Web Design & Development Lecture 5

Common Errors

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

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

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(); }

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

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

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

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

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

How to Use Java Doc

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

More OOP

Inheritance

Employee class hierarchy Teacher Manager Person

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

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 ….

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;

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

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

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

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 }

Compile & Execute

Polymorphism

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

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 }

Type Casting

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();

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