Class java.lang.Exception

Figure 531. Method printStackTrace() Slide presentation 
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)

Figure 532. Ascending inheritance ordering Slide presentation 
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

Figure 533. Wrong ordering Slide presentation 
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

Figure 534. Implementing convert Slide presentation 
/* 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;
  }
}

Figure 535. Problem: “Silent” errors Slide presentation

  • 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.


Figure 536. Step 1: Find exception base class Slide presentation

Figure 537. Step 2: Derive CardinalException Slide presentation 
public class CardinalException extends IllegalArgumentException {

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

Figure 538. Step 3: Throwing CardinalException Slide presentation 
/* 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");
}

Figure 539. Step 4: Unit test throwing CardinalException Slide presentation 
@Test public void testRegular() {
  Assert.assertEquals("second", Cardinal.convert("two"));
}

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

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