Lect 1-class and object
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This document introduces classes and objects in Java. It defines a class as a collection of fields and methods that operate on those fields. It shows how to define a Circle class with fields for the x and y coordinates and radius, and methods to calculate the circumference and area. It demonstrates how to create Circle objects, access the object fields and methods, and use the Circle class in a sample program.
![5
Classes
ïź A class is a collection of fields (data) and methods
(procedure or function) that operate on that data.
ïź The basic syntax for a class definition:
ïź Bare bone class â no fields, no methods
public class Circle {
// my circle class
}
class ClassName [extends
SuperClassName]
{
[fields declaration]
[methods declaration]
}](https://image.slidesharecdn.com/lect-1-classandobject-180910003356/85/Lect-1-class-and-object-5-320.jpg)
![17
Using Circle Class
// Circle.java: Contains both Circle class and its user class
//Add Circle class code here
class MyMain
{
public static void main(String args[])
{
Circle aCircle; // creating reference
aCircle = new Circle(); // creating object
aCircle.x = 10; // assigning value to data field
aCircle.y = 20;
aCircle.r = 5;
double area = aCircle.area(); // invoking method
double circumf = aCircle.circumference();
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf);
}
}
[raj@mundroo]%: java MyMain
Radius=5.0 Area=78.5
Radius=5.0 Circumference =31.400000000000002](https://image.slidesharecdn.com/lect-1-classandobject-180910003356/85/Lect-1-class-and-object-17-320.jpg)
Lect 1-class and object
- 1. 1 Classes and Objects in Java Basics of Classes in Java
- 2. 2 Contents ïź Introduce to classes and objects in Java. ïź Understand how some of the OO concepts learnt so far are supported in Java. ïź Understand important features in Java classes.
- 3. 3 Introduction ïź Java is a true OO language and therefore the underlying structure of all Java programs is classes. ïź Anything we wish to represent in Java must be encapsulated in a class that defines the âstateâ and âbehaviourâ of the basic program components known as objects. ïź Classes create objects and objects use methods to communicate between them. They provide a convenient method for packaging a group of logically related data items and functions that work on them. ïź A class essentially serves as a template for an object and behaves like a basic data type âintâ. It is therefore important to understand how the fields and methods are defined in a class and how they are used to build a Java program that incorporates the basic OO concepts such as encapsulation, inheritance, and polymorphism.
- 4. 4 Classes ïź A class is a collection of fields (data) and methods (procedure or function) that operate on that data. Circle centre radius circumference() area()
- 5. 5 Classes ïź A class is a collection of fields (data) and methods (procedure or function) that operate on that data. ïź The basic syntax for a class definition: ïź Bare bone class â no fields, no methods public class Circle { // my circle class } class ClassName [extends SuperClassName] { [fields declaration] [methods declaration] }
- 6. 6 Adding Fields: Class Circle with fields ïź Add fields ïź The fields (data) are also called the instance varaibles. public class Circle { public double x, y; // centre coordinate public double r; // radius of the circle }
- 7. 7 Adding Methods ïź A class with only data fields has no life. Objects created by such a class cannot respond to any messages. ïź Methods are declared inside the body of the class but immediately after the declaration of data fields. ïź The general form of a method declaration is: type MethodName (parameter-list) { Method-body; }
- 8. 8 Adding Methods to Class Circle public class Circle { public double x, y; // centre of the circle public double r; // radius of circle //Methods to return circumference and area public double circumference() { return 2*3.14*r; } public double area() { return 3.14 * r * r; } } Method Body
- 9. 9 Data Abstraction ïź Declare the Circle class, have created a new data type â Data Abstraction ïź Can define variables (objects) of that type: Circle aCircle; Circle bCircle;
- 10. 10 Class of Circle cont. ïź aCircle, bCircle simply refers to a Circle object, not an object itself. aCircle Points to nothing (Null Reference) bCircle Points to nothing (Null Reference) null null
- 11. 11 Creating objects of a class ïź Objects are created dynamically using the new keyword. ïź aCircle and bCircle refer to Circle objects bCircle = new Circle() ;aCircle = new Circle() ;
- 12. 12 Creating objects of a class aCircle = new Circle(); bCircle = new Circle() ; bCircle = aCircle;
- 13. 13 Creating objects of a class aCircle = new Circle(); bCircle = new Circle() ; bCircle = aCircle; P aCircle Q bCircle Before Assignment P aCircle Q bCircle Before Assignment
- 14. 14 Automatic garbage collection ïź The object does not have a reference and cannot be used in future. ïź The object becomes a candidate for automatic garbage collection. ïź Java automatically collects garbage periodically and releases the memory used to be used in the future. Q
- 15. 15 Accessing Object/Circle Data ïź Similar to C syntax for accessing data defined in a structure. Circle aCircle = new Circle(); aCircle.x = 2.0 // initialize center and radius aCircle.y = 2.0 aCircle.r = 1.0 ObjectName.VariableName ObjectName.MethodName(parameter-list)
- 16. 16 Executing Methods in Object/Circle ïź Using Object Methods: Circle aCircle = new Circle(); double area; aCircle.r = 1.0; area = aCircle.area(); sent âmessageâ to aCircle
- 17. 17 Using Circle Class // Circle.java: Contains both Circle class and its user class //Add Circle class code here class MyMain { public static void main(String args[]) { Circle aCircle; // creating reference aCircle = new Circle(); // creating object aCircle.x = 10; // assigning value to data field aCircle.y = 20; aCircle.r = 5; double area = aCircle.area(); // invoking method double circumf = aCircle.circumference(); System.out.println("Radius="+aCircle.r+" Area="+area); System.out.println("Radius="+aCircle.r+" Circumference ="+circumf); } } [raj@mundroo]%: java MyMain Radius=5.0 Area=78.5 Radius=5.0 Circumference =31.400000000000002
- 18. 18 Summary ïź Classes, objects, and methods are the basic components used in Java programming. ïź We have discussed: ïź How to define a class ïź How to create objects ïź How to add data fields and methods to classes ïź How to access data fields and methods to classes