Download Java Programming class – Department of Network

Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
1. Overriding Methods
A subclass can modify behavior inherited from a parent class.
• A subclass can create a method with different functionality than the
parent’s method but with the same:
• Name
• Return type
• Argument list
Then the new method is said to override the old one.
So, what is the objective of subclass?
Example
Public class Employee {
protected String name;
protected double salary;
protected Date birthDate;
public String getDetails() {
return “Name: “ + name + “\n” +
“Salary: “ + salary;
}
}
public class Manager extends Employee {
protected String department;
Public String getDetails() {
return “Name: “ + name + “\n” +
“Salary: “ + salary + "\n" +
“Manager of: “ + department;
}
}
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Date: September 19, 2013
Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
The Manager class has a getDetails method by definition because
it inherits one from the “Employee class”. However, the original has
been replaced, or overridden by the child class ‘version.
2. Overridden Methods Cannot Be Less Accessible
Public class Parent {
Public void doSomething() {
}
}
Public class Child extends Parent {
Private void doSomething() {} // illegal
}
public class UseBoth {
public void doOtherThing() {
Parent p1 = new Parent();
Parent p2 = new Child();
p1.doSomething();
p2.doSomething();
}
}
The
Java
programming
language
semantics
dictate
that
p2.method () results in the child version of the method being
executed, because the method is declared private, p2 ( declared
as parent ) cannot access it , this the semantics language is
violated
3. Invoking Overridden Methods
A subclass method may invoke a superclass method using the
super keyword:
• The keyword super is used in a class to refer to its superclass.
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Date: September 19, 2013
Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
• The keyword super is used to refer to the members of
superclass, both data attributes and methods.
• Behavior invoked does not have to be in the superclass; it can
be further up in the hierarchy.
public class Employee {
private String name;
private double salary;
private Date birthDate;
public String getDetails() {
return "Name: " + name + "\nSalary: " + salary;
}
}
public class Manager extends Employee {
private String department;
public String getDetails() {
// call parent method
return super.getDetails()
“\nDepartment: " + department;
}
}
The Object Class
The object class is the root of all classes in the Java technology
programming language. If a class is declared with no extends
clause, then the compiler adds implicitly the code extend Object to
the declaration for example
Public class Employee {
// code here
}
Is equivalent to
Public class Employee extends Object {
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// code goes here }
Date: September 19, 2013
Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
Two important methods are:
• equals
• toString
1. The equals Method
 The == operator determines if two references are identical to
each other (that is, refer to the same object).
 The equals method determines if objects are equal but not
necessarily identical.
 The Object implementation of the equals method uses the ==
operator
 User classes can override the equals method to implement a
domain-specific test for equality.
Note: You should override the hashCode method if you override the
equals method.
Example
public class MyDate1 {
private int day;
private int month;
private int year;
// constructor
public MyDate1(int day2, int month2, int year2) {
this.day = day2;
this.month = month2;
this.year = year2;
}
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Date: September 19, 2013
3rd class – Department of Network
College of IT- University of Babylon
Java Programming
Mehdi Ebady Manaa
public boolean equals(Object o) {
boolean result = false;
if ( (o != null) && (o instanceof MyDate1) )
{
MyDate1 d = (MyDate1) o;
if ( (day == d.day) && (month == d.month)&& (year ==
d.year) ) {
result = true;
} // end if
}// end if
return result;
} // end method
public int hashCode() {
return (day ^ month ^ year);
} // end method hashCode
}
public class TestEquals {
public static void main(String[] args) {
MyDate1 date1 = new MyDate1(10, 10, 1976);
MyDate1 date2 = new MyDate1(10, 10, 1976);
if ( date1 == date2 ) {
System.out.println("date1 is identical to date2");
} else {
System.out.println("date1 is not identical to date2");
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Date: September 19, 2013
Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
}
if ( date1.equals(date2) ) {
System.out.println("date1 is equal to date2");
} else {
System.out.println("date1 is not equal to date2");
System.out.println("set date2 = date1;");
}
date2 = date1;
if ( date1 == date2 ) {
System.out.println("date1 is identical to date2");
} else {
}
System.out.println("date1 is not identical to date2");
}
}
This example generates the following output:
date1 is not identical to date2
date1 is equal to date2
set date2 = date1;
date1 is identical to date2
2. The toString method
It has the following characteristics:
• This method converts an object to a String.
• Use this method during string concatenation.
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Date: September 19, 2013
Java Programming
Mehdi Ebady Manaa
3rd class – Department of Network
College of IT- University of Babylon
• Override this method to provide information about a userdefined object in readable format.
class Bank
{
String n;
String add;
int an;
int bal;
int dep;
public Bank(String n,String add,int an,int bal)
{
this.add=add;this.bal=bal;
this.an=an;this.n=n;
}
public String toString()
{
return "Name of the customer.:" + this.n+",, "+"Address of the
customer.:"+this.add +",, "+"A/c no..:"+this.an+",, " +"Balance in
A/c..:"+this.bal;
}
}
Q/ how we convert primitive types to a String using toSting
method??
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Date: September 19, 2013