JAVA COURSE LESSON2

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JAVA COURSE LESSON2

• BY OMPUTER ENGINEEING ASSOCIATION

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What we can learn today?

• Review of OO programming
• Exception concept
• Class concept
• hashtable
• Basic event handling
• Basic jswing uses

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OO programming

• OO programming includes encapsulation,
  inheritance and polymorphism.
• We will talk more about polymorphism.
• Java call the methods based on the type of the
  class it is most of the time in compile time
• But, with the use of inheritance, we can use the
  superclass type and call the methods from the
  class inherited from it.
• This is called polymorphism.
• Which the calling of method is resolved in run
  time, but not in compile time (dynamic!)

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Example

• e.g There are three classes, like employee,
  hourly empoloyee and contract employee
• Which now the last two inherit employee class
• Employee class do not have payment() method
• But the other two have!
• When we type
• Employee temp new ConEmployee()
• We cannot call temp.payment()! As JAVA resolve it
  by type
• But if the super class employee also has this
  payment() method, then we can call
  temp.payment()!

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Exception Handling

• Exception is not something program errors.
• It maybe a matter of runtime unpredictable
  hardware problems, or usage or program.
• In JAVA, exception are finely classified into
• We need to handle
• And we don't need to handle type
• We would demonstrate in two examples

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The type we must handle

• Public static void main(String args)
• Try
• Thread.sleep(100)
• Catch (Exception e)
• System.out.println(caught)
• In the above example, we must do something for
  the Thread.sleep(100) method as it throws the
  exception and it is the type we must handle.
  First type!.

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Something we can ignore

• Public static void main(String args)
• Int a 1,2,3,4,5
• A6 9
• It compiles well!
• But it throws ArrayIndexOutOfBoundsException in
  runtime.
• BUT WE CAN STILL NOT CATCH IT!

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Two ways we can deal with exception

• First, code it to deal with it
• Try
• Statement may throw exception
• Catch (Exception e)
• Finally
• The catch block is used to catch certain type
  exception and code the problem. (As Exception is
  the SUPERCLASS of all exception, so the
  statements above catch all exception.)
• The finally block must execute before any return
  and after the try and catch blocks.

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• Even you type return in try block, the statements
  in finally block is executed!
• After the try block, finally or catch blocks must
  be executed!

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Second method

• We don't handle the exception, but we throw it
  away.
• Like
• void a() throws Exception
• Thread.sleep(100)
• Public static void main(String args)
• Test temp new Test() try
• temp.a()catch (Exception)//we catch it
• We catch it in other block as the exception is
  threw in a() method.

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Class concept

• All the classes in JAVA are inherited from the
  Object class.
• And thus all classes have all the methods in
  Object (if they are stating as right modifier in
  Object!)
• Eight types are not classes
• Stated in last lesson
• But they can be class also
• Integer temp new Integer(6)
• The storage are not the same, will not explained
  here

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Hashtable

• Each element is a pair of
• We just the key to find the element we want!
• In java, hashtable is given in java.utils.Hashtabl
  e
• We will use two methods in our examples
• get(key)
• And put(key, object)
• Example shown

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Basic event handling

• IN java Jswing, we can handle the action, like
  mouse action, window closing......with the Jswing
  elements to write interactive program
• See the demonstration

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WE would now teach the Jswing

• Game.java is demonstrated.

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The end