraction

Java provides many levels of abstraction.

Class abstraction is the separation of class implementation from the use of a class.

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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 next figure. circleArray references to the entire array. circleArray[1] references to a Circle object.

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Array of Objects, cont.

reference

Circle object 0 circleArray[0]

…

circleArray circleArray[1]

circleArray[9]

Circle object 9

Circle object 1

Circle[] circleArray = new Circle[10];

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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

// One class combines/includes both: // ADT - user defined class Counter // + // tester class functionality as method Main(....) //

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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

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Practical session

// Two separate classes, same package: // one for ADT - user defined class Counter // // one tester class as method Main(....) //

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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

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Thank Youfor

Your attention!