Download Advanced Java - Systems and Computer Engineering

94.204* Object-Oriented Software
Development
Unit 4(b)
Advanced Java
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
1
Advanced Java
• There are a few areas in Java that are
either completely new or that merit a
good review due to their complexity or
importance.
– Strings
– Class Methods and Variables
– Visibility, Packages and Directory
Structures
– Exceptions
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Java Strings
• A string is a contiguous sequence of
Unicode char
– Like arrays, the compiler gives special
support for strings to convert string
literals ("This is a literal string") into
String objects
– There is no primitive type for a string
• Recall, all primitive types fit into a
computer word
Copyright © 2002, Systems and Computer Engineering,
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Java Strings
• Two classes support strings in Java
– String class
• Supports immutable string objects
• This means that the string value
cannot change in size or content
once instantiated
– StringBuffer class
• Supports a string with content and
size that can change (e.g. insert,
append)
• When do you use String vs. StringBuffer?
– When one of the sets of methods is
more appropriate for you than the other
– When one behaviour is more efficient
for you than the other
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String Constructors
• String ()
– Create a new String with value ""
• String (String value)
– Creates new String that is a copy of the
String object value
• The copy has a different reference
• StringBuffer () and
StringBuffer (String value)
– Behave the same as String constructors
String myString = new String
(“Hello");
StringBuffer myString =
new StringBuffer (“ World”);
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String Methods
• Many methods are defined for strings
– length () returns the length of the string
– charAt (int position) returns the Java char
at the given position (from 0)
– There are many more string methods
• While entering your program, you can use
JBuilder to browse the methods associated
with a class
– e.g. highlight the word StringBuffer in a
source file. Right click over the highlighted
name and select Browse Symbol at Cursor
• The source code for the StringBuffer class
will be brought up
• Click on Doc in source browser window to
see extensive documentation on the class
• To return to editing your source files, click
on the
above the project browser
window
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Using String Variables
• Print a concatenated string – a list of
strings joined together
System.out.println
( "My string is " +
myString +
“ and has length " +
myString.length() +
“ bytes");
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Using String Variables
• Count the number of occurrences of each
char
String str = new String
("Here is a string");
int [] counts = new int
[numOfUnicodeChars];
for (int i=0;
i <str.length();
i++)
{
counts[str.charAt( i )]++;
}
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Comparing Strings
• Two strings are equal if both have the same
length and the same Unicode characters.
• The == operator cannot be used for
comparing strings.
• You must use the equals method
public boolean equals
(Object anObject);
– Performs a deep comparison of objects
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Comparing Strings
• Class Exercise : For each statement below,
say whether it returns True or False.
Explain.
String s = new String
("Hello there");
String t = “Hello there";
String u = new String (t);
a) s.equals( t ))
b) s == t
c) t == u
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Class Methods and Variables
• The variables and methods that we have
looked at so far (with the exception of
main() methods) are often referred to as
instance variables and instance methods
– each object (an instance of a class) has
its own copy of the instance variables
– instance methods (defined in the class)
are invoked by sending messages to the
object
• e.g. anObject.msg ( data);
Copyright © 2002, Systems and Computer Engineering,
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Class Methods and Class Variables
• We can also define methods and variables
with the static modifier
• These are often called class methods and
class variables
• Class methods & variables are associated
with the class itself, not with the instances
of the class
• Class methods & variables may be used
before any instances of the class have been
created (as well as after) .
• Class methods & variables are usually
accessed via the class’s name, not an object
instance
eg. ClassName.classMethod( arg );
eg. x = ClassName.classVariable
Class methods can also be accessed via an
instance
eg. objectRef.classMethod( arg );
Copyright © 2002, Systems and Computer Engineering,
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Class Methods
• Class methods can only access static
members of the class
– It belongs to the class, so it can’t “see”
any of the instances (and their variables
or methods)
• Class methods are useful for :
– Work independent of any instances
– Helping in creating instances of the
class
– Calculations of the form y = f(x), where
x has a primitive type
• because x does not have class type
(i.e., it is not an object) it does not
have methods bound to it
• Many examples in java’s Math class
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Example: Using the Math Class
• Example: add a method to class Complex
that calculates the magnitude of a complex
number
public double magnitude()
{
return Math.sqrt(
Math.pow(real, 2) +
Math.pow(imag, 2))
}
– sqrt(x) returns the square root of x
– pow(x, y) returns x raised to the
power y
Copyright © 2002, Systems and Computer Engineering,
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Example: Using the Math Class
• Notice the invocation of sqrt() in
magnitude()
• sqrt() is a static method in class
java.lang.Math
• Its argument is a double and it returns a
double
• sqrt() is invoked by sending the
sqrt() message to its class; e.g.,
y = Math.sqrt(x);
• Notice that we are not sending the
sqrt() message to a Math object
• pow() is also a static method in class
Math
Copyright © 2002, Systems and Computer Engineering,
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Class Variables
• There is one copy of each class variable,
regardless of the number of instances of
the class that are created
• Class variables are allocated usually at
program startup or when the class is first
used
– before any objects of the class are
created !
