[java] Data access object (DAO) in Java

I am going to be general and not specific to Java as DAO and ORM are used in all languages.

To understand DAO you first need to understand ORM (Object Relational Mapping). This means that if you have a table called "person" with columns "name" and "age", then you would create object-template for that table:

type Person {
name
age
}

Now with help of DAO instead of writing some specific queries, to fetch all persons, for what ever type of db you are using (which can be error-prone) instead you do:

list persons = DAO.getPersons();
...
person = DAO.getPersonWithName("John");
age = person.age;

You do not write the DAO abstraction yourself, instead it is usually part of some opensource project, depending on what language and framework you are using.

Now to the main question here. "..where it is used..". Well usually if you are writing complex business and domain specific code your life will be very difficult without DAO. Of course you do not need to use ORM and DAO provided, instead you can write your own abstraction and native queries. I have done that in the past and almost always regretted it later.