A Brief Introduction to Java

1
A Brief Introduction to Java

• Programming the World Wide Web

2
Overview of Java

• In comparison with C
• Based on C
• Support for object-oriented programming
• No user-defined overloading
• Implicit deallocating heap objects
• No pointers
• Java is an object-oriented language.
• Java does not support procedure-oriented
  programming.
• Subprograms in Java can appear only as methods
  defined in class definitions.
• All data and functions are associated with
  classes, and also with objects.

3
Overview of Java

• Java does not allow user-defined operator
  overloading.
• Garbage collection is used to reclaim heap
  storage that has been deallocated.
• Java does not support multiple inheritance but
  uses interfaces instead.
• Java does not include pointers. Instead, it
  provides references.
• Java does not allow narrowing coercions.
• Control expressions in control statement in Java
  must have Boolean values.

4
Overview of Java

• Output to the screen from a Java application
  appears through the object System.out.
• The values of non-String variables that appear in
  the parameter to System.out.print or
  System.out.println.
• The string parameter to print and println is
  often specified as a catenation of several
  strings, using the catenation operation.
• Examples
• System.out.println(Apples are good for you)
• System.out.println(You should eat numApples
  apples each week)
• System.out.print(Grapes )
• System.out.println(are good, too)
• Apples are good for you
• You should eat 7 apples each week
• Grapes are good, too

5
Overview of Java

• Naming conventions used in Java are as follows
• Class and interface names begin with uppercase
  letters.
• Variable and method names begin with lowercase
  letters.
• Package names use all lowercase letters.
• Constant names use all uppercase letters.
• The first letter of all embedded words is
  capitalized.

6
Data Types and Structures

• Java has only one way to reference an object
  through a reference variable.
• The Java primitive types are int, float, double,
  char, and boolean.
• Each primitive type has a corresponding wrapper
  class, which is used when it is convenient to
  treat a primitive value as object.
• For example, the wrapper class for double is
  Double.
• Integer wrapsum new Integer (sum)

7
Data Types and Structures

• To convert the float value speed to a String
  object, the following can be used
• Float speedObj new Float(speed)
• String SpeedStr speedObj.toString()
• Reference variables are defined the same way as
  primitive variables
• String str1
• An array of characters can be created using the
  String and StringBuffer classes in two ways
• String greet1 new String(Guten Morgan)
• String greet2 Guten Morgan

8
String in Java

• Both Java String and StringBuffer objects use 2
  bytes per character because they use the Unicode
  character coding.
• String catenation can be done with the plus ()
  operator.
• greet3 greet3 New Year
• The String object has methods such as charAt,
  substring, concat, and indexOf.
• The equals method of String must be used to
  compare two strings of equality ( can not be
  used.)
• If a string must be manipulated, it cannot be a
  String object. A Stringbuffer object can be used.
• The Stringbuffer class has methods such as
  append, delete, and insert.

9
Arrays in Java

• In Java, arrays are objects of a class that has
  some special functionality. Array objects can be
  created with statements
• element_type arary_name new
  element_typelength
• Consider the following statements
• int list1 new int100
• Float list2 new float10
• Int list3
• list3 new int200
• When a subscript is detected that is out of
  range, the exception ArraryIndexOutOfBoundsExcepti
  on is thrown.
• Java does not have the struct and union data
  structures. It also does not have the unsigned
  types or the typedef declaration.

10
Control Flow

• Almost identical that of C language if-else,
  do-while, for, while, switch, break and continue,
  etc.
• No infamous goto
• Unlike C, we can define any local variable
  anywhere in methods
• Block scope - define the scope of your variable,
  can be nested

11
Switch Conditions

• Format
• switch (test)
• case valueOne
• resultOne
• break
• case valueTwo
• resultTwo
• break
• case valueThree
• resultThree break
• ...
• default defaultresult

12
Switch Conditions

• Example
• switch (x)
• case 2
• case 4
• case 6
• case 8
• System.out.println("x is an even
  number.") break default
• System.out.println("x is an odd
  number.")

