Java Classes
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A class defines a template for objects with fields (data) and methods (functions). An object is an instance of a class that exists in memory. The Circle class is defined with double fields for the x and y coordinates of the center and the radius. Methods like circumference() and area() can be declared within the class. Objects are created using the new keyword and access fields and methods using dot notation.
Java Classes
- 1. CLASS
- 2. CLASSES A class is a collection of fields (data) and methods (procedure or function) that operate on that data. Circle centre radius circumference() area()
- 3. A Class is a template It provides a new data type It is logical entity , does not exist physically
- 4. OBJECT The variable of the new data type i.e class is Called object It is an instance of the class It exists physically i.e take space in memory To access the class object is necessary
- 5. SYNTAX TO DECLARE CLASS… class demo { } class demo2 {type instance var; type maethod(parameter lists) { } }
- 6. public class Circle { double x, y; // centre coordinate double r; // radius of the circle } The fields (data) are also called the instance varaibles.
- 7. A class with only data fields has no life. Objects created by such a class cannot respond to any messages. Methods are declared inside the body of the class but immediately after the declaration of data fields. The general form of a method declaration is type MethodName (parameter-list) { Method-body; }
- 8. public class Circle { double x, y; // centre of the circle 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; } }
- 9. Declare the Circle class, have created a new data type – Data Abstraction Can define variables (objects) of that type: Circle aCircle; // Variable of class Circle bCircle;
- 10. aCircle, bCircle simply refers to a Circle object, not an object itself. null
- 11. 11 CREATING OBJECTS OF A CLASS Objects are created dynamically using the new keyword. aCircle and bCircle refer to Circle objects bCircle = new Circle() ;aCircle = new Circle() ;
- 12. 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 After Assignment
- 14. 14 ACCESSING OBJECT/CIRCLE DATA 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)
- 15. 15 EXECUTING METHODS IN OBJECT/CIRCLE Using Object Methods: Circle aCircle = new Circle(); double area; aCircle.r = 1.0; area = aCircle.area(); sent ‘message’ to aCircle