Copyright © 2002, Systems and Computer Engineering,
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Class Variables
• Class variables are useful for :
– Constants (final class variables)
public static final double
PI = 3.1415;
– “Global data” or more specifically, data
that is shared among all instances of the
class
eg. State variables, counters (next
page)
Copyright © 2002, Systems and Computer Engineering,
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Example: Counting Complex Objects
• Add the following code to class Complex
private static int count = 0;
private static void incrementCount()
{
count++;
}
public static int numberCreated()
{
return count;
}
public Complex()
{
…
Complex.incrementCount();
}
Class exercise : What does it do ?
Copyright © 2002, Systems and Computer Engineering,
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Example: Counting Complex Objects
Recall : Class methods can be invoked by
sending messages either to the class or to
an object of the class.
int nc;
nc = Complex.numberCreated();
or :
Complex c = new Complex();
nc = c.numberCreated();
But, an instance method can only be invoked
by sending a message to an object:
instanceName.methodname();
Copyright © 2002, Systems and Computer Engineering,
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Why is main() Static?
• When we execute a Java program: java
SomeClass
– no instances of SomeClass exist, so
the Java interpreter can't send a
message to a SomeClass object
– main() is static, so it can be invoked
before any SomeClass objects have
been created
– therefore, the Java interpreter invokes
SomeClass.main()
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Static Initializers
• A static initializer is a statement sequence
that is executed when a class is first loaded
• Used to initialize class variables and
perform other initialization for the class
class TV
{
// Array of channel numbers
private static int [] channels = new int [nChan];
// Static initializer
static
{
// For all channels allocate a channel number
for (int i = 0; i < nChan; i++)
{
channels [i] = TunerClass.getChanNum(i);
}
}
} © 2002, Systems and Computer Engineering,
Copyright
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Carleton University. 94.204-04b-JavaAdvanced.ppt
Class Methods & Variables :
Summary
• Why does Java support class methods and
variables?
– sometimes, we think of a class as a
factory that produces objects
• class variables represent the state of
the factory (not the state of the
objects produced by the factory)
• class methods implement the
behaviours of the factory
– to perform computation that involves
values of primitive types (not objects)
• e.g., java.lang.Math contains
methods for performing floating
point trigonometry, calculation of
absolute values, exponentiation, etc.
Copyright © 2002, Systems and Computer Engineering,
Carleton University. 94.204-04b-JavaAdvanced.ppt
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Class Methods & Variables:
Test Your Understanding
• For 5 marks, explain why the following
will not compile :
public class WontWork
{
private int ns;
// Instance variable
public static void fns ()
{
ns = 0;
}
}
Copyright © 2002, Systems and Computer Engineering,
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Visibility, Packages and Directories
• We’ve already seen that variables, methods and
classes have access modifiers that limit their
visibility to objects of other classes.
public class Complex
{
private double real;
private double imag;
public Complex()
{…}
public Complex plus
(Complex c)
{
return new Complex
(real + c.real,
imag + c.imag);
}
}
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A Note About Instance Variable
Visibility
• Notice that plus() directly accesses the
real and imag variables
– of the Complex object that receives
the message
– of the Complex object referenced by
parameter c
• Students sometimes think that a method
can access the private variables of the
object that received the message, but
cannot access the private variables of other
instances of the same class
• This is not true
– A method defined in one class can
access the private variables of any
instance of the same class
– A method defined in one class cannot
access the private variables of objects
that are instances of other classes
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Visibility, Packages and Directories
• Java has another mechanism for
controlling the scope and visibility of
variables, methods and classes : Packages.
Copyright © 2002, Systems and Computer Engineering,
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Java Packages
• A package represents a loose affiliation
of classes (and interfaces - later..)
• A package provides a name and scope
for the group of classes
• Many classes can be part of the same
package
• We could invent a package houseWares,
containing classes such as: TV, Couch,
Lamp and Fridge
– These classes would typically all be
stored in different files
TV
Couch
Lamp
Fridge
package houseWares
Copyright © 2002, Systems and Computer Engineering,
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Declaring Packages
• Classes in a source file become part of a
given package by using the package
statement before any classes are declared
In file TV.java
// Claim membership in the houseWares package
package houseWares;
public class TV {…}
In file Couch.java
// Claim membership in the houseWares package
package houseWares;
public class Couch {…}
• All classes in that source file become
members of the specified package
Copyright © 2002, Systems and Computer Engineering,
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Declaring Packages
• If the package statement is not used, then
all the classes in that file become members
of Java's default anonymous package
– All such classes are visible to each
other
• A class can be a member of only 1 package
Copyright © 2002, Systems and Computer Engineering,
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Packages and Visibility
• By default, a class is friendly (visible) to
other classes in the same package (unless
the class is nested in an enclosing class)
• To gain visibility to the public classes
stored with another package, you must
import the class (or its entire package)
• (An) import statement(s) must be
declared after the package statement, and
before any classes are declared
Copyright © 2002, Systems and Computer Engineering,
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Packages and Visibility
// Gain visibility to just the TV class
import houseWares.TV;
public class TvUser
{
TV myTv = new TV ();
…
}
OR
// Import all classes in houseWares
import houseWares.*;
public class HouseUser
{
Lamp myLamp = new Lamp( );
TV myTV = new TV( );
…
}
Copyright © 2002, Systems and Computer Engineering,
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Files and Packages
• Each file can have only one public
class (visible to classes in other files)
– Can place several non-public classes
in the same file with a public class
• Java Packages provide a mechanism to:
– Group functionally related classes for
reuse by other programs
• We can have more than one
public class per package
• Grouping several classes to form a
package is is analogous to how
data and methods are grouped to
form a class
– Make non-public classes, in different
files, accessible to each other (and only
to each other) by claiming membership
in the same package
Copyright © 2002, Systems and Computer Engineering,
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Files and Packages
• Continued from previous slide ….
