Calendario

int qtDiasMes = new GregorianCalendar().getActualMaximum(Calendar.DAY_OF_MONTH);
		System.out.println(qtDiasMes);

Calendar cal = new GregorianCalendar();
		cal.set(Calendar.MONTH, 8); //janeiro = 0; dezembro=11
		int qtDiasMes = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
		System.out.println(qtDiasMes);

Calendar.JANUARY
        ...
        Calendar.DECEMBER

SimpleDateFormat formatador = new SimpleDateFormat("dd/MM/yyyy");
Calendar cal = GregorianCalendar.getInstance();
		
cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
int mes = cal.get(Calendar.MONTH);
		
		
while(cal.get(Calendar.MONTH)==mes){
   System.out.println(formatador.format(cal.getTime()));
   cal.add(Calendar.DAY_OF_MONTH, 1);
}