Return all birthdays of the month chosen by the user

Let's say you have the following list of friends:

Amigo[] agenda = {
    new Amigo("João", "20/01/1983"),
    new Amigo("Marcia", "12/08/1990"),
    new Amigo("Lucas", "02/12/1989"),
    new Amigo("Tereza", "30/08/1985")
}

You want to get the birthday list for month 08, then call the method:

String aniversariantes = objeto.aniversariarNoMes(8);

Considering the initial implementation of the method:

public static String aniversariarNoMes(int mes) {
    String saida = "";

    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida = amigo.getNome() + "\n";

                return saida;
            }

            return saida+=amigo.getNome()+"\n";
        }
    }

    return null;
}

Follow the steps that will be performed (table test):

As returned, the method is terminated and therefore the returned value would be saida . In addition to having returned before the time, it returned an unexpected value, since João celebrates his birthday in month 01. By this you already know that this amigo does not make sense to be there. Removing it, it stays:

public static String aniversariarNoMes(int mes) {
    String saida = "";

    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida = amigo.getNome() + "\n";

                return saida;
            }
        }
    }

    return null;
}

Remaining the table test:

As returned, the method is terminated again. In this case, the return would be amigo . In fact this record is birthday in month 8, but it is not the only one, so we can not finalize the method at this time. This shows us that again this if is wrong. Removing it, it stays:

public static String aniversariarNoMes(int mes) {
    String saida = "";

    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida = amigo.getNome() + "\n";
            }
        }
    }

    return null;
}

Remaining the table test:

We have already noticed that now it is always returning if , so there must be another for in the code that returns the correct value. As we have seen that inside amigo does not work, just put it outside:

public static String aniversariarNoMes(int mes) {
    String saida = "";

    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida = amigo.getNome() + "\n";
            }
        }

        return saida;
    }

    return null;
}

But if you look at item 5.14 of the last table test, you will see that the value of Amigo("Tereza", "30/08/1985") ends with amigo . This is not what we want, but the list of names of the anniversaries. That is, within the if , we can not only make the simple assignment to the value of saida , but rather concatenate the current value. To do this:

public static String aniversariarNoMes(int mes) {
    String saida = "";

    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida += amigo.getNome() + "\n";
            }
        }

        return saida;
    }

    return null;
}

In this way, the code should return amigo , which is expected, or for if there are no birthday parties. I leave the table test of this version as an activity for you. Do it and see if it really matches what the code does.

// Method that receives a month and returns all birthday friends

public static String aniversariarNoMes(int mes) {
    String saida = "";
    //Faz a validação do mes escolhido
    if (mes < 01 || mes > 12) {
        return "Mês invalido.";
    } else {
        //Percorre o ArrayList a procura do mes informado
        for (Amigo amigo : agenda) {
            if (mes == amigo.getMesNas()) {
                saida += amigo.getNome() + "\n";

            }

        }
        return saida;
    }



}