The Data Access Object is basically an object or an interface that provides access to an underlying database or any other persistence storage.
That definition from:
http://en.wikipedia.org/wiki/Data_access_object
Check also the sequence diagram here:
http://www.oracle.com/technetwork/java/dataaccessobject-138824.html
Maybe a simple example can help you understand the concept:
Let’s say we have an entity to represent an employee:
public class Employee {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
The employee entities will be persisted into a corresponding Employee
table in a database.
A simple DAO interface to handle the database operation required to manipulate an employee entity will be like:
interface EmployeeDAO {
List<Employee> findAll();
List<Employee> findById();
List<Employee> findByName();
boolean insertEmployee(Employee employee);
boolean updateEmployee(Employee employee);
boolean deleteEmployee(Employee employee);
}
Next we have to provide a concrete implementation for that interface to deal with SQL server, and another to deal with flat files, etc.