Java Interfaces

Interfaces are used so that any object that implements it, will use certain method signatures. Consider the following classes: Person, Employee, and Customer. Persisting a Person, Employee, and Customer object will always involve connecting to a data store. However, the implementation for persisting a Person, Employee and Customer may differ. If we were to persist these objects into a relational database, such as MySQL, the SQL syntax would more than likely be different because each object may have different attributes.

Unlike Abstract Classes, Interfaces contain only method signatures. Also, a class can implement multiple interfaces. Whereas, only a single Abstract class can be inherited.

IDataHandler

public interface IDataHandler
{
    //Create, Read, Update, Delete
    public void insert(Object obj);
    public Object select(int id);
    public void update(Object obj);
    public void delete(Object obj);
}

PersonDAO

public class PersonDAO implements IDataHandler
{
    //Database Connection fields
    
    public PersonDAO() {
        //Initialize the DB fields here...
        
    }
    
    public void insert(Object obj) {
        Person person = (Person)obj;
        System.out.println("Inserting " + person.name + " into the database.");
    }
    
    public Object select(int id) {
        System.out.println("Selecting " + id + " from the database.");
        Person person = new Person();
        return person;
    }
    
    public void update(Object obj){
        Person person = (Person)obj;
        System.out.println("Updating " + person.name + " from the database.");
    }
    
    public void delete(Object obj) {
        Person person = (Person)obj;
        System.out.println("Deleting " + person.name + " from the database.");
    }
    
}

Person

public class Person
{
    public String name;
    public int age;
    
    public Person() {
        this.name = "John Doe";
        this.age = 0;
    }
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Driver

public class Driver
{
    public static void main(String[] args) {
        PersonDAO dao = new PersonDAO();
        Person rob = new Person("Rob", 22);
        Person jess = new Person("Jess", 20);
        
        dao.insert(rob);
        dao.insert(jess);
        
        //Make changes to jess
        dao.update(jess);
        dao.delete(rob);
    }
}