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• Month.JANUARY is the same as Month.of(1)
• Month is an enum
• Month.JANUARY == 1 does not compile since Month is an enum and enums cannot be compared to an int
• Month.JANUARY.getValue() returns 1 because getValue is implemented as return ordinal() + 1
• ZonedDateTimes can be created from a LocalDate, LocalTime and a ZoneId:

  LocalDate date = LocalDate.of(2018, Month.JANUARY, 31);
  LocalTime time = LocalTime.of(15, 30);
  LocalDateTime dateTime = LocalDateTime.of(date, time);
  ZoneId zone = ZoneId.of("Europe/Paris");
  ZonedDateTime zonedDateTime = ZonedDateTime.of(date, time, zone);

• You should not chain Period creation like Period.ofYears(2).ofWeeks(1); This will return P1D and you will not have your two years.
• System.out.println(Period.ofWeeks(2)); will print out P14D cause period is in YMD format.
• Period is a day or more of time
• Duration is for units of time smaller than a day
• Period notation starts with P and Duration notation starts with PT
• Instant represents a specific moment (GMT)
• You can convert a ZonedDateTime to Instant with toInstant()
• The maximum Unit of Time you can use in the plus method of Instant is ChronoUnit.DAYS because Java implemented the plus method as:

  public Instant plus(long amountToAdd, TemporalUnit unit) {
      if (unit instanceof ChronoUnit) {
          switch ((ChronoUnit) unit) {
              case NANOS: return plusNanos(amountToAdd);
              case MICROS: return plus(amountToAdd / 1000_000, (amountToAdd % 1000_000) * 1000);
              case MILLIS: return plusMillis(amountToAdd);
              case SECONDS: return plusSeconds(amountToAdd);
              case MINUTES: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_MINUTE));
              case HOURS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_HOUR));
              case HALF_DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY / 2));
              case DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY));
          }
          throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
      }
      return unit.addTo(this, amountToAdd);
  }

• Java accounts for Daylight saving time
• When constantly updating the value of a String it might be better to use StringBuilder since String is immutable
• If you need a threadsafe StringBuilder use StringBuffer instead
• Locale can be created in different ways

  Locale.FRANCE;
  new Locale("fr");
  new Locale("fr_CA");
  new Locale.Builder()
     .setLanguage("fr")
     .setRegion("CA")
     .build();

• A resource bundle contains the local specific values that are used in your application
• You can specify a default value when fetching a property from a resource bunde with getString()
• You cannot do that when using get()
• Resource files for a Shop program can be named like Shop_fr_FR.java, Shop_fr_FR.properties, Shop_fr.java, Shop_fr.properties … Shop.properties
• The order above is also the order in which java will search through the different resource files
• When using a  NumberFormatter you use format() to turn a number into a String and parse() to turn a String into a number

• LocalDateTime ldt = LocalDateTime.now();
  DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT);
  
  // both of the following lines produce the same result
  System.out.println(formatter.format(ldt));
  System.out.println(ldt.format(formatter));
  

   

Code Examples: https://github.com/Nxtra/OCP-Examples/tree/master/5_DatesStringsLocalization