gistfile1.java

import java.util.ArrayList;
import java.util.List;

class Employee {
	private Double dailyRate;
	
	public Employee(Double dailyRate) {
		this.dailyRate = dailyRate;
	}
	
	public Double getDailyRate() {
		return this.dailyRate;
	}
}

interface Calculator {
	public Double calculate(Double dailyRate, Integer noOfDays);
}

interface Calendar {
	public Integer noOfDays();
}

abstract class SalaryCalculationEngine<E extends Employee> {

	interface Context {
		public Calculator getCalculator();
		public Calendar getCalendar();
	}
	
	protected Context ctx; // This is the point of injection.
	
	public Double calculate(Double dailyRate) {
		return ctx.getCalculator().calculate(dailyRate,
			ctx.getCalendar().noOfDays());
	}
	
	public void payroll(List<E> employees) {
		for(E emp : employees)
			System.out.println(calculate(emp.getDailyRate()));
	}
}

abstract class Class1SalaryConfig {
	public Calculator calculator = new DefaultCalculator();
	public Calendar calendar = new DefaultCalendar();

	class DefaultCalculator implements Calculator {
		public Double calculate(Double basic, Integer noOfDays) {
			return basic * noOfDays;
		}
	}
	
	class DefaultCalendar implements Calendar {
		public Integer noOfDays() {
			return 30;
		}
	}
}

class DI {
	public static void main(String[] args) {
		List<Employee> emps = new ArrayList<Employee>();
		emps.add(new Employee(500d));
    
		SalaryCalculationEngine<Employee> sal = new SalaryCalculationEngine<Employee>() {
			
			{ ctx = new Class1SalaryConfigWithContext(); }
			
			class Class1SalaryConfigWithContext extends Class1SalaryConfig implements Context {
				public Calculator getCalculator() {
					return this.calculator;
				}
				public Calendar getCalendar() {
					return this.calendar;
				}
			}
		};
		sal.payroll(emps);
	}
}