Object Associations

Composition: the Employee is encapsulated within the Company . There is no way for the outside world to get a reference to the Employee. The Employee is created and destroyed with the company

final class Company{
private final Employee Employee;
{
    Company(EmpDetails details) {
    engine = new Employee(details);
  }
   void assign() {
      emp.work();
   }
}

Aggregation: The Company also performs its functions through an Employee, but the Employee is not always an internal part of the Company . Employees can be exchanged, or even completely removed. As the employee is injected, the Employee reference can live outside the Company.

final class Company{
  private Employee engine;
  void addEmployee(Employee emp) {
    this.emp = emp;
  }
  void assign() {
    if (emp != null)
      emp.work();
  }
}

Dependency: The company does not hold the employee reference. It receives an employee reference only to the scope an operation. Company is dependent on the Employee object to perform an operation

final class Company{
  void assign(Employee emp) {
    if (emp != null)
      emp.work();
  }
}

Abstraction: Defines the basic operations the implementer should adher to. Employee interface lists the general behavior of an employee

public interface Employee{
 public void work();
 public void off();
 public void quit();
}

Realisation: A class implements the behavior defined by the other other class or interface

public abstract class Engineer implements Employee{
 public void work();
 public void off();
 public void quit();
}

Generalization: A class which is a special form of a parent class

 public class SWEngineer extends Engineer {
 public void work()
 {
  System.out.println('SW Engineer working');
 }
 public void off()
 {
  System.out.println('SW Engineer is off today');
 }
 public void quit()
 {
  System.out.println('SW Engineer is quitting');
 }

}

UML Relationship Pointers

ref:http://techie-experience.blogspot.gr/2013/01/quick-summary-object-associations.html