CS 46B - Lecture 5

Cay S. Horstmann

Pre-class reading

• Sections 11.1 - 11.3

Text I/O

• Use PrintWriter for output, Scanner for input
• Construct with File parameter
  • new PrintWriter(new File("output.txt"))
  • new Scanner(new File("input.txt"), "UTF-8")
  • Be sure to specify the character encoding—the book doesn't mention it
• Can throw exception—add throws FileNotFoundException to method
• Choose between
  • number: in.nextInt()/in.nextDouble()
  • line: in.nextLine()
  • word: in.next()
  • character: (rare, call in.useDelimiter(""), then in.next())

Mixing next and nextLine

• Input file

  1729
  Harry
  1730
  Wilma

• Initially, the input contains
  
• Call

  int studentID = in.nextInt(); 

• Now the input contains
  
• If you call nextLine, you don't read Harry reads an empty string!
• Remedy: a call to nextLine after reading the ID:

  int studentID = in.nextInt(); 
  in.nextLine(); // Consume the \n 
  String name = in.nextLine();

Lecture 5 Clicker Question 1

Suppose the input contains the characters 6,995.00 12. What is the value of price and quantity after these statements?

double price = in.nextDouble(); 
int quantity = in.nextInt();

1. price is 6995 and quantity is 12
2. price is "6,995.00" and quantity is 12
3. price is 6, then an exception is thrown because the comma is not a valid part of an integer
4. an exception is thrown because a comma is not a valid part of a floating-point number

Text Input Tricks

• String methods trim, indexOf, lastIndexOf
• Character predicates isDigit, isWhitespace
• Read a line at a time, then make another scanner for each line

  String line = in.nextLine(); // Reads line such as United States 303824646
  Scanner lineIn = new Scanner(line);
  String country = lineIn.next();
  while (!in.hasNextDouble()) country = country + " " + lineIn.next(); 
  double area = lineIn.nextDouble(); 

• Or just use Integer.parseInt/Double.parseDouble (but remember to trim!)
• Nonstandard delimiters: in.useDelimiter("[^A-Za-z]+")

Lecture 5 Clicker Question 2

An input file contains lines such as

Fred 40
Wilma 28
Mary Ann 30

Consider this code  to process a line:

String line = in.nextLine();
int n = line.lastIndexOf(' ');
String name = line.substring(0, n);
int age = Integer.parseInt(line.substring(n));

What is the problem with this code?

• The name includes the trailing space
• The call to Integer.parseInt throws an exception

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 5 Clicker Question 3

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

Lab

• Continue Lab 2
• You will work with a buddy
• Buddy list is on schedule page
• No buddy? Come to the front of the room
• One of you (driver) fusses with the code, the other (scribe) types up answers
• Switch roles each class meeting
• Submit lab work to Desire2Learn