• Error Handling 
final int public ❶ = 33;

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

❶

Compile time error: public is a Java™ keyword not to be used as variable's name.

❷

Run time error: De-referencing null yields a NullPointerException. 

final String s = null;
System.out.println(s.length());
Exception in thread "main" java.lang.NullPointerException
  at exceptionhandling.Npe.main(Npe.java:7)
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...
if (somethingBadHappens) {
  throw new NullPointerException();
}
...

Note

Without countermeasures your program will terminate

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 ...
  1. 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 
public static void main(String[] args) {
  final Path
    sourcePath = Paths.get("/tmp/test.txt"),
    destPath = Paths.get("/tmp/copy.java");

  // Compile time error:
  // Unhandled exception:
      java.io.IOException
  Files.copy(sourcePath, destPath);
...
 
public static void
  main(String[] args) {

final String s = null;

// No problem
System.out.println(s.length());
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  • 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
}
  1. 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()

❶

Class must implement interface AutoCloseable.

❷

Variable scanner's scope limited to block.

❸

close() method will be called automatically before leaving block scope. 

public class Scanner
  implements AutoCloseable ❶, ... {

  ...

  public void close() {...} ❷

}
 
Interface AutoCloseable {
  public void close(); // Signature, no
                       // implementation
}

❶

Promise to implement all methods being declared in AutoCloseable.

❷

Actually implementing a close() method. 

try (final String s = new String()) { // Error: Required type: AutoCloseable; Provided: String
   ...
}
package exceptionhandling;
public class StackTrace {
  public static void main(
       String[] args){
    a();
  }
  static void a() { b();}
  static void b() { c();}
  static void c() {
    String s = null;
    s.length();
  }
}
 
Exception in thread "main"
   java.lang.NullPointerException
 at ex.Trace.c(Trace.java:10)
 at ex.Trace.b(Trace.java:7)
 at ex.Trace.a(Trace.java:6)
 at ex.Trace.main(Trace.java:4)
 
try {
  FileInputStream f = new FileInputStream(
     new File("test.txt"));
} 
catch(final FileNotFoundException e) {
  System.err.println( "File not found");
  }
catch (final IOException e) {
  System.err.println( "IO error");
  }
catch(final Exception e) {
  System.err.println("General error");
  }
Ascending inheritance ordering
 
try {
  FileInputStream f = new FileInputStream(
     new File("test.txt"));
} 
catch(Exception e) {
      System.err.println("General error");
  }
catch (IOException e) {          // Already caught above!
      System.err.println( "IO error");
  }
catch(FileNotFoundException e) { // Already caught above!
      System.err.println("File not found");
 }
Wrong ordering
 
/* Turn cardinals {"one", "two", "three"} into ordinals {"first", "second", "third"}
 *
 * @param input The cardinal to be translated.
 * @return The corresponding ordinal or error message.
 */
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;
  }
}

  • Return false result, application continues.

  • Solution: Throw an exception. Steps:

    1. Find a suitable exception base class.

    2. Derive a corresponding exception class

    3. Throw the exception accordingly.

    4. Test correct behaviour.

public class CardinalException extends IllegalArgumentException {

  public CardinalException(final String msg) {
    super(msg);
  }
}
/* Turn cardinals {"one", "two", "three"} into ordinals {"first", "second", "third"}
 *
 * @param input The cardinal to be translated.
 * @return The corresponding ordinal or error message.
 */
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
}