13
For Loops

• Format
• for (initialization test increment)
  statements
• Multiple Initializers and Incrementers
• Example
• for (int i 1, j 100 i lt 100 i i1, j
  j-1) System.out.println(i j)

14
While Loops

• Format
• while (condition) bodyOfLoop
• Example
• int count 0
• while ( count lt array1.length array1count
  !0)
• array2count (float) array1count

15
Do .. While Loops

• Format
• do bodyOfLoop while (condition)
• Example
• int x 1
• do
• System.out.println("Looping, round " x)
  x
• while (x lt 10)

16
Classes, Objects, and Methods

• The parent of a class is specified in the class
  definition with the extends reserved word.
• modifier class class_name extends
  parent_class
• Three different modifiers can appear at the
  beginning of a class definition public,
  abstract, and final.
• The public modifier makes the class visible to
  classes that are not in the same package.
• The abstract modifier specifies that the class
  cannot be instantiated.
• The final modifier specifies that the class
  cannot be extended.
• The root class of all Java classes is Object.

17
Classes, Objects, and Methods

• The visibility of variables and member functions
  (methods) defined in classes is specified by
  placing their declarations in public, private,
  and protected.
• A variable declaration can include the final
  modifier to specify that the variable is a
  constant.
• Java class methods are specified by including the
  static modifier to their definitions.
• Any method without static is an instance method,
  that is any method that must be invoked with
  respect to an instance of a class.
• The abstract modifier specifies that the method
  is not defined in the class.
• The final modifier specifies that the method
  cannot be overridden.

18
Classes, Objects, and Methods

• Static methods are allocated when the class is
  loaded and invoked through the class but not an
  instance of a class.
• Java includes the package at a level above
  classes. Packages can contain more than one class
  definition.
• Packages often contain libraries and can be
  defined in hierarchies.
• A package can be included as the following way
• Package cars
• A static variable or method in a package can be
  specified in the following way
• weatherpkg.WeatherData.avgTemp

19
Classes, Objects, and Methods

• An import statement provides a way to abbreviate
  such imported names.
• import weatherpkg.WeatherData
• Now the variable avgTemp can be accessed directly
  with just its name. The import statement can
  include an asterisk to indicate all classes in
  the package are imported
• import weatherpkg.

20
Classes, Objects, and Methods

• A Java application program is a compiled class
  that includes a method named main.
• Public class Hello
• public static void main (String args)
• System.out.println(Hello World)
• The modifier on the main method are always the
  same.
• The public modifier indicates the class must have
  public accessibility.
• The void modifier indicates that main does not
  return a value.
• The only parameter to main is an array of strings
  that has command-line parameters from the user.

21
Classes, Objects, and Methods

• In Java, method definitions must appear in their
  associated classes.
• Java does not have destructors.
• In Java, the method is dynamically bound by
  default.
• Objects of user-defined classes are created with
  new.
• MyClass myObject1
• myObject1 new MyClass()
• MyClass myObject2 new MyClass()
• Numeric variables are implicitly initialized to
  zero. Boolean variables are initialized to false.
  Reference variables are initialized to null.

22
Classes, Objects, and Methods

• Instance variables are referenced through the
  reference variables that point to their
  associated object for example
• Class MyClass extends Object
• public int sum
• MyClass myObject new MyClass()
• The instance variable sum can be referenced with
  this
• myObject.sum

23
Classes, Objects, and Methods

• import java.io.
• class Stack_class
• private int stack_ref
• private int max_len, top_index
• public Stack_class() // A constructor
• stack_ref new int 100
• max_len 99
• top_index -1
• public void push (int number)
• if (top_index max_len)
• System.out.println("Error in push-stack is
  full")
• else stack_reftop_index number
• public void pop ()
• if (top_index -1)
• System.out.println("Error in push-stack is
  empty")
• else --top_index