– “Naming Disambiguation”
• You can have two classes with the
same name as long as they live in
different packages.
• Collision is avoided by importing
the right package, or by calling the
class with its long name.
• Of course it is not desirable to use
the same name for two classes, but it
sometimes happens...
Copyright © 2002, Systems and Computer Engineering,
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Packages and Directories
• Each package is typically a separate
directory that contains the files that are
within that package
• Nested packages can be created by
layering directories and using hierarchical
package names
package myPkg.mySubPkg;
would be stored in directory
myPkg\mySubPkg
and can be imported using
// Import 1 class from the sub package
import myPkg.mySubPkg.Myclass;
// Import all classes from the sub
package
import myPkg.mySubPkg.*;
Copyright © 2002, Systems and Computer Engineering,
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Standard Java Packages
• Java supplies many packages
– java.lang
• Basic classes and interfaces used by most
Java programs
– java.util
• Utility classes and interfaces such as
date/time, randomizers, string manipulation
– java.io
• Classes that provide input/output for Java
programs
– java.text, java.applet,
java.awt, java.net
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The System Class in java.lang
• About System.out.println(…)
– System is a class in package
java.lang that, by default, is imported by
every Java file
• out is an class variable (of type
PrintStream defined in java.io)
within the class System and is the
Standard output stream
• Similarly:
– The System.err is a class variable of type
PrintStream, can be used to report errors
• Convenient because err is redirected to a
file different than out
– The System.in is a class variable of type
InputStream also defined in java.io
• in, out, and err are byte streams
– A byte stream is a sequence of bytes
Copyright © 2002, Systems and Computer Engineering,
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Study Exercise
• Given the descriptions on the last slide,
sketch out the code for the class System,
including :
– Package declarations and imports
– Access modifiers
– Instance Variables & methods
• For full marks, use proper coding
conventions
– Indentation
– Naming conventions
• Hint : Attempt the code first and then
check the code by browsing for the code in
Jbuilder
Copyright © 2002, Systems and Computer Engineering,
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What is an Exception ?
• The compilation process will detect
programming mistakes such as syntax
errors and type mismatches. Such errors
can therefore be referred to as
compilation errors.
• But once code is compiled and running,
it will have to face the real world of
erroneous input, inexistent files,
hardware failure… Such problems are
commonly known as runtime errors,
which are likely to cause the program to
abort.
• It is therefore important to anticipate
such problems and handle them
correctly, by avoiding loss of data or
premature termination, and by notifying
the user.
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Exceptions in this Course
• Exceptions are an integral part of
defensive programming
– We want you to be in the habit, from
the beginning, in using exceptions
– We want to teach it now
• But, exceptions are a complex topic,
requiring an understanding of
inheritance not yet taught.
• At this point, we shall teach the basic
exception mechanism.
– You should be able to throw and
catch any exceptions that we define
for you.
• Later, we shall examine the exception
hierarchy, and teach you how to define
your own exceptions.
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Throwing Exceptions
the throw statement
• Suppose you have a class with a
setLatitude() mutator (for example, the
Geopoint class in the 94.202 notes):
• You have to check whether the argument is within
the degree range [-90 .. +90]
• If this is not the case you can throw an
IllegalArgumentException
public void setLatitude(double l)
{
if ((l < -90) || (l > 90))
// is it within the range
{
throw new
IllegalArgumentException();
}
else
{
latitude = l;
}
}
Copyright © 2002, Systems and Computer Engineering,
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40
Exception handling: catching
•
Now the calling method needs to do something
with the exception that has been thrown by the
invoked method, or the program will terminate
abruptly! You need to catch the exception:
•
To catch an exception, you set up a
try/catch block:
public void setLatandLong(double lat,
double long)
{
try
{
setLatitude(lat);
setLongitude(long);
}
catch (IllegalArgumentException e)
{
System.out.println(“Wrong
input!")
}
}
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41
Exception handling: catching
from Core Java Vol I:
• “If any of the code inside the try block
throws an exception of the class specified
in the catch clause, then:
– The program skips the remainder of the
code in the try block.
– The program executes the handler code
inside the catch clause.
• If none of the code inside the try block
throws an exception, then the program
skips the catch clause”
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