A Factory Pattern or Factory Method Pattern says that just defines an interface or abstract class for creating an object but lets the subclasses decide which class to instantiate. In other words, subclasses are responsible to create the instance of the class.
The Factory Method Pattern is also known as Virtual Constructor.
Advantages of Factory Design Pattern
Factory Method Pattern allows the sub-classes to choose the type of objects to create.
It promotes the loose-coupling by eliminating the need to bind application-specific classes into the code. That means the code interacts solely with the resultant interface or abstract class so that it will work with any classes that implement that interface or that extend that abstract class.
Usage of Factory Design Pattern
When a class doesn't know what sub-classes will be required to create
When a class wants that its sub-classes specify the objects to be created.
When the parent classes choose the creation of objects for its sub-classes.
Calculate Electricity Bill: A Real World Example of Factory Method
Step 1: Create a Plan abstract class.
import java.io.*;
abstract class Plan{
protected double rate;
abstract void getRate();
public void calculateBill(int units){
System.out.println(units*rate);
}
}
Step 2: Create the concrete classes that extends Plan abstract class
class DomesticPlan extends Plan{
//@override
public void getRate(){
rate=3.50;
}
}
class CommercialPlan extends Plan{
//@override
public void getRate(){
rate=7.50;
}
class InstitutionalPlan extends Plan{
//@override
public void getRate(){
rate=5.50;
}
Step 3: Create a GetPlanFactory to generate object of concrete classes based on given information
class GetPlanFactory{
//use getPlan method to get object of type Plan
public Plan getPlan(String planType){
if(planType == null){
return null;
}
if(planType.equalsIgnoreCase("DOMESTICPLAN")) {
return new DomesticPlan();
}
else if(planType.equalsIgnoreCase("COMMERCIALPLAN")){
return new CommercialPlan();
}
else if(planType.equalsIgnoreCase("INSTITUTIONALPLAN")) {
return new InstitutionalPlan();
}
return null;
}
}
Step 4: Generate Bill by using the GetPlanFactory to get the object of concrete classes by passing a piece of information such as type of plan DOMESTICPLAN or COMMERCIALPLAN or INSTITUTIONALPLAN.
import java.io.*;
class GenerateBill{
public static void main(String args[])throws IOException{
GetPlanFactory planFactory = new GetPlanFactory();
System.out.print("Enter the name of plan for which the bill will be generated: ");
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
String planName=br.readLine();
System.out.print("Enter the number of units for bill will be calculated: ");
int units=Integer.parseInt(br.readLine());
Plan p = planFactory.getPlan(planName);
//call getRate() method and calculateBill()method of DomesticPaln.
System.out.print("Bill amount for "+planName+" of "+units+" units is: ");
p.getRate();
p.calculateBill(units);
}
}
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