Java Exceptions: try, catch, finally, throw, throws (ISC Class 11 Computer Science)

Understand Errors and Exceptions: try, catch, finally, throw, throws and Propagation

try {
    int a = 10 / 0;
    System.out.println("skipped");
} catch (ArithmeticException e) {
    System.out.println("Err");
} finally {
    System.out.println("Done");
}

static int div(int a, int b) { return a / b; }
// in main:
try { System.out.println(div(10, 2)); System.out.println(div(5, 0)); System.out.println("After"); }
catch (ArithmeticException e) { System.out.println("Cannot divide"); }
finally { System.out.println("End"); }

Errors and Exceptions: try, catch, finally, throw, throws and Propagation: key facts

Block order

try	the risky code
catch (Type e)	handler for that exception
finally	always runs

Structure.

Raising and declaring

throw: raise an exception | throws: declare that a method may pass one on

Do not confuse.

Common exceptions

• •ArithmeticException
• •ArrayIndexOutOfBoundsException
• •NumberFormatException
• •NullPointerException
• •IOException

Causes differ.

How to answer Errors and Exceptions: try, catch, finally, throw, throws and Propagation questions

1. 1Mark the statement that fails and the exception type.
2. 2Skip the rest of the try block.
3. 3Find the first catch that matches; run it.
4. 4Run finally; then continue after the whole try-catch.

Errors and Exceptions: try, catch, finally, throw, throws and Propagation in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Errors and Exceptions: try, catch, finally, throw, throws and Propagation

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Try, catch, finally

  Attempt, handle, always.

  Use it when: Blocks.

• throw acts, throws warns

  throw raises; throws declares.

  Use it when: Keywords.

• Skipped after the failure

  Rest of try is not executed.

  Use it when: Tracing.

Errors and Exceptions: try, catch, finally, throw, throws and Propagation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

try {
    System.out.println(div(10, 2));
    System.out.println(div(5, 0));
    System.out.println("After");
} catch (ArithmeticException e) {
    System.out.println("Cannot divide");
} finally {
    System.out.println("End");
}

Where students lose marks in Errors and Exceptions: try, catch, finally, throw, throws and Propagation

The slips examiners see most often, and what to write instead.

• ✗Mistake: Executing statements after the failure in try

  Ignoring control transfer.

  Wrong working

  After is printed

  Right working

  After is skipped

  ✓Do this: Control jumps to the matching catch.

  Usually costs: 2 marks.

• ✗Mistake: Writing catch for a different exception

  No match.

  Wrong working

  catch (NumberFormatException e) for division by zero

  Right working

  catch (ArithmeticException e)

  ✓Do this: Match the type.

  Usually costs: 1-2 marks.

• ✗Mistake: Mixing throw and throws

  Similar words.

  Wrong working

  void m() throw IOException

  Right working

  void m() throws IOException

  ✓Do this: Header uses throws.

  Usually costs: 1 mark.

Practice questions on Errors and Exceptions: try, catch, finally, throw, throws and Propagation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Integer.parseInt("12a") throws:
   (a) ArithmeticException (b) NumberFormatException (c) NullPointerException (d) IOException

   Hint

   Wrong number format.

   Answer

   (b) NumberFormatException

2. Q2 · 3 marks · Short answerBoard level

   What is an exception? Name two run-time exceptions with a cause for each. (3 marks)

   Hint

   Abnormal event; examples.

   Answer

   An exception is an object signalling an abnormal event at run time. ArithmeticException: division by zero; ArrayIndexOutOfBoundsException: index outside the array.

3. Q3 · 3 marks · Short answerHarder board level

   Give the output: String s = null; try { System.out.println(s.length()); } catch (NullPointerException e) { System.out.println("N"); } catch (Exception e) { System.out.println("E"); } finally { System.out.println("F"); } (3 marks)

   Hint

   s is null.

   Answer

   N then F. s.length() throws NullPointerException; the first matching catch runs, then finally.

4. Q4 · 4 marks · Short answerHarder board level

   Write a method that reads an integer from a string and returns it, returning -1 when the string is not a valid number; test with "45" and "4x5". (4 marks)

   Hint

   try-catch around parseInt.

   Answer

   static int toInt(String s) { try { return Integer.parseInt(s); } catch (NumberFormatException e) { return -1; } } toInt("45") returns 45; toInt("4x5") returns -1.

5. Q5 · 3 marks · Short answerHOTS / competency

   A programmer places the whole body of main inside try { ... } catch (Exception e) { } with an empty catch block to "avoid crashes". Evaluate this practice with two disadvantages and a better approach. (3 marks)

   Hint

   Hidden errors.

   Answer

   Errors are hidden so the user and programmer do not know that something failed, and the program may continue with wrong data. A better approach is to catch specific exceptions, report or log a clear message and recover or stop safely.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Always runs?Show answer

   finally.

2. 2.Division by zero exception?Show answer

   ArithmeticException.

3. 3.throws goes in?Show answer

   The method header.

4. 4.Invalid number string?Show answer

   NumberFormatException.

5. 5.Uncaught exception goes to?Show answer

   The calling method.

Errors and Exceptions: try, catch, finally, throw, throws and Propagation: frequently asked questions