JAVA COURSE LESSON2 BY OMPUTER ENGINEEING ASSOCIATION

What we can learn today?  Review of OO programming  Exception concept  Class concept  hashtable  Basic event handling  Basic jswing uses

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!)

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()!^^

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

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!.

Something we can ignore  Public static void main(String args[]){  Int a[] = {1,2,3,4,5};  A[6] = 9;  }  It compiles well!  But it throws ArrayIndexOutOfBoundsException in runtime.  BUT WE CAN STILL NOT CATCH IT!

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.

 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!

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.

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^^

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.Hashtable  We will use two methods in our examples  get(key)  And put(key, object)  Example shown

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

WE would now teach the Jswing  Game.java is demonstrated.

The end