Java session04

Java Programming Language
Objectives

In this session, you will learn to:
Declare and create arrays of primitive, class, or array types
Explain how to initialize the elements of an array
Determine the number of elements in an array
Create a multidimensional array
Write code to copy array values from one array to another
Define inheritance, polymorphism, overloading, overriding, and
virtual method invocation
Define heterogeneous collections

Ver. 1.0 Session 4 Slide 1 of 26

Arrays

Declaring Arrays:
Group data objects of the same type.
An array is an object.
Declare arrays of primitive or class types:
• char s[ ];
• Point p[ ];
• char[ ] s;
• Point[ ] p;
– The declaration of an array creates space for a reference.
– Actual memory allocation is done dynamically either by a new
statement or by an array initializer.


Creating Arrays

In order to create an array object:
– Use the new keyword
– An example to create and initialize a primitive (char) array:
public char[] createArray()
{
char[] s;
s = new char[26];
for ( int i=0; i<26; i++ )
{
s[i] = (char) (’A’ + i);
}
return s;
}


Creating Arrays (Contd.)

Creating an Array of Character Primitives:


Creating Reference Arrays

The following code snippet creates an array of 10
references of type Point:
public Point[] createArray()
{
Point[] p;
p = new Point[10];
for ( int i=0; i<10; i++ )
{
p[i] = new Point(i, i+1);
}
return p;
}


Creating Reference Arrays (Contd.)

Creating an Array of Character Primitives with Point
Objects:


Demonstration

Lets see how to declare, create, and manipulate an array of
reference type elements.


Multidimensional Arrays

A Multidimensional array is an array of arrays.
For example:
int [] [] twoDim = new int [4] [];
twoDim[0] = new int [5];
twoDim[1] = new int[5];
– The first call to new creates an object, an array that contains
four elements. Each element is a null reference to an element
of type array of int.
– Each element must be initialized separately so that each
element points to its array.


Array Bounds

• All array subscripts begin at 0.
• The number of elements in an array is stored as part of the
array object in the length attribute.
• The following code uses the length attribute to iterate on
an array:
public void printElements(int[] list)
{
for (int i = 0; i < list.length; i++)
{
System.out.println(list[i]);
}
}


The Enhanced for Loop

• Java 2 Platform, Standard Edition (J2SE™) version 5.0 has
introduced an enhanced for loop for iterating over arrays:
public void printElements(int[] list)
{
for ( int element : list )
{
System.out.println(element);
}
}
– The for loop can be read as for each element in list do.


Array Resizing

You cannot resize an array.
You can use the same reference variable to refer to an
entirely new array, such as:
int[] myArray = new int[6];
myArray = new int[10];
In the preceding case, the first array is effectively lost unless
another reference to it is retained elsewhere.


Copying Arrays

• The Java programming language provides a special method
in the System class, arraycopy(), to copy arrays.
For example:
int myarray[] = {1,2,3,4,5,6}; // original array
int hold[] = {10,9,8,7,6,5,4,3,2,1}; // new
larger array
System.arraycopy(myarray,0,hold,0,
myarray.length); // copy all of the myarray array to
the hold array, starting with the 0th index
– The contents of the array hold will be : 1,2,3,4,5,6,4,3,2,1.
– The System.arraycopy() method copies references, not
objects, when dealing with array of objects.


Inheritance

Inheritance means that a class derives a set of attributes
and related behavior from a parent class.
Benefits of Inheritance:
Reduces redundancy in code
Code can be easily maintained
Extends the functionality of an existing class


Inheritance (Contd.)

Single Inheritance
The subclasses are derived from one super class.
An example of single inheritance is as follows:


Inheritance (Contd.)

Java does not support multiple inheritance.
Interfaces provide the benefits of multiple inheritance
without drawbacks.
Syntax of a Java class in order to implement inheritance is
as follows:
<modifier> class <name> [extends
superclass>]
{ <declaration>* }


Access Control

Variables and methods can be at one of the following four
access levels:
public
protected
default
private
Classes can be at the public or default levels.
The default accessibility (if not specified explicitly), is
package-friendly or package-private.


Overriding Methods

A subclass can modify behavior inherited from a parent
class.
Overridden methods cannot be less accessible.
A subclass can create a method with different functionality
than the parent’s method but with the same:
Name
Return type
Argument list


Overriding Methods (Contd.)

• 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.
– 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.



• Invoking Overridden Methods using super keyword:
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;
}
}


Demonstration

Lets see how to create subclasses and call the constructor of
the base class.


Polymorphism

Polymorphism is the ability to have many different forms;
for example, the Manager class has access to methods
from Employee class.
An object has only one form.
A reference variable can refer to objects of different forms.
Java programming language permits you to refer to an object
with a variable of one of the parent class types.
For example:
Employee e = new Manager(); // legal


Virtual Method Invocation

Virtual method invocation is performed as follows:
Employee e = new Manager();
e.getDetails();
Compile-time type and runtime type invocations have the
following characteristics:
The method name must be a member of the declared variable
type; in this case Employee has a method called getDetails.
The method implementation used is based on the runtime object’s
type; in this case the Manager class has an implementation of the
getDetails method.


Heterogeneous Collections

Heterogeneous Collections:
Collections of objects with the same class type are called
homogeneous collections. For example:
MyDate[] dates = new MyDate[2];
dates[0] = new MyDate(22, 12, 1964);
dates[1] = new MyDate(22, 7, 1964);
Collections of objects with different class types are called
heterogeneous collections. For example:
Employee [] staff = new Employee[1024];
staff[0] = new Manager();
staff[1] = new Employee();
staff[2] = new Engineer();


Summary

In this session, you learned that:
– Arrays are objects used to group data objects of the same
type. Arrays can be of primitive or class type.
– Arrays can be created by using the keyword new.
– A multidimensional array is an array of arrays.
– All array indices begin at 0. The number of elements in an
array is stored as part of the array object in the length
attribute.
– An array once created can not be resized. However the same
reference variable can be used to refer to an entirely new
array.
– The Java programming language permits a class to extend one
other class i.e, single inheritance.


Summary (Contd.)

Variables and methods can be at one of the four access levels:
public, protected, default, or private.
Classes can be at the public or default level.
The existing behavior of a base class can be modified by
overriding the methods of the base class.
A subclass method may invoke a superclass method using the
super keyword.
Polymorphism is the ability to have many different forms; for
example, the Manager class (derived) has the access to
methods from Employee class (base).
Collections of objects with different class types are called
heterogeneous collections.


Java session04

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