Question

3 6 7 10 11 QUESTION 4: Object Orientation (a) Given the provided code: [08] 1 public class Base{ public void doSomething() {} /* Remainder of class omitted */ public class Derived extends Base{ public void do something() {} * Remainder of class omitted */ public class Container<t extends Base> { private T data; public void dort() { data.doSomething(); /* Remainder of Class omitted */ 16 public class Main { public static void main(String[] args) { Container<Derived> instance = new Container<>(); 19 instance.doit(); 20 21} Identify four (4) Java programming constructs/principles present in the code and list the line number that it is found at. For example: Line 99 - Exception Handling (b) Is there a problem with the provided code? If so, explain the reason for the problem. [02] 12 13 } 17 18 } Total: 10

Answer 1

(a)
Four constructs/principles of Java present in the code are:

1. Line 5 - Inheritance. class Derived inherits from class Base using the extends keyword.
2. Line 6 - Method overriding. class Derived is overriding the method doSomething() from class Base.
3. Line 9 - Generic classes. class Container is using a generic datatype T, that can be any datatype that extends class Base.
4. Line 10 - Data Encapsulation. The instance variable data of class Container is set to private, so the value of data can only be changed or accessed using a method the class Container.

(b) The problem with the code is, there is constructor for class Container. Since there is no constructor, when the object of class Container is created in main(), the instance variable data is set to null since it is an object of class Derived, as seen in line 18. Now, when the code calls instance.doIt(), it will call data.doSomething(). But data is null, and that will cause an NullPointerException.