1. The Problem Unexpected erroneous situations are often discovered by code which is NOT prepared to remedy the error………. For example.. String in = JOptionPane.showInputDialog(“enter side of square “); side = Integer.parseInt(in); area = side * side; The parseInt method will not be able to convert an input string, such as ‘abccd’ to a integer value !! SO … WHAT SHOULD THIS METHOD DO??

2. What should parseInt do if an integer value can not be determined from the parameter string?? Prompt the user for another value?? (System.out, JOptionPane, FileReader ??) Substitute a default value?? Output an error message and cause program to exit?? parseInt does not have the ‘knowledge’ to make this decision … there MAY be another method which does know how this should be handled…….

3. This is where Exception objects come in. Integer.parseInt cannot ignore the error. parseInt must do something. So it stops executing, creates an exception object (in particular a NumberFormatException object), and throws this back to the caller. WHY??? Perhaps the caller knows what the appropriate action is. Code to throw an exception object to indicate failure: if (failure){ throw new NumberFormatException(“For input” + param); }

4. If no method ‘handles’ the exception, the exception will eventually reach the main method … and crash picture of console***********

5. Your code can throw exceptions: public class BankAccount { public void withdraw(double amount) { if (amount > balance) throw new IllegalArgumentException( "Amount exceeds balance"); balance = balance - amount; }... }

7. Checked Exceptions Some exceptions are unavoidable -- and fatal to program: file not found error in file Java puts these exceptions into a category called ‘checked’ exceptions. That means that the compiler will check that your code either ‘handles’ the situation, or specifies (declares) that it will occurs. Checked exceptions are subclasses of Exception that are not subclasses of RuntimeException

8. Checked and Unchecked Exceptions

9. Exception Specifications BufferedReader.readLine may throw IOException So the read method specifies this: public class Coin { public void read(BufferedReader in) throws IOException { value = Double.parseDouble(in.readLine()); name =in.readLine(); }... }

10. Exception Specifications Need to tag caller of Coin.read as well Stop at main or with handler (see below) Can have multiple exception types public void read() throws IOException, ClassNotFoundException throws specifier not a sign of irresponsible programming Better to declare exception than to handle it incompetently

11. Designing Your Own Exception Types if (amount > balance) throw new InsufficientFundsException(...); Make it an unchecked exception--programmer could have avoided it by calling getBalance first Extend RuntimeException Supply two constructors

12. Designing Your Own Exception Types previousprevious | start | nextstartnext File Purse.java previousprevious | start | nextstartnext public class InsufficientFundsException extends RuntimeException { public InsufficientFundsException() { } public InsufficientFundsException(String reason) { super(reason); } }