Reviewing Exceptions

Handling Exceptions

        try {
            // protected code
        } catch (IOException e) {
            // handler
        } catch (ArithmeticException | IllegalArgumentException e) {
            // multi catch
        } finally {
            // allways run this
        }

throw throws

public String getDataFromDatabase() throws SQLException {
    throw new UnsupportedOperationException();
}

Exception category

• all exception inherit from Throwable
• handling exceptions from Exception
• checked exception MUST be handled or declared
• unchecked exceptions does not need to be handled or declared

Unchecked Exceptions inherit RUntomeException

• ArithmeticException
• ArrayIndexOutOfBoundsException
• ArrayStoreException
• ClassCastException
• IllegalArgumentException
• IllegalStateException
• MissingResourceException
• NullPointerException
• NumberFormatException

• UnsupportedOperationException

• ! NumberFormatException inherits from IllegalArgumentException

Checked Exception

• FileNotFoundException
• IOException
• NotSerializableException
• ParseException

• SQLException

• ! FileNotFoundException and NotSerializableException inherits from IOException

• broad to specific exception=> DNC

try {
   throw new IOException();
} catch (IOException | FileNotFoundException e) {} // DOES NOT COMPILE (parent + child...)

try {
   throw new IOException();
} catch (IOException e) {
} catch (FileNotFoundException e) {} // DOES NOT COMPILE (Order...)

Creating Custom Exceptions

When creating your own exception, you need to decide whether it should be a checked or unchecked exception. While you can extend any exception class, it is most common to extend Exception (for checked) or RuntimeException (for unchecked).

class CannotSwimException extends Exception {}

  class DangerInTheWater extends RuntimeException {}

  class SharkInTheWaterException extends DangerInTheWater {}

  class Dolphin {
   public void swim() throws CannotSwimException {

   }
}


// constructors

    public class CannotSwimException extends Exception {

    public CannotSwimException() {
      super();  // Optional, compiler will insert automatically
    }

    public CannotSwimException(Exception e) {
      super(e);
    }

    public CannotSwimException(String message) {
          super(message);
    }
}

Printing Stack Traces

• e.printStackTrace();

Automating resource managment

• constructing with try-with-resources statements
• AutoClosable interface
• OMGEKEERDE volgorde van opruimen...
• try scope blijft in try scope!!

public class MyFileReader implements AutoCloseable { private String tag;
    public MyFileReader(String tag) { this.tag = tag;}
    @Override public void close() {
      System.out.println("Closed: "+tag);
    }
}

try (var bookReader = new MyFileReader("monkey")) {
    System.out.println("Try Block");
} finally {
   System.out.println("Finally Block");
}

//Try Block
//Closed: monkey
//Finally Block

// scoping

try (Scanner s = new Scanner(System.in)) {
 s.nextLine();
 } catch(Exception e) {
 s.nextInt(); // DOES NOT COMPILE
 } finally {
 s.nextInt(); // DOES NOT COMPILE
}

effectively Final Feature

it is possible to use resources declared prior to the try‐with‐resources statement, provided they are marked final or effectively final.

public void relax() {

final var bookReader = new MyFileReader("4");
MyFileReader movieReader = new MyFileReader("5");

try (bookReader;
     var tvReader = new MyFileReader("6");
movieReader) {
    System.out.println("Try Block");
} finally {
   System.out.println("Finally Block");
}

Suppressed Exceptions

• de eerste wordt als main exception gezien
• die daarna erbij komen als suppressed

public class TurkeyCage implements AutoCloseable { public void close() {
      System.out.println("Close gate");
   }
public static void main(String[] args) {
    try (var t = new TurkeyCage()) {
         System.out.println("Put turkeys in");
    }
} }

Assertions

• assert test_value;
• assert test_value : message;
• assert => false => AssertionError
• AssertionError is fatal and ends the program
• JVM heeft moet -enableassertions of -ea flag hebben
• -ea zonder iets dan geldt t voor alles in de running package
• -ea:com.demos... voor alles in demos en eronder

• -ea:comdemos.TestColors my.program.Main voor specifiek die Class

• -disableassertions of -da om uit te zetten

// correct!
assert 1 == age;
assert(2 == height);
assert 100.0 == length : "Problem with length";
assert ("Cecelia".equals(name)): "Failed to verify user data";


// DNC
assert(1);
assert x -> true;
assert 1==2 ? "Accept" : "Error";
assert.test(5> age);

public class Party {

    public static void main(String[] args) {
       int numGuests = -5;
       assert numGuests> 0;
       System.out.println(numGuests);


    }
 }

Assertions should never alter outcomes.

int x = 10;
assert ++x> 10;

Dates and Times

Dates beginnen met een 1 NIET een 0... gebruik daarom Month.October

.now()

// hebben allemaal .now()
System.out.println(LocalDate.now());     // 2020-10-14
System.out.println(LocalTime.now());     // 12:45:20.854
System.out.println(LocalDateTime.now()); // 2020-10-14T12:45:20.854
System.out.println(ZonedDateTime.now()); // 2020-10-14T12:45:20.854-04:00[America/New_York]

.of()

LocalDate date1 = LocalDate.of(2020, Month.OCTOBER, 20);
LocalDate date2 = LocalDate.of(2020, 10, 20);

formatting

• format()
• getDayOfWeek()
• getMonth()
• getYear()
• getDayOfYear()

DateTimeFormatter

LocalDate date = LocalDate.of(2020, Month.OCTOBER, 20);
System.out.println(date.format(DateTimeFormatter.ISO_LOCAL_DAT E));

// or custom
var f = DateTimeFormatter.ofPattern("MMMM dd, yyyy 'at' hh:mm");
System.out.println(dt.format(f)); // October 20, 2020 at 11:12

