Transcript 26-May-05 PowerPointPresentation

CIS3931 - Intro to JAVA
Lecture Notes Set 5
26-May-05
Method Overloading
• Enables methods to have the same name
• They must have different parameter lists
• The parameter list allows the
compiler/interpreter to distinguish between
which to use.
Method Overloading Example
• int MyMethod(int x)
• int MyMethod(double y)
• int MyMethod(int a, double b)
• Since they all have different parameter
lists, they can all be used in the same
class and an error will not be raised.
Is this correct?
• int MyMethod(int x)
• double MyMethod(int a)
• Are these two methods allowed together?
Math Class
• Very useful. Provides many methods in a
pre-built class.
• The following are available in the Math
class
• Trigonometry functions:
sin, cos, tan, acos, atan, asin
Exponent Methods
•
•
•
•
exp – raise e to a power
sqrt – returns the square root
pow – raise a number to a power
log – natural log of a number
Rounding in Math Class
• ceil – round up to nearest integer
• floor – round down to nearest integer
Helpful Math
• random – Returns a random number
greater than or equal to 0.0 and less than
1.0
• abs – return absolute value of a number
• min – return minimum of two numbers
• max – return max of two numbers
Advice
• The previous were all important, but are
only half of what the Math class can
actually do. Make sure you know how to
use the methods I mentioned here. How to
call them, and what they do. I would
recommend playing around with the Math
class and experimenting with at least the
ones mentioned. View the API for the Math
class.
Exception Handling
• What is an exception?
– An exception is an event that occurs during
the execution of a program that disrupts the
normal flow of instructions during the
execution of a program.
• In normal programming, an exception or
error would cause a program to crash.
Exception Handling
• JAVA offers methods to handle exceptions
• Exception objects – Created by a method
when an error occurs. This object contains
information about the error (type, state of
program when error occurred …)
• Creating an exception object = throwing an
exception
Exception Handling
• When an exception is thrown, the system
trying to find a way to handle it.
• The system searches through the ordered
list of methods that have been called prior
to the exception (call stack)
Exception Handling
• Block of code than can handle exception =
exception handler
• An exception handler must be able to
handle the type of exception thrown
• A chose exception handler “catches” the
exception
• If no handler is found, the program will
terminate.
Exceptions
• Some reasons an exception may be
thrown …
– Unable to open a file
– Array is out of range
– Dividing by zero
– Unable to parse a stream
Exceptions : Throw / Catch
• Similar to the raise/handle model in other
languages
• Exceptions are caused in one of two ways
– Program does something illegal
– Program execute the “throw” keyword
Exceptions : Throw / Catch
• The following code will crash a program …
public class DivideByZero
{
public static void main(String[] args)
{
int i = 1, j =0, k;
//Force an error …
k = i / j;
}
}
Exceptions : Throw / Catch
• Result :
Exception in thread “main”
java.lang.ArithmeticException: / by zero
at DivideByZero.main(DivideByZero.java:14)
Exceptions : Throw / Catch
• Error message shows that a
“ArithmeticException” was thrown by the
program
• Program died because there was no
exception handler available to catch the
exception
• Use a try / catch block to handle the
exception
Exceptions : Throw / Catch
Try / Catch Block example
try
{
//Some block of code that you could potentially
//throw an exception
}
catch (Exception e)
{
//Block of code to handle the exception
}
Exceptions : Throw / Catch
• When an exception is thrown in the try
block, control is immediately passed to the
associated catch block.
• Try blocks must have at least one catch
block (sometimes multiple are needed).
• Every catch block must have one try block
associated with it.
Throw / Catch : Advantages
• Error conditions dealt with only where it makes
sense to deal with them
– Don’t have to deal with error at ever level between
occurrence and where it is handled
• Code can be written as if everything will work
– Separates error handling from the normal flow of
control
• Reduced program complexity
– Calling functions do not need to error check returned
values
Exception handling …
• Example program …
• “Finally” statements
• Users can create user defined exceptions
…
• … will go over this next class.
Scanner
• Good news, Scanner can now be used on
program since 1.5 was added. This can be
used instead of Buffered Readers.