24
Classes, Objects, and Methods

• public class Tst_Stack
• public static void main (String args)
• Stack_class myStack new Stack_class()
• myStack.push(42)
• myStack.push(29)
• System.out.println("29 is "
  myStack.top())
• myStack.pop()
• System.out.println("42 is "
  myStack.top())
• myStack.pop()
• myStack.pop() // Produces an error message

25
Interfaces and Applets

• Java supports only single inheritance. An
  interface can contain only named constants and
  method declarations.
• Applets are programs that are interpreted by a
  Web browser after being downloaded from a Web
  server.
• import java.applet.Applet
• import java.awt.Graphics
• public class HelloWorld extends Applet
• public void paint(Graphics g)
• g.drawString("Hello world!", 50, 25)

26
Applets

• lthtmlgt
• ltheadgt
• lttitlegtHelloWorld Appletlt/titlegt
• lt/headgt
• ltbodygt
• ltcentergt
• This is the HelloWorld Applet. ltbrgt
• ltapplet code "HelloWorld.class" width 150
  height 50gt
• lt/appletgt
• lt/centergt
• lt/bodygt
• lt/htmlgt

27
Input/Output
Source Example of Source Class Result
Disk file File (file.data) FileInputStream FileOutputStream Stream of bytes
Disk file File (file.data) FileReader Stream of chars
Stream of bytes FileInputStream FileOutputStream InputStreamReader PrintStream Stream of chars
Stream of chars InputStreamReader BufferedReader Lines of strings
28
Input/Output

• import java.io.
• public class DisplayFile
• public static void main (String args) throws
  Exception
• BufferedReader in
• new BufferedReader (new InputStreamReader
  (new FileInputStream(args0)))
• BufferedReader keyboard
• new BufferedReader (new InputStreamReader
  (System.in))
• String line
• System.out.print("Output File Name ")
• line keyboard.readLine()
• PrintStream out new PrintStream (new
  FileOutputStream(line))
• while((line in.readLine()) ! null)
• System.out.println(line)
• out.println(line)

29
Exception Handling

• All Java exceptions are objects of classes that
  are descendants of the Throwable class.
• The Java system includes two system-defined
  exception classes that are subclasses of
  Throwable Error and Exception.
• There are two system-defined direct descendants
  of Exception RuntimeException and IOException.
• User-defined exceptions are subclasses of
  Exception.

30
Exception Handlers

• An instance of the exception class is given as
  the operand of the throw statement.
• class MyException extends Exception
• public MyException()
• public MyException(String message)
• super(message)
• The first constructor does nothing
• The second sends its parameter to the parent
  class (specified with super) constructor.

31
Exception Handlers

• The exception can be thrown with this statement
• throw new MyException()
• Creating the instance of the exception for throw
  can be done separately from the throw statement.
• MyException myExceptionObject new
  MyException()
• throw myExceptionObject
• A new exception could be thrown with the
  following statement.
• Throw new MyException (a message to specify the
  location of the error)

32
Exception Propagation

• When a handler is found in the sequence of
  handlers in a try construct, that handler is
  executed and program execution with the statement
  following the try construct.
• If none is found, the handlers of enclosing try
  constructs are searched, starting from the
  innermost. If no handler is found, the exception
  is propagated to the caller of the method.
• To ensure that exceptions thrown in a try clause
  are always handled in a method, a special handler
  can be written that matches all exceptions
• Catch (Exception genricObject)

33
The throws Claus

• When a method specifies that it can be throw
  IOException, it can throw an IOException object
  or an object of its descendent.
• Exceptions of class Error and RuntimeException
  and their descendants are called unchecked
  exceptions. All are called checked exceptions.
  The compiler ensures that all checked exceptions
  are either listed in its throws clause or handled
  in the method.
• Java has no default handlers, and it is not
  possible to disable exceptions.