Lab Week 06: Exceptions

Plan

1. Exceptions
2. Input Stream
3. Starting Lab 04: Exceptions

Exceptions

A Problem You Brought Up

    public class Fraction {
        private int num;
        private int den;
        
        Fraction (int num, int den) {
            this.num = num;
            this.den = den;
            reduce();
        }
    }

What stops us from calling new Fraction(1, 0)?

Nothing Stops Us!

Unique problem:

• Nothing formally wrong with new Fraction(1, 0)
  • valid Java syntax, no error
• Problem is with semantics
  • $1 / 0$ is not a well defined fraction
  • not a programming error, but a logical error

Enter Exceptions

Exceptions give a way of:

1. Signaling that a method could introduce undesireable behavior
2. Detecting that undesireable behavior occurred
3. Handling the undesireable behavior
   • make the method caller deal with it!

Declaring Methods that Throw Exceptions

        Fraction (int num, int den) throws ArithmeticException {
            if (den == 0) {
                throw new ArithmeticException("division by zero");
            }
            this.num = num;
            this.den = den;
            reduce();
        }

When throw ... is executed, method call halts

• like return statement, but nothing returned; no Fraction created

What happens

• Now new Fraction(1, 0) throws the exception
• This is good because we see that there was an error

Catching Exceptions

When an exception is thrown it can be caught

• Catching exceptions allows us to handle bad behavior without crashing the program

Fraction f;
try {
    // some code that could throw an exception
    f = newFraction(getNum(), getDen());
} catch (ArithmeticException e) {
    // code to be executed if exception is thrown
    System.out.println(e);
    System.out.println("Using default value of 0/1");
    f = newFraction(0, 1);
}

• This is a try-catch block

Input Streams

How Does Your Computer Read (Keyboard) Input?

• 1 character at a time
• With each keystroke, it decides what to do next
  • print a letter on the screen?
  • wait for another keystroke (e.g., shift key)?

Accessing Individual Characters

• Use InputStream in
  • in particular System.in
• in.read() returns one character at a time from terminal input
  • returns an int
  • 0 through 255 represents char
  • -1 represents end of stream
  • throws IOException if something goes wrong

Lab 04: Exceptions

Terminal Input

• Just a sequence (stream) of characters
• Sometimes might represent an integer:
  • 123, -23 represent numbers
  • Mammoth, 12a3 don’t represent numbers

Your Task

Write a program that

1. Parses integers from input stream
   • stream may contain many values separated by whitespace
   • e.g., 1 yes 2 no -321
2. Returns values of valid integers as int
3. Throws an exception if value is not an integer

Program Demo

How?

• Use in.read() to get individual characters
• Each time a character is read, decide what to do
  • update int value?
  • return value?
  • throw exception?
  • ???

Suggestion

Implement solution with no exception handling first

• Assume all input consists of valid numbers
• Once this solution works, modify it to handle exceptions