final int public ❶ = 33;

final String s = null;
System.out.println(s.length())❷ ;

final String s = null;
System.out.println(s.length());

Exception in thread "main" java.lang.NullPointerException
  at exceptionhandling.Npe.main(Npe.java:7)

...
if (somethingBadHappens) {
  throw new NullPointerException();
}
...

final String s = null;
try {
  System.out.println(s.length()) ;
} catch (final NullPointerException e) {
  System.out.println("Dear user, something bad just happened");
}
System.out.println("Business as usual ...");

Dear user, something bad just happened
Business as usual ...

176. Mind your prey

try {
 [code that may throw an exception]
}[catch (ExceptionType-1 e) {
 [code that is executed when ExceptionType-1 is thrown]
}] [catch (ExceptionType-2 e) {
 [code that is executed when ExceptionType-2 is thrown]
}]
  ...
} [catch (ExceptionType-n e) {
  [code that is executed when ExceptionType-n is thrown]
}]
[finally {
  [code that runs regardless of whether an exception was thrown]]
}]

• Error Handling
  • ➟ Checked vs unchecked exceptions

• Error Handling
  • ➟ Exceptions and Junit

@Test(expected = FileAlreadyExistsException.class)
public void copyFile() throws IOException {
  final Path
    source = Paths.get("/tmp/source.txt"),
    dest   = Paths.get("/tmp/dest.txt");

  Files.copy(source, dest);  // May work.
  Files.copy(source, dest);  // Failure: FileAlreadyExistsException
}

177. Expected exception test failure

• Error Handling
  • ➟ Variants

Scanner scanner = null;
try {
  scanner = new Scanner(System.in);
   ... // Something may fail
} finally {
  if (null != scanner) {
    scanner.close(); // Clean up, save resources!
  }
}

try (final Scanner❶  scanner❷ = new Scanner(System.in)) {
   ... // Something may fail
}❸ // implicitly calling scanner.close()

try (final String s = new String()) { // Error: Required type: AutoCloseable; Provided: String
   ...
}

• Error Handling
  • ➟ Class java.lang.Exception

/* Translate {"one", "two", "three"} to {"first", "second", "third"}
 * @param input The input String to be translated.
 * @return See above explanation. */
static public String convert(final String input) {
 switch (input) {
   case "one": return "first";
   case "two": return "second";
   case "three": return "third";
  default: return "no idea for " + input;
  }
}

public class CardinalException
  extends IllegalArgumentException {

  public CardinalException(final String msg) {
    super(msg);
  }
}

/**
 * Translate {"one", "two", "three"} to {"first", "second", "third"}
 * @param input The input String to be translated.
 * @return See above explanation.
 * @throws CardinalException If input not from list.
 */
static public String convert(final String input)
  throws CardinalException {

  switch (input) {
    case "one": return "first";
    case "two": return "second";
    case "three": return "third";
  }
  throw new CardinalException(
            "Sorry, no translation for '" + input + "' on offer");
}

@Test public void testRegular() {
  Assert.assertEquals("second", Cardinal.convert("two"));
}

@Test(expected = CardinalException.class)
public void testException() {
  Cardinal.convert("four"); // No assert...() required
}