How to transform String from time to an integer?

I do not think you need a class like Calendar to solve, just a split and do a math operation, like this:

String horacompleta = "12:39";
String[] horamin = horacompleta.split(":");

int hora = Integer.parseInt(horamin[0]);
int min = Integer.parseInt(horamin[1]);

int minutos = (hora * 60) + min; //Pega o total

You can put in a function like this:

public static int hour2minutes(String fullhour)
{
    String[] parts = fullhour.split(":");

    int hour = Integer.parseInt(parts[0]);
    int min = Integer.parseInt(parts[1]);

    return (hour * 60) + min;
}

And to use it, do so:

hour2minutes("12:39"); //759
hour2minutes("16:20"); //980
hour2minutes("23:59"); //1439

See a test at IDEONE: link

There are several ways to do this. Another one besides Guilherme would be, for example, using SimpleDataFormat to convert the string into date format, and then use the class TimeUnit converting milliseconds in minutes using the toMinutes() method. Here's how it would look:

public int hour2min(String hour) {
    try {
        Date date = new SimpleDateFormat("kk:mm").parse(hour);
        return (int) (TimeUnit.MILLISECONDS.toMinutes(date.getTime())-180);
    } catch (ParseException e) {
        e.printStackTrace();
        return 0;
    }
}

To use just do so:

Log.wtf("",""+hour2min("18:23"));

Output:

1103