Scanner
• Scanner is contained in java.util.Scanner
• You will have to import that class if you
wish to use Scanner.
import java.util.Scanner;
Scanner
• You must first create Scanner to obtain
input from command window.
• This is done with the following command:
Scanner input = new Scanner(System.in);
Scanner
• Next, you must parse the input into
something meaningful, as it is now of type
Scanner. The example below shows how
to parse it into an int
int num1 = input.nextInt();
Scanner Example
import java.util.Scanner;
//Class and main declarations
Scanner input = new Scanner( System.in );
System.out.print(“Enter digit: “);
int num1 = input.nextInt();
Scanner Exceptions
• With Scanner, you do not need to throw
any Exceptions as you do with Buffered
Readers. In other words, you do not need
the throws Exception after declaring main.
Strings
• You have already played around with
Strings somewhat…
String input = br.readLine();
The above says to create a String variable
called input and place the input from the
Buffered Reader into it.
Scanner & Strings
• To grab a String using Scanner, just use
the following:
Scanner input = new Scanner(System.in);
String name = input.nextLine();
Strings overview
• A String is an object in Java, it comes from
the String class
• The String class is part of java.lang and
doesn’t need to be imported since
java.lang is imported automatically with
every java program.
String Methods
• There are over 50 different methods which
can be called when you use the String
class. Some of the more important ones
follow.
charAt()
• This returns the char at the location you
specify.
String greet = new String(“Hello”);
greet.charAt(1); //returns e
Remember, you must start counting at 0!
concat()
• This concatenates one string to another
string.
• Example:
String st1 = “Hello”;
String st2 = “ World”;
String st3 = st1.concat(st2);
//st3 now equals “Hello World”
equals()
• Returns a boolean value – true or false
• Check to see if 2 strings are equal
if( st1.equals(st2) )
System.out.println(“Equal”);
equals()
• The equals() method takes case into
consideration. To ignore case, yet still
check for equals you should use:
equalsIgnoreCase()
length()
• Returns the length of the String
• Example:
String st1 = “Hello”
int Stlen = st1.length();
//Stlen now equals 5
trim()
• Removes all leading and trailing
whitespace characters. This does not
remove any whitespace characters in the
middle, just at either end.
Conversions
• toLowerCase()
• toUpperCase()
These convert the String to upper/lower
case letters and ignore all the
nonalphabetic characters.
Main
• Recall the following:
public static void main( String args[] )
The String args[] that is in the parameters is
an array of Strings. This is so the user can
enter input to start the program.
Main
• For instance, if the program was started
with:
java Project foo bar
Then args[0] = foo & args[1] = bar
args.length = 2
Objects
• Encapsulation of Data along with functions
that act upon that data
• An object consists of 3 things:
1. Name – which we give it
2. Attributes – set of data fields and their
current values
3. Methods – set of methods that define
the behavior of the object
Class
• Blueprint for objects
• Describes and defines objects of the same
type
• Contains data and method declarations
• An object is a single instance of a class
• You can create many objects from the
same class type
Creating Objects
• Objects are created from a class using the
new operator.
• They must be attached to a reference
variable.
Example
• Suppose we have a class we built called
Square. Inside we have methods for
computing area and circumference.
• Format:
Square MySquare;
My Square = new Square();
Or
Square MySquare = new Square();
Using the object
• Square MySquare = new Square();
• MySquare.side = 10;
• The above sets side to 10.
Constructors
• Special member function of a class.
• Purpose is to initialize the members of an
object.
How to spot a constructor?
• It’s very simple since it must meet the
following criteria:
1. It has the same name as the class
2. It has no return type.
Default Constructor
• A constructor without any parameters is
the default constructor.
• A constructor can have parameters, but no
return type.
• A constructor is invoked when an object is
created with new.
MySquare = new Square() //default
MySquare = new Square(2) //also valid
Visibility
• We can declare members of a class to be
public or private.
• Public can be accessed inside or outside
of the class it is in.
• Private can only be used by the object
itself.
• You can hide data this way for security,
simpler interface, etc…