1. Web Design & Development Lecture 5

2. Common Errors

3. Common Errors NoClassDefFoundError NoSuchMethodError: main
Cannot resolve symbol
Local variables must be initialized
ArrayIndexOutofBounds & NumberFormat exception

4. Error: Exception in thread “main” NoClassDefFoundError
public class CommonErrors{
public static void main (String args[]) {
System.out.println(“Hello World”); }

5. Error: Exception in thread “main” NoSuchMethodError: main
public class CommonErrors{
public static void display () {
System.out.println(“hello world”); }
public static void main (String args[]) {
CommonErrors.display(); }

6. Error: cannot resolve symbol
public class CommonErrors{
public static void main (String args[]) {
int number;
System.out.println(number1); }
number);

7. Error: Local variable not initialized
public class CommonErrors{
public static void main (String args[]) {
int number ;
System.out.println(number); }
= 10 ;

8. Error: ArrayIndexOutofBoundsException
public class CommonErrors{
public static void main (String args[]) {
System.out.println(args[0]); }

9. Error: NumberFormatException
public class CommonErrors{
public static void main (String args[]) {
String s = “20s”;
int number = Integer.parseInt(s); }
= “20” ;

10. Error: NullPointerException
public class CommonErrors{
public static void main (String args[ ]) {
Student s = null;
s.print( ); }

11. How to Use Java Doc

12. How to use Java Doc public class DocDemo{
public static void main (String args[]) {
String s = “Hello World”;
// I want to convert this string into Upper case? }
String upper = s.toUpperCase(); //upper contains “HELLO WORLD”
System.out.println(upper); //will print HELLO WORLD

13. More OOP

14. Inheritance

15. Employee class hierarchy
Teacher
Manager
Person

16. Employee. java (super class)
public class Employee{
protected int id;
protected String name;
//parameterized constructor
public Employee(int id, String name){
this.id = id;
this.name = name; }
//default constructor
public Employee(){
this (10, “not set”);
//continue

17. Employee. java (super class)
public void setId (int id) {
this.id = id; }
public int getId () {
return id;
public void setName (String name) {
this.name = name;
public String getName () {
return name;
//continue ….

18. Employee. java (super class)
public void display(){
System.out.println(“in employee display method”);
System.out.println("Employee id:" + id + " name:" + name); }
//overriding object class toString method
public String toString() {
System.out.println(“in employee toString method”);
return "id:" + id + "name:" + name;

19. Teacher. java (sub class)
public class Teacher extends Employee{
private String qual;
//default constructor
public Teacher () {
//implicit call to superclass default construct
qual = ""; }
//parameterized constructor
public Teacher(int i, String n, String q){
super(i,n); //call to superclass const must be first line
qual = q;
//continue

20. Teacher. java (sub class)
//accessors for qual
public void setQual (String qual){
this.qual = qual; }
public String getQual(){
return qual;
//continue

21. Teacher. java (sub class)
//overriding dispaly method of Employee class
public void display(){
System.out.println("in teacher's display method");
super.display(); //call to superclass display method
System.out.println("Teacher qualification:" + qual); }
//overriding toString method of Employee class
public String toString() {
System.out.println("in teacher's toString method");
String emp = super.toString();
return emp +" qualification:" + qual;
}// end of class

22. Test. java (Driver class)
public class Test{
public static void main (String args[]){
System.out.println("making object of employee");
Employee e = new Employee(89, "khurram ahmad");
System.out.println("making object of teacher");
Teacher t = new Teacher (91, "ali raza", "phd");
e.display(); //call to Employee class display method
t.display(); //call to Teacher class display method
System.out.println("Employee: " +e.toString());
System.out.println("Teacher: " + t);
} //end of main }

23. Compile & Execute

24. Polymorphism

25. Polymorphism
“Polymorphic” literally means “of multiple shapes” and in the context of OOP, polymorphic means “having multiple behavior”
A parent class reference can point to the subclass objects. For example Employee reference
Can point to the Employee Object
Can point to the Teacher Object
Can point to the Manager Object
A polymorphic method results in different actions depending on the object being referenced
Also known as late binding or run-time binding

26. Modify Test. java (of Inheritance Example)
public class Test {
public static void main (String args[]){
// Make employee references
Employee ref1, ref2;
ref1 = new Employee(89, "khurram ahmad");
//upcasting, is-a relationship
ref2 = new Teacher (91, "ali raza", "phd");
ref1.display(); //call to Employee class display method
ref2.display(); //call to Teacher class display method
System.out.println("Employee: " +ref1.toString());
System.out.println("Teacher: " + ref2.toString());
} //end of main }

27. Type Casting

28. Type Casting Java is strongly typed language Up-casting Implicit
No loss of information
Example of
Primitives
int a = 10;
double b = a;
Classes
Employee e = new Teacher();

29. Type Casting Downcasting Explicit Possible loss of information
Example of
Primitives
double a = 7.65;
int b = (int) a;
Classes
Employee e = new Teacher(); //upcasting
Teacher t = (Teacher) e; //downcasting