1. JAVA COURSE LESSON2 BY OMPUTER ENGINEEING ASSOCIATION

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

3. 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!)

4. 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()!^^

5. 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

6. 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!.

7. 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!

8. 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.

9.  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!

10. 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.

11. 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^^

12. 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

13. 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

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

15. The end