How to program according to good OO design practices?

Question

I did this program, simple and nothing complicated. My problem is that although this is doing exactly what I want, it does not follow correct patterns of object-oriented design. What should I do to, for example, end variable statics, extends, gets and sets...?

This is the code I have.

package com.mycompany.exerciciocontadores;

public abstract class Contador {

private String cliente;
private String id;
private double consumo;

private static String CLIENTE_POR_OMISSAO = "sem cliente";
private static String ID_POR_OMISSAO = "sem ID";
private static double CONSUMO_POR_OMISSAO = 0;

public Contador(String cliente, String id, double consumo) {
    this.cliente = cliente;
    this.id = id;
    this.consumo = consumo;
}

public Contador() {
    cliente = CLIENTE_POR_OMISSAO;
    id = ID_POR_OMISSAO;
    consumo = CONSUMO_POR_OMISSAO;
}

public String getCliente() {
    return cliente;
}

public String getId() {
    return id;
}

public double getConsumo() {
    return consumo;
}

public void setCliente(String cliente) {
    this.cliente = cliente;
}

public void setId(String id) {
    this.id = id;
}

public void setConsumo(double consumo) {
    this.consumo = consumo;
}

@Override
public String toString() {
    return "Contador{" + "cliente=" + cliente + ", id=" + id + ", consumo=" + consumo + '}';
}

public abstract double calcularCusto();

}

The subclass:

package com.mycompany.exerciciocontadores;

public class ContadorGas extends Contador {

private static final String PREFIXO = "GAS-";
private static int contador = 0;
private double custoUnitario = 0.8;

public ContadorGas(String cliente, double consumo) {
    super(cliente, PREFIXO + (++contador), consumo);
}

public static int getContador() {
    return contador;
}

public double getCustoUnitario() {
    return custoUnitario;
}

public static void setContador(int contador) {
    ContadorGas.contador = contador;
}

public void setCustoUnitario(double custoUnitario) {
    this.custoUnitario = custoUnitario;
}

@Override
public double calcularCusto() {
    return getConsumo() * custoUnitario;
}

}

Answer 1

The basic part you’ve already done. You can think about whether to use get and set in all attributes because there are things the class shouldn’t change. You may ask why to change the counter id, if you cannot change, you cannot have a set.

Another thing that can change and have a method that increases and maybe one that lowers the counter. Can you let me touch the counter anyway? Can you reset, can you put a high number? And can have a validation as the friend above wrote.

You might want to make consumption protected so you wouldn’t have to call getConsumo.

Ah I remembered that the costUnitario should be Static too.

Answer 2

Maybe it’s not exactly what you’re looking for, but a good practice is to use the set method to control the consistency of the data and prevent negative values from being saved where they shouldn’t.

Rather:

public void setConsumo(double consumo) 
{
    this.consumo = consumo;
}

That:

public void setConsumo(double consumoCliente )

     {
         if(consumoCliente >= 0.0)
            consumo = consumoCliente;
         else
           consumo = 0.0;
    }

And about the use of inheritance in your code, honestly I do not see anything out of rule, alias is an excellent practice. An alternative to inheritance would be composition, but there will be repetition of code that is not a good practice.