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 (and also use unchecked exceptions)
Throwing Different Exceptions
/*
* REQUIRES: accountName has a non-zero length and initialBalance >= 0
* //...
*/
public Account(String accountName, double initialBalance)
} throws IllegalNameException, IllegalBalanceException {- If we have different things that can go wrong, we should have different types of exceptions
- At most one exception can be thrown out of any given call
- We can then address them individually in the caller...
if (name == null || name.isEmpty()) {
throw new IllegalNameException();
}if (initialBalance < 0) {
throw new IllegalBalanceException();
}
// ...Throwing Different 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 eException 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");
}
}Unchecked Exceptions
- Maybe we don't want the compiler to check for some exceptions?
public class CouldNotStartException
extends RuntimeException {}Note: unchecked exceptions normally indicate bad programming / code smells. We normally don't want to explicitly use/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 and declaring throws becomes optional!
Lecture Ticket Review
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
-eabalance >= 0assert balance >= 0;
Lecture Lab
C2: Advanced Exception Handling
The End - Thank You!
C2 Advanced Exception Handling
By Steven Wolfman
