What is "granularity level" and how to identify it?

The level of granularity would be basically to define a criterion that we will adopt to separate our classes, thinking about the concept of object orientation. For example:
If we have to register an employee with the data of Name, CPF, Date of Birth and Salary; we would think at first of creating the following class Funcionário :

public class Funcionário
 {
     private String nome;
     private String cpf;
     private date datanasc;
     private double salario;

    /*restante do código*/
}

But if we are thinking about reusing this code, we can define a Pessoa class that will contain the attributes that are basic to all people:

public class Pessoa
 {
     private String nome;
     private String cpf;
     private date datanasc;

    /*restante do código*/
}

And so, our Class Funcionário would inherit from person and would only have the field Salary more.

public class Funcionário extends Pessoa
 {
     private double salario;

    /*restante do código*/
}

So the definition of granularity is what helps us define which attribute will be contained in each class, thus making our code reusable.

Granularity can be understood as the level of division into which objects are built. To address in terms of high and low classification:

Lowgranularityisthedivisionintomanydetailsofobjects,commonlywithmanycompositionsandfewattributesperobject;highgranularityisthecoarserlevel,objectsgetmanyattributesandfewrelationships.

Imakeaparallelwiththerepresentationofinvoicesinsomecorporatesystems,maintainedwithtwoobjects:NotaFiscalNaoEmitidaandNotaFiscalEmitida.Itseemstomeagoodexamplebecauseitdealswithtwoextremesofthesamedataandisconsistentwith this brief reference :

  

Building objects at the lowest level of granularity provides optimal flexibility, but may be unacceptable in terms of performance and memory usage. (free translation)

The first has only a few fields of the invoice and refers to dozens of other entities such as Cliente , Produto , Empresa , Endereco , Imposto . It has low granularity .

Once issued and no longer modified, and for performance purposes, historical consistency, and storage simplicity, NotaFiscalEmitida has a copy of all relationship information stored in a single row in the database. The object has an absurd configuration with more than a hundred fields and no relation, but it is totally independent of any other entity and self-contained in its data. It has high granularity .