sahar mosleh california state university san marcospage 1 classes and objects and methods

Sahar Mosleh California State University San Marcos

Classes and Objects and Methods


Class Fundamentals

• A class is a template that defines the form of an object

• It includes both the attributes (data) and a set of methods (routines and code) that operate on the data

• Methods and attributes that constitute a class are called members

• A class specification is used to construct objects . Objects are instances of a class

• You may think of a class as a type, and object as variables instantiated with a specific type (class)


• The general form of a class is:

class className{

type var1;type var2;…type varN;//--------------------------------type method1 (parameters){

Body of the method}.…type methodN (parameters){

Body of the method}

}


• So far the only type of class we have seen is the one that contains main() routine

• Note that, the general form of a class does not specify a main() method

• A main() method is required only if that class is the starting point for your program

• Eventually, when you learn about APPLETS in Java, you will notice that applets is Java do not require main() routine


• Consider the following example:

class Vehicle{

int passenger; // number of passengersint fuelcap // Fuel capacity in gallonsint mpg; // fuel consumption in miles per

gallon}

• This is an example of a simple class.

• The above class definition creates a new data type but nothing is allocated in memory yet.

• So, the class declaration is only a type description, it does not create an actual object


Creating Objects

• To create a vehicle object, you will use a statement like the following:

Vehicle minivan = new Vehicle();

• At this stage, minivan will be an instance of vehicle and has location in the memory;

• Each object that is instantiated from the vehicle class has its own copy of data members (passengers, fuelcap, mpg)


Accessing Data Members

• The general form of accessing a member of an object is object.member

• For example, to set the fuelcap of minivan to 16 we do:minivan.fuelcap = 16;

• In general you can use the dot operator to access both instance variables and methods. We will talk about methods shortly.


• Consider the following example:

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

{ Vehicle minivan = new Vehicle();

int range;

minivan.passengers = 7; minivan.fuelcap = 16;minivan.mpg = 21;

range = minivan.fuelcap * minivan.mpg;System.out.println(“Minivan can carry ” + minivan.passengers + “ with a range of ” + range);

}}

class Vehicle{ int passengers;

int fuelcap;int mpg;

}


Some points to remember about this example

• First, you should call this file VehicleDemo.java because the main() routine is in the VehicleDemo class and not in Vehicle class

• After you compile this program, you will notice that you have two names with extension class in your directory. They are:

Vehicle.class and VehicleDemo.class

• Although in this example both classes, Vehicle and VehicleDemo are in the same source file, it does not have to be this way.

• You may place these two classes in two separate source files. If you compile VehicleDemo.java, the other class will automatically be complied


How objects are created

• Remember that we mentioned, in order to create an object of type Vehicle, we do the following:

Vehicle minivan = new Vehicle();

• This declaration does two things at the same time:• It declares a variable called minivan of class type Vehicle• Second, the new operator creates a physical copy of object

and assign a reference to that object.

• The above two steps can be written for the minivan object as follows:

Step1 : Vehicle minivan;Step2: minivan = new Vehicle();

• In step one, the value of minivan is assigned NULL automatically by Java


Reference Variables and Assignments• In an assignment operation, object reference variables act

differently than the variables of a simple type such as int.

• For example, int x, y;x = 5;y = x;

• In this case, x and y refer to different locations in memory but their contents (values) are the same. Thus, if we change x to 6 y still remains 5.

• Now consider, Vehicle car1 = new Vehicle;Vehicle car 2 = car1;

• In this case, both car1 and car2 refer to the same memory location. If the fuelcap attribute of car1 changes, the fuelcap of car2 has changed and vice versa.


Methods

• So far the classes we have introduced did not contain methods (except the main routine , main())

• Methods are routines with a set of operations that change the state (values) of the attributes (data members)

• The general form of a method is:

returnType methodName(parameter list){

// body of the method}

• Where the return type is the type returned from the method.• methodName is the name of the routine (method) and • parameterList is a sequence of types and identifiers separated

by commas


Adding method to our Vehicle classclass Vehicle{ int passengers; int fuelcap; int mpg;

void range() { System.out.println(“Range is ” + fuelcap*mpg);}

}//----------------------------class AddMeth{public static void main(String args[])

{ Vehicle minivan = new Vehicle();

Vehicle sportscar = new Vehicle(); int range1, range2; minivan.passengers = 7; sportscar.passengers = 2; minivan.fuelcap = 16; sportscar.fuelcap = 14; minivan.mpg = 21; sportscar.mpg = 12;

System.out.println(“Minivan can carry ”+minivan.passengers + “. ”); minivan.range();

System.out.println(“Sportscar carry ” + sportscar.passengers + “. ”); sportscar.range();}

}


• In the previous example, the first line of range() is void range()

• That declares a method called range(). This method has no parameters and return type void (i.e. nothing is returned)

• The body of the range method is:System.out.println(“Range is: ” + fuelcap*mpg)

• The range method ends when its closing brace is encountered. This causes the control to be transferred back to the caller of the function


• In the example, the range method is called as follows:minivan.range()

• This calls the range method on minivan object

• The call to minivan.range() displays the range of vehicle defined by minivan.

• Similarly, sportscar.range() displays the range of vehicle defined by sportscar

• So each time range() is called, it displays the range for specific objects

• But, note that the data fuelcap, and mpg in the range() method are called directly (i.e. without using the dot (.) operator and object name). Why?

• The answer is, a method is always invoked relative to some object of its class. Once this invocation has occurred, the object in known. There is no reason to specify the object name again.


Returning a Method

• A method terminates under two conditions:• if it completes and closing brace } is encountered• if “return” statement is executed somewhere in the middle

of the method

• If the return type of a method is void, simple return statement:return;

returns the method to the caller

• Otherwise for the none-void methods return statements returns a value to the caller:

return value;


Example of void method

void myMeth( ){

int i;for (i=0; i<10; i++){

if (i==5) return;System.out.println();

}}

• This is an example of a void method. The method does not return any value at all. It only prints something and terminates

• When the value of “i” becomes 5, the loop terminates and the control goes back to the caller of the method


class Vehicle{ int passengers; int fuelcap; int mpg;

int range() { return (fuelcap*mpg); }}//----------------------------class RetMeth{public static void main (String args[])

{ Vehicle minivan = new Vehicle(); Vehicle sportscar = new Vehicle(); int range1, range2;

minivan.passengers = 7; sportscar.passengers = 2; minivan.fuelcap = 16; sportscar.fuelcap = 14; minivan.mpg = 21; sportscar.mpg = 12; range1 = minivan.range(); range2 = sportscar.range(); System.out.println(“Minivan can carry ”+minivan.passengers + “ with range of .” + range1 + “ Miles”); System.out.println(“Sportscar can carry ” + sportscar.passengers + “with range of ” + range2 + “ Miles”);}

}


• In the previous example, notice that range() has a return type of int.

• It means, it returns an integer value to the caller

• The type of the data returned by a method must be compatible with the return type specified by the method

• When a particular non-void function is called, the result of the function can be used in three ways:

• It can be stored. For example:int result = minivan.range();

• It can be printed directly. For example, System.out.println(“The range is ” + minivan.range());

• It can be used in an expression:int y, x, zy = x +z + minivan.range();Orif (minivan.range() > 50)

….….

sahar mosleh california state university san marcospage 1 classes and objects and methods

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