C2: Advanced Exception Handling

CPSC 210

Learning Goals

To design appropriate exception hierarchies to provide flexibility in handling exceptions
To identify and declare exceptions as checked or unchecked
To understand what it means to throw unchecked exceptions

Throwing Multiple Exceptions

/*
 * REQUIRES: accountName has a non-zero length, initialBalance >= 0
 * //...
 */
public Account(String accountName, double initialBalance) {








}

AND

  throws IllegalNameException, IllegalBalanceException {

If we have different things that can go wrong, we should have different types of exceptions
We can then address them individually in the caller...

if (name == null || name.isEmpty()) {
  throw new IllegalNameException();
}

if (initialBalance < 0) {
  throw new IllegalBalanceException();
}
// ...

Throwing Multiple Exceptions (2)

private void init() {
  try {
    cheq = new Account("Joe", 145.00);
    sav = new Account("Joe", 256.50);
  } catch (IllegalBalanceException e) {
    System.err.println("Illegal balance given to one of the accounts");
  } catch (IllegalNameException e) {
    System.err.println("Illegal name given to one of the accounts");
  }
}

What should we put within one try block?

private void init() {
  try {
    cheq = new Account("Joe", 145.00);
  } catch (IllegalBalanceException e) {
    System.err.println("An illegal balance was given to CHECKING account");
  } catch (IllegalNameException e) {
    System.err.println("An illegal name was given to CHECKING account");
  }
  try {
    sav = new Account("Joe", 256.50);
  } catch (IllegalBalanceException e) {
    System.err.println("An illegal balance was given to SAVINGS account");
  } catch (IllegalNameException e) {
    System.err.println("An illegal name was given to SAVINGS account");
  }
}

It depends. Sometimes we want to differentiate.

Catching both exceptions!

Exception Hierarchies

private void init() {
  try {
    cheq = new Account("Joe", 145.00);
    sav = new Account("Joe", 256.50);
  } catch (Exception e) {
    System.err.println("Something went wrong...");
  }
}

What if we don't care about the type of exception?

Both our exceptions extend the Exception class!

But Exception can really be ANYTHING. How about any exception related to creation of accounts?

public class AccountCreationException 
	extends Exception {}

public class IllegalBalanceException 
	extends AccountCreationException {}

public class IllegalNameException 
	extends AccountCreationException {}

AccountCreationException e

Exception Hierarchies (2)

Throwable

Object

Error

Exception

RuntimeException

...

Unchecked Exceptions

Checked Exceptions

Rethrowing Exceptions

private void init() throws CouldNotStartException {
  try {
    cheq = new Account("Joe", 145.00);
    sav = new Account("Joe", 256.50);
  } catch (AccountCreationException e) {
    System.err.println("Something went wrong during account creation");
    throw new CouldNotStartException();
  }
}

Let's say we don't want to start our program at all if something went wrong during account creation

public static void main(String[] args) {
  try {
    new TellerApp();
  } catch (CouldNotStartException e) {
    System.err.println("Could not start the app");
  }
}

Lecture Ticket Review

Unchecked Exceptions

Maybe we don't want the compiler to check for some exceptions?

public class CouldNotStartException 
	extends RuntimeException {}

Note: unchecked exceptions in many cases indicate bad programming. We often don't want to explicitly throw/catch these!

private void init() throws CouldNotStartException {
  try {
    cheq = new Account("Joe", 145.00);
    sav = new Account("Joe", 256.50);
  } catch (AccountCreationException e) {
    System.err.println("Something went wrong during account creation");
    throw new CouldNotStartException();
  }
}

public static void main(String[] args) {
  try {
    new TellerApp();
  } catch (CouldNotStartException e) {
    System.err.println("Could not create accounts exception");
  }
}

Catching is optional!

Invariants / Assert Statements

A class invariant is a property that is always true about every instance of that class
For example, an invariant for our Account class would be
Invariants can be checked using Java assert statements
These are part of the Java language and not of jUnit!
Assert statements are ignored if program is not run with flag

-ea

balance >= 0

assert(balance >= 0);

C2: Advanced Exception Handling

CPSC 210

Learning Goals

Throwing Multiple Exceptions

Throwing Multiple Exceptions (2)

Exception Hierarchies

Exception Hierarchies (2)

Rethrowing Exceptions

Lecture Ticket Review

Unchecked Exceptions

Invariants / Assert Statements

Quick Real-World Demo...

Lecture Lab

edX > C2 > Lecture Lab

C2: Advanced Exception Handling

The End - Thank You!