Lecture 27
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Log into Windows/ACENET. Start MS VS and
open the PointClassDemo project.
Reminder: Project 1 is due on Wednesday.
Questions?
Monday, March 21
CS 205 Programming for the Sciences - Lecture 27
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Outline
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Classes – Chapter 4
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Instance methods
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Static methods
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Properties
Exceptions and exception handling
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Static Methods
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The methods we wrote for our console
programs used the static modifier. This is
used when the method is to be called without a
class object. Since it is defined in a class, it is
called with the class name. E.g., Math.Sqrt( )
One important static method is Parse( ), which
is used when a user is expected to enter values
of the class from the keyboard. For example,
we have used double.Parse( ) and int.Parse( )
to interpret user input strings into double and int
values, respectively.
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Parse( ) Method
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A Parse( ) method basically ignores any
punctuation in the user entered string and
converts the rest into attribute values.
For the Point class, we want to parse strings of
the form "(x,y)", where x and y are numbers.
This involves find the substrings between the
punctuation and convert them into the
coordinate values and construct a Point object
with them.
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Parse( ) Method
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For example, for (12.5,-3.14)
find these substrings and convert to numbers for x and y
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Two string methods are useful for doing this
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FindIndexOf( ) receives a character to search for
and returns the index of the first occurrence of the
character if it is present, or -1 if it is not.
Substring( ) receives the starting index of a
substring and the number of characters in the
substring. It returns a (new) substring consisting of
the character at the starting index and the next
number of characters or to the end of the string,
whichever is reached first.
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Parse( ) Method
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For now, we will assume that the user enters a
well-formed string, i.e., '(' is the first character,
')' is the last character, and there is a ',' in the
middle of the string between two substrings that
are valid numbers. Here is the basic algorithm:
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Find the index of the ',' using FindIndexOf( )
Compute the start index and length of the two
substrings
Use Substring( ) to get the substrings
Use double.Parse( ) to convert them into numbers
Create and return a Point with these coordinates
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Parse( ) Method
static Point Parse (string input)
{
int commaIndex = input.FindIndexOf(',');
int xStartIndex = 1;
int xLength = commaIndex – 1;
int yStartIndex = commaIndex + 1;
int yLength = input.Length – commaIndex – 2;
string xString = input.Substring(xStartIndex, xLength);
string yString = input.Substring(yStartIndex, yLength);
double newX = double.Parse(xString);
double newY = double.Parse(yString);
return new Point(newX, newY); // create a Point
}
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Testing Parse( )
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To test the Parse( ) methods, we will write
handlers for the Enter buttons.
Double-click on the left Enter button to go to the
button handler this button. This handler does
the following:
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Use Point.Parse( ) to parse txtPoint1.Text and
assign the result to variable p1
Set the Text property of lblPoint1 to p1.ToString( )
Clear txtPoint1.Text
Add an item to lbxResults.Items indicating the result
of the handler (this is already completed)
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Testing Parse( )
private void btnEnterP1_Click(object sender,
EventArgs e)
{
p1 = Point.Parse(txtPoint1.Text);
lblPoint1.Text = p1.ToString();
txtPoint1.Text = "";
lbxResults.Items.Add
("Entered p1: " + p1.ToString());
}
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Testing Parse( )
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Build and run your program. You should be
able to input a Point in "(x,y)" form in the left
textbox, click on the left Enter button, and see
the label above the textbox change to the input
and the textbox become empty, along with a
message in the results area.
Repeat these steps for right Enter button
handler. This handler is exactly the same,
except that is uses p2, txtPoint2, and lblPoint2.
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Magnitude( ) Method
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The magnitude of a Point (x,y) is its distance
from the origin (0,0) computed using the
formula:
distance=  x  y
2
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2
The Point class instance method Magnitude( )
returns the magnitude its Point object.
Write the method definition for Magnitude in the
file Point.cs.
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Testing Magnitude( )
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Double-click on the Magnitude button to get a
stub for its handler.
Write code to compute the magnitude of Points
p1 and p2, and display these results in the
results area ListBox.
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Getters and Setters
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As noted last time, we make the attributes
private so that the class can control access to
them, preventing applications from creating
invalid objects.
However, often an application would like to
read or change an attribute. To support such
actions a class can provide a getter method
that returns the attribute and a setter method
that receives and assigns a new value to the
attribute.
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Getters and Setters
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E.g., to allow an application to get and set the
x-coordinate of a Point, we could write instance
methods:
// Getter
double GetX ()
{
return x;
}
// Setter
void SetX(double newX)
{
x = newX;
}
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Getters and Setters
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To use these methods, we call them as usual.
