Applets & Applications CSC 171 FALL 2001 LECTURE 15

History: FORTRAN John Backus proposed the development of a programming language that would allow uses to express their problems in commonly understood mathematical formulae -- later to be named FORTRAN

Change in Schedule 11/12 – Chapter 14 – Exception Handling 11/16 – Midterm – Hoyt – 8AM 11/19 – Chapters 11, 12, & 13 – Graphics & GUIs (optional) – Enjoy the Turkey 11/26 – Chapter 16 – Files & Streams 11/29 – Project due 12/3 – Chapter 19 – Data Structures 12/11 – last lecture 12/19 – FINAL EXAM - 12/19 4PM

Exam Friday 11/16 8 AM - 9 AM Hoyt Chapters 1-10 – – end of chapter questions make good study materiel Labs Projects Workshops Multiple choice 2%) Some “Write a method/class that...” 6%)

Throwing Exceptions What should a method do when a problem is detected? Traditionally, methods return some special code to indicate failure However, – The caller may forget to check the return value – The caller may not be able to fix it

Problems with return signals If the caller forgets to check – Bad data is processed String input = myTextBox.getText(); Double d = new Double(input); // what if I type “hello” i = d.doubleValue(); If the caller can’t fix it – We have to punt to the caller’s caller x.doStuff(); // becomes If (!x.doStuff()) return false; //man, that’s a lot of code

Exception-handling mechanism to the rescue Exceptions can’t be overlooked Excepts can be handled by a competent handler – Not just the caller

Simple Try Block Syntax try { statement; } catch ( ExceptionClass ExceptionObject ) { statement; }

Example: Console Input import java.io.*; public class ReadingDoubles { public static void main (String[] args) { double i, j ; try { // open the console for bufferd I/O InputStreamReader reader = new InputStreamReader(System.in); BufferedReader console = new BufferedReader(reader);

Alternately // instead of InputStreamReader reader = new InputStreamReader(System.in); BufferedReader console = new BufferedReader(reader); // could use nested constructors BufferedReader console = new BufferedReader(new InputStreamReader(System.in));

What happens? public static void main (String[] args){ BufferedReader console = new BufferedReader(InputStreamReader(System.in)); System.out.print("Please enter a floating point number : "); String input = console.readLine(); Double d = new Double(input); double d1 = d.doubleValue(); }

This happens cd d:/courses/CSC171/CSC171FALL2001/code/ d:/devenv/jdk1.3/bin/javac Console1Bad.java Console1Bad.java:14: unreported exception java.io.IOException; must be caught or declared to be thrown String input = console.readLine(); //read a String ^ 1 error Compilation exited abnormally with code 1 at Mon Nov 12 21:14:52

Deal with the potential exception try { String input = console.readLine(); } catch (IOException e) { System.out.println(e + “bad read”); System.exit(0); }

Example //get the value System.out.print("Please enter a floating point number : "); try { input = console.readLine(); //read a String } catch (IOException e) { System.out.println(e + " bad read "); System.exit(0); }

Now check the number format try{ Double d = new Double(input); double d1 = d.doubleValue(); // Calculate the sum & printout System.out.println(d1 + "^2 == " + (d1*d1)); } catch (NumberFormatException e) { System.out.println(e + " : What you entered was not a double"); }

Checked & Unchecked Exceptions Java Exceptions fall into two categories – Checked You MUST tell the compiler what you are going to do about the exception – Unchecked (sub-classes of RuntimeException ) NumberFormatException IllegalArgumentException NullPointerException

Exception Class Hierarchy

Why Checked & Unchecked? Checked Exceptions are not your fault – So, you have to deal with them Unchecked Exceptions are your fault – So, we trust you to deal with them You have to deal with things you cannot prevent!

Cheap hacks 1. Lazy empty clauses try {System.in.read()} catch (IOException e){} 2. Punt the exception to the caller public static void main(String[] args) throws IOException

Alternate version I import java.io.*; public class ReadingDoubles3{ public static void main (String[] args) throws IOException { InputStreamReader reader = new InputStreamReader(System.in); BufferedReader console = new BufferedReader(reader); //get the first value System.out.print("Please enter a floating point number : "); String input = console.readLine(); //read a String Double d = new Double(input); double d1 = d.doubleValue();

Alternate Version II // Calculate the sum & printout System.out.println(d1 + "^2 == " + (d1*d1)); // wait for user to end System.out.println(" Hit return to exit"); System.in.read(); }

Throwing your own exeptions public static double myfun(double ValueBetweenZeroAndOne) { if ((ValueBetweenZeroAndOne < 0) || ( ValueBetweenZeroAndOne > 1)) throw new IllegalArgumentException("RTFM - you dolt!"); return ValueBetweenZeroAndOne * ValueBetweenZeroAndOne; }

Making them deal with it public static double myfun(double ValBetZeroAndOne) throws Exception { // now, they have to write a handler if ((ValBetZeroAndOne 1)) throw new IllegalArgumentException("RTFM – you dolt!"); return ValBetZeroAndOne * ValBetZeroAndOne; }

Defining your own exceptions public class DivideByZeroException extends RuntimeException { public DivideByZeroException() { super(“Attempt to divide by Zero”); } public DivideByZeroException( String msg) { super(msg); }

Defining your own checked exceptions public class myCheckException extends Exception { public myCheckException() { super(“Attempt to divide by Zero”); } public myCheckException( String msg) { super(msg); }

Finally try { statements; } catch ( ExceptionClass ExceptionObject ) { statements; //exception specific } finally { statements; //clean up }

Finally The Finally block executes regardless of what happens in the try/catch Good for cleaning up resources Good for conditions where the catch blocks may throw exceptions.