// of de oude classes
DateFormat s = new SimpleDateFormat("MMMM dd, yyyy'at' hh:mm");
System.out.println(s.format(new Date()));  // October 20, 2020 at 06:15

• y Year 20, 2020
• M Month 1, 01, Jan, January
• d Day 5, 05
• h Hour 9, 09
• m Minute 45
• s Second 52
• a a.m./p.m. AM, PM
• z Time Zone Name Eastern Standard Time,EST
• Z Time Zone Offset ‐0400

M Month m minute

Supported data/time symbols

• LocalDate : YMd
• LocalTime : hsma
• LocaldateTime : YMdhmsa
• ZonedDateTime : alles

Custom Text values

• concat
• use '' 'var f = DateTimeFormatter.ofPattern("MMMM dd, yyyy 'at' hh:mm");'
• escape ' met ' 'var g1 = DateTimeFormatter.ofPattern("MMMM dd', Party''s at' hh:mm");'

Internationalization i18n

Locale

Locale locale = Locale.getDefault();
System.out.println(locale);

• first lowercase language
• language is allways required
• then _ with UPPERcase country
• Locale.GERMAN => de
• Locale.GERMANY => de_DE

met builder()

Locale l1 = new Locale.Builder()
                .setLanguage("en")
                .setRegion("US")
                .build();

specific for your program

in scope van program, niet machine

System.out.println(Locale.getDefault()); // en_US
Locale locale = new Locale("fr");
Locale.setDefault(locale); // change the default
System.out.println(Locale.getDefault()); // fr

Localing Dates

• DateTimeFormatter.ofLocalizedDate(dateStyle)
• DateTimeFormatter.ofLocalizedTime(timeStyle)

• DateTimeFormatter.ofLocalizedDateTime(dateStyle, timeStyle)

• .SHORT .MEDIUM .FULL

Localizing Numbers

• java.text
• eerst formatten dan pas number va maken

Factory methods to get a NumberFormat |Description| Using default Locale and a specified Locale| |-|-| |A general‐purpose formatter|NumberFormat.getInstance()| | | NumberFormat.getInstance(locale)| |Same as getInstance|NumberFormat.getNumberInstance()| | |NumberFormat.getNumberInstance(locale)| |For formatting monetary amounts|NumberFormat.getCurrencyInstance()| | |NumberFormat.getCurrencyInstance(locale)| |For formatting percentages|NumberFormat.getPercentInstance()| | |NumberFormat.getPercentInstance(locale)| |Rounds decimal values before displaying|NumberFormat.getIntegerInstance()| | |NumberFormat.getIntegerInstance(locale)| Once you have the NumberFormat instance, you can call format() to turn a number into a String, or you can use parse() to turn a String into a number.

double price = 48;
var myLocale = NumberFormat.getCurrencyInstance();
System.out.println(myLocale.format(price));

String s = "23.45";
var en = NumberFormat.getInstance(Locale.US);
System.out.println(en.parse(s)); // 23.45

// LET OP!!

String s = "23.45";
var en = NumberFormat.getInstance(Locale.FRANCE);
System.out.println(en.parse(s)); //  !! 23 vanwege de .

// currency

String s = "$23.45"; // "23.45" DNC !!!
var en = NumberFormat.getCurrencyInstance(Locale.US);
System.out.println(en.parse(s));

Custom Number Formatter

• # , Omit the position if no digit exists for it. , $2.2

• 0 - Put a 0 in the position if no digit exists for it - $002.20

 double d = 1234567.467;
 NumberFormat f1 = new DecimalFormat("###,###,###.0");
 System.out.println(f1.format(d)); // 1,234,567.5

 NumberFormat f2 = new DecimalFormat("000,000,000.00000");
 System.out.println(f2.format(d)); // 001,234,567.46700

 NumberFormat f3 = new DecimalFormat("$#,###,###.##");
 System.out.println(f3.format(d)); // $1,234,567.47

Locale Category

• DISPLAY
• FORMAT
• Locale.setDefault() doet DISPLAY en FORMAT

    public static void printCurrency(Locale locale, double money) {
        System.out.println(
                NumberFormat.getCurrencyInstance().format(money) + ", " + locale.getDisplayLanguage());
    }

    public static void main(String[] args) throws ParseException {
        var spain = new Locale("es", "ES");
        var money = 1.23;

        // Print with default locale
        Locale.setDefault(new Locale("en", "US"));
        printCurrency(spain, money); // $1.23, Spanish

        // Print with default locale and selected locale display
        Locale.setDefault(Locale.Category.DISPLAY, spain);
        printCurrency(spain, money); // $1.23, espaÑol  ! let op NUMBERFORMAT is changed...

        // Print with default locale and selected locale format
        Locale.setDefault(Locale.Category.FORMAT, spain);
        printCurrency(spain, money); // 1,23 €, espaÑol
    }

Resource Bundle

• op goede locatie...

ResourceBundle.getBundle("name");          // default locale
ResourceBundle.getBundle("name", locale);

1. Lookfortheresourcebundlefortherequestedlocale,followedbytheone for the default locale.
2. Foreachlocale,checklanguage/country,followedbyjustthelanguage.
3. Usethedefaultresourcebundleifnomatchinglocalecanbefound.

having: - Zoo_en.properties - Zoo_hi.properties - Zoo.properties

Locale.setDefault(new Locale("hi"));
ResourceBundle rb = ResourceBundle.getBundle("Zoo", new Locale("en"));

Worden ALLE drie geladen!

Formatting Messages

• {0} {1} als markers

Properties Class

• HashMap<String, String>

System.out.println(props.getProperty("camel")); // null
System.out.println(props.getProperty("camel", "Bob")); // Bob is default waarde omdat camel er niet is

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