E.g. for Point object p1:
// In btnGetX_Click
lbxResults.Items.Add("p1's x­coordinate is: " +
p1.GetX().ToString());
// In btnSetX_Click
// Get new coordinate from input textbox, if exists
if (txtPoint1.Text != "")
{
double newX = double.Parse(txtInput.Text);
p1.SetX(newX);
txtPoint1.Text = "";
lblPoint1 = p1.ToString();
}
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Properties
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While getter and setter methods work, they are
tedious write and use, especially needing to
type the ( )'s everywhere. It would be more
convenient for the application programmer if the
syntax for this use could be as if the attribute
were a public data member.
E.g., for the Point class, we might like to be
able to write:
lbxResults.Items.Add("p1's x­coordinate is: " + p1.X)
p1.X = newX;
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Properties
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C# has a special method definition that allows
us to define getters and setters that are called
using this syntax called a property.
By convention, a property's name is the same
as the attribute it accesses only capitalized.
(This assumes the convention that the attribute
variables start with a lowercase letter.) E.g.,
Point attribute x is accessed using property X.
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Properties
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The getter and setter for a property are defined
together. Either is optional. (E.g., you might
want to allow applications to read an attribute,
but not change it.)
public double X // name of property
{
get // getter definition
{
return x;
}
set // setter definition
{
x = value; // value is an implicit argument } // from right side of assignment
}
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Properties
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Write a property with getters and setters for the
y-coordinate attribute in Point.cs.
Properties are accessed as shown previously.
In particular, there are no ( )'s to call them.
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Properties
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Complete the GetX, GetY, SetX, and SetY
button handlers for PointClassDemo.cs.
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GetX is to display the x-coordinate of Point objects
p1 and p2 in the results area using the X property
GetY is to do the same for the y-coordinates of
each point.
SetX is to convert the input strings in each textbox
(if they are not empty strings) into double values
and set the x-coordinate using the X property for
each point. SetY is to do the same for the ycoordinates of each point.
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Exception Handling
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So far we have ignored what happens when a
user types in input that is ill-formed. In the
absence of any error handling, a program
usually will abort with an unhandled
exception.
An exception is an object that is thrown by a
method when an error occurs. To handle an
exception, it is caught by the code that called
the method. For GUI programs, the usual
action is to inform the user and go back to
waiting for input.
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Exception Handling
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When an exception is thrown, if the calling
method does not catch it, the caller is
immediately aborted and the exception is
rethrown.
For example, most Parse( ) methods will throw
a FormatException if the string being parsed is
not well-formed. In the Point.Parse( ) method,
we call double.Parse( ), but we can ignore any
exceptions that it throws and they will be
passed on to Point.Parse( )'s caller.
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Exception Handling
unhandled exception dialog
incorrect input
name of exception
under "Details"; also
a backtrace of calls
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Exception Handling
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Sometimes a caller wants to do something
when an error occurs, such as inform the user
that an error has happened and tell them what
to do.
For example, in the handlers for the Enter,
SetX, and SetY buttons we would like to inform
the user of the error via a MessageBox and
then go back to waiting for user input.
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Try-Catch Statement
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Exceptions are handled by putting calls in a trycatch statement. This statement has the
following syntax:
try
{
// method calls that may throw an exception
// there may be more than one such call
}
catch (<exception type> ex)
{
// do something about the error
}
// more than one catch block can be attached
// the first one that matches is executed
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Try-Catch Statement
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In btnEnterP1_Click, we can catch any
FormatExceptions as follows:
try
{ p1 = Point.Parse(txtPoint1.Text);
}
catch (FormatException ex)
{
MessageBox.Show ("Input is invalid. Must be (#,#).",
"Point Class Demo");
txtPoint1.Text = ""; // clear textbox
return; // go back to waiting
}
// rest of handler goes here
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Exception Handling
incorrect input
MessageBox shown when
FormatException is thrown
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Throwing Exceptions
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Our Point.Parse( ) method now throws a
FormatException when the numeric portions of
the input are incorrect. But there are other
possible input errors:
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The first character must be '(' and the last character
must be ')'. Currently, any character can be in
those places.
There must be a ',' in the middle. If not, the code
that computes the two number substrings will be
incorrect.
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Throwing Exceptions
incorrect input
the second substring
has computed length < 0,
which is not allowed
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Throwing Exceptions
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The exception thrown by the Substring method
is not meaningful to Point.Parse( ) callers. We
want a FormatException thrown instead.
To throw an exception, we use the throw
operator with a new exception object as shown:
int commaIndex = input.FindIndexOf(',');
if ((input[0] != '(') || (input[input.Length ­ 1] != ')') ||
(commaIndex == ­1))
throw new FormatException("Invalid Point");
// rest of Parse method code goes here
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