Dependancy Injection (Spring Note 1)

Just assume a situation where we have to develop a module that must generate different types of reports such as PDF reports, Excel reports. One of a worst design for such scenario is design classes for each report separately without any form of polymorphism. Such a design may may result in increasing coupling of the program.
example :

ExcelReport excelReport=new ExcelReport();
PDFReport pdfReport=new PDFReport();

excelReport.initialize();
excelReport.createReport();
excelReport.saveReport();

pdfReport.initialize();
pdfReport.createReport();
pdfReport.saveReport();

We can use polymorphism to reduce the coupling of the design.

Report excelReport=new ExcelReport();
Report pdfReport=new PDFReport();

excelReport.initialize();
excelReport.createReport();
excelReport.saveReport();

pdfReport.initialize();
pdfReport.createReport();
pdfReport.saveReport();

But still the caller have to know about the type of the objects. still there is no much advantage of using above design to reduce coupling.

There is another solution to reduce coupling dramatically. Which is called "Dependency Injection" . What we do here is, using a helper class to separate dependencies.

Report.java

package model;

public interface Report {
 public void initialize();
 public void createReport();
 public void saveReport();
}

ExcelReport.java

package model;

public class ExcelReport implements Report {
 @Override
 public void initialize() {
  System.out.println("-= Initializing Excel Report =-");
 }

 @Override
 public void createReport() {
  System.out.println("-= Creating Excel Report =-");
 }

 @Override
 public void saveReport() {
  System.out.println("-= Saving Excel Report =-");
 }
}

PDFReport.java

package model;

public class PDFReport implements Report{
 @Override
 public void initialize() {
  System.out.println("-= Initializing PDF Report =-");
 }

 @Override
 public void createReport() {
  System.out.println("-= Creating PDF Report =-");
 }

 @Override
 public void saveReport() {
  System.out.println("-= Saving PDF Report =-");
 }

}

ReportGenerator.java

package model;

public class ReportGenerator {
 Report reoprt;
 
 public ReportGenerator(Report report){
  this.reoprt=report;
 }
 
 public void generateReport(){
  this.reoprt.createReport();
  this.reoprt.saveReport();
 }
 
 public void setReport(Report report){
  this.reoprt=report;
 }
}

DependancyInjectionDemo.java

import model.ExcelReport;
import model.PDFReport;
import model.Report;
import model.ReportGenerator;

public class DependancyInjectionDemo {

 /**
  * @param args
  */
 public static void main(String[] args) {
  // without dependancy injection
  System.out
    .println("========Generating Reports Using Without Fependancy Injection========");
  Report excelReport = new ExcelReport();
  Report pdfReport = new PDFReport();

  excelReport.initialize();
  excelReport.createReport();
  excelReport.saveReport();

  pdfReport.initialize();
  pdfReport.createReport();
  pdfReport.saveReport();

  System.out
    .println("========Generating Reports Using Fependancy Injection========");
  // using dependancy injection
  ReportGenerator rpeortGen = new ReportGenerator(excelReport); // dependancy
                  // injection
                  // via
                  // constructor
  rpeortGen.generateReport();

  rpeortGen.setReport(pdfReport); // dependancy injection via mutator
  rpeortGen.generateReport();
 }
}

The above process can be simplified by using Spring.