Data access object (DAO) in Java

Question

I was going through a document and I came across a term called DAO. I found out that it is a Data Access Object. Can someone please explain me what this actually is?

I know that it is some kind of an interface for accessing data from different types of sources, in the middle of this little research of mine I bumped into a concept called data source or data source object, and things got messed up in my mind.

I really want to know what a DAO is programmatically in terms of where it is used. How it is used? Any links to pages that explain this concept from the very basic stuff is also appreciated.

Answer 1

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.