CS 46B - Lecture 6

Pre-class reading

• How To 11.1, Worked Example 11.1, Sections 11.4 - 11.5

If You Can't Do it By Hand...

• If you can't do it by hand...
• ...you can't program it.
• Don't even try.
• Or you'll waste your time.
• And whatever you do, don't do this.
• Instead
  • Make sample inputs
  • Trace through them by hand
  • Write pseudocode
• New Piazza rule: You can only post pseudocode

HOWTO 11.1 - Processing Text Files

• Understand the task
• Write pseudocode!
• File names
  • Hard-coded
  • Ask the user
  • Command-line
• Read lines, words, or characters?
• Extract information from lines
  • Convert strings to numbers
  • Pay attention to number formats (dollars, commas)
  • Or use secondary scanner and nextInt/nextDouble
• Use classes and methods to factor out common tasks
  • Example: AikenReader in Homework 4A

Lecture 6 Clicker Question 1

Have a look at Homework 4C.

We need to read two files with the same format.

That's an obvious case for a method that reads a file and returns the data in the file (boy and girl names).

What should it return?

1. An array ArrayList<String>[] of length 2, holding a list of boy names and a list of girl names
2. An array String[][] of String[] arrays of length 2, holding an array of boy names and an array of girl names
3. An array list ArrayList<ArrayList<String>> of length 2, holding a list of boy names and a list of girl names
4. An ArrayList<Person>, where Person is a class whose instances have a name and a gender

Throwing Exceptions

• Exception handling: transfer control from error location to competent handler
• When you detect an error, just throw an exception

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

Hierarchy of Exception Classes

Lecture 6 Clicker Question 2

public static Comparable smallest(Comparable[] values)
{
   if (values.length == 0) throw new XXXException(); // What should we throw?
   Comparable smallestSoFar = values[0];
   for (int i = 1; i < values.length; i++)
      if (values[i].compareTo(smallestSoFar) < 0)
         smallestSoFar = values[i];
   return smallestSoFar;
}   

1. ArrayIndexOutOfBoundsException
2. IllegalArgumentException
3. IllegalStateException
4. NoSuchElementException

Checked Exceptions

• new Scanner(file) can throw FileNotFoundException
• Checked exception—can't ignore
• IO exceptions are checked since they are beyond the control of the programmer
• Use throws to alert the caller

  public static void process(String filename) throws FileNotFoundException
  
  public static void main(String[] args) throws FileNotFoundException

Lecture 6 Clicker Question 3

When you call a[i] on an array, your code might throw an ArrayIndexOutOfBoundsException. Why don't you have to tag methods with throws ArrayIndexOutOfBoundsException?

• Because ArrayIndexOutOfBoundsException is a checked exception
• Because the ArrayIndexOutOfBoundsException is checked at compile-time
• Because you could have checked whether i is valid
• Because the code could have also thrown a NullPointerException (if a is null)

Catching Exceptions

try
{
   String filename = . . .;
   FileReader reader = new FileReader(filename);
   Scanner in = new Scanner(reader);
   String input = in.next();
   int value = Integer.parseInt(input);
   . . .
}
catch (IOException exception)
{
   exception.printStackTrace();
}
catch (NumberFormatException exception)
{
   System.out.println("Input was not a number");
}

Exceptions and Inheritance

• Can use superclass in throws, catch

Lecture 6 Clicker Question 4

try 
{
   process(filename)
}
catch (FileNotFoundException ex) { println("File not found"); }
catch (IOException ex) { println("I/O error"); }
catch (RuntimeException ex) { println("Something went wrong"); }

Suppose process throws a EOFException. What gets printed?

The finally Clause

• Exception terminates current method
• Danger: Can skip over essential code
• Example:
  

  reader = new FileReader(filename);
  Scanner in = new Scanner(reader);
  readData(in);
  reader.close(); // May never get here

• Must execute reader.close() even if exception happens
• Use finally clause for code that must be executed no matter what

The finally Clause

Scanner in = new Scanner(file);
try
{
   readData(in);
}
finally {
   in.close(); 
}

• Executed when try block is exited in any of three ways:
  1. After last statement of try block
  2. After last statement of catch clause, if this try block caught an exception
  3. When an exception was thrown in try block and not caught

Lecture 6 Clicker Question 5

public void read(String filename) 
{
   Scanner in = null; 
   try
   {
      in = new Scanner(new File(filename));
      process(in);
   }
   catch (IOException ex) { println("I/O Error"); }
   finally { in.close(); }
}

Which of the following is true if the file does not exist?

1. The message "I/O error" is printed
2. A FileNotFoundException is thrown to the caller of read
3. A NullPointerException is thrown to the caller of read
4. The read method returns without printing anything and without an exception

The Try-With-Resources Statement

• try (Scanner in = new Scanner(file))
  {   
     readData(in);
  } // in.close() called automatically—Scanner is AutoCloseable

• Special Topic 11.6
• The scope of in is the inside of the {...}
• Can define more than one variable

  try (Scanner in = new Scanner(...); PrintWriter out = new PrintWriter(...))

• Use it!
  • In each of Homework 4A, 4B, 4C Final
  • Always

Designing with Exceptions

• There is no shame in throwing an exception
• If something isn't the way it should be, it's better to abort than to continue
  • Pick an appropriate exception class
  • Maybe a custom class
• Don't catch an exception unless you know what to do with it
• Throw early, catch late!
• Example: Homework 4B Final
  • AikenReader throws exception when there is a problem
  • AikenReaderDemo catches it, displays, and terminates cleanly
  • In another application, it might ask the user to intervene

Before Next Class

• Read Sections 13.1 - 13.2, How To 13.1
• Take the quiz