Department of Computer Science
Faculty of Physical Sciences
Ahmadu Bello University, Zaria

COSC 211 : Object Oriented Programming I - LAB02

Objectives:

To gain experience with:

• Creating objects from classes
• Fundermental Data Types
• The String class

1.  Creating Objects from classes

Java is an object-oriented programming language, meaning problems are solved by interaction among objects.

An Object is a software-entity that models some real-world entity – e.g. A particular rectangle, a particular student, etc.

Each object has a state (represented by its fields) and behaviors (represented by its methods).

Objects are created from classes, thus to create an object, we need first to design a class.  We can also use classes from the Java library.  The Java library consists of hundreds of classes organized into related groups called packages.

A class is a general blueprint or specification for a set of objects in which fields and methods are defined.

To create an object from a class, we use the new operator as follows:

Rectangle myRectangle = new Rectangle(5,10, 20, 30);

In the above, Rectangle is a class provided in the Java library under java.awt package.  When creating a rectangle object, we need to provide values for the top-left corner (x,y), with and height of the rectangle object.  The new operator creates the rectangle object and returns the address of the object in memory which is then assigned to the reference variable myRectangle.

Once an object is created, we can call its methods using the dot operator.  For example, to call the translate method, we write:

myRectangle.translate(10, 15).

The following is a complete program showing the above points.  Notice that if we use a Java library class that is not in the java.lang package, we must import the class using the import statement.

Example1:

import java.awt.Rectangle;

/*  Creates a rectangle object, calls its translate method and then print it. */

public class MoveRectangle {

        public static void main(String[] args) {

               Rectangle myRectangle=new Rectangle(5, 5, 10, 10);

               System.out.println("Before Translation: "+myRectangle);

               myRectangle.translate(15,25);

                System.out.println("After Translation: "+myRectangle);

        }

}

2.  The String Class.

A string is a sequence of characters enclosed in double quotes.

String processing is one of the common applications of computers.  Java recognizes this fact and therefore provides special support for string processing through a class called String in the java.lang package, which has many useful methods.

To create a string object from the string class we can use the new operator as follows:

String s = new String(“the content of the string in quates”);

However, because of its common use, Java allows strings objects to be created without using the new operator as in:

String greeting = “Hello, World!”;

In a string object, the characters are indexed starting from 0 as shown below:

The table below shows some of the most frequently used methods of the String class:

int length()

int i = myString.length(); 

// i = 8

String toUpperCase()

String upper=myString.toUpperCase(); 

// upper == "EXAMPLES" 

String toLowerCase()

String lower=myString.toLowerCase();

// lower == "examples" 

boolean equals(String s)

Boolean equalsIgnoreCase

           (String s)

if (myString.equals("EXAMPLES") )

// false

int compareTo(String s)

int compareToIngoreCase

        (String s)

if (myString.compareTo("Yes")>0 )

// true

char charAt(int index)

char c = myString.charAt(6);

// c = ‘e’

int indexOf(int ch)

int lastIndexOf(int ch)

//also overloaded to 

accept String parameter

int i = myString.indexOf("amp");

// i == 2

if (myString.indexOf("Y") > 0)

// returns -1 and is false

String substring

        (int from)

String substring

      (int from, int to)

String s=myString.substring(5)

// returns "ample" and "les"

String s=myString.substring(2, 7);

// returns "les"

String trim()

String s = “    125     “;

s = s.trim();

//returns  “125”

static String valueOf

   (any primitive type)

String s = String.valueOf(3.14F);

// numbers to Strings "3.14"

String concat(String s)

//equivalent to + symbol

String s = myString.concat(“ below”)

// returns “examples below”

Example 2:

The following example generates a password for a student using his initials and age.  (note:  do not use this for your actual passwords).

                 lastName.substring(3,4);

3.  Assignments

4.  Home Work

1.        The class Random of the java.util package can be used to create an object which can then be used to generate a random integer number from 0 to a given maximum.

Example:

Random rand = new Random();

int randomInt = rand.nextInt(1000);

Improve the password example by generating a random number,  multiply the random number with the age before appending it to the initials.

2.        Write a program to evaluate and print the value of the following mathematical function :

      f(x,y) = 2x² - y  +   3

Vary the values of x and y according to the following table and prints the values of f(x,y) as shown in the table.

Very important guidelines:

1. You should submit both printed copy and soft copy in a flash drive bofore the next lab.
2. All your programs files should be saved in a folder HW1 on your flash drive.
3. Code your programs according to the Java naming conventions  -- check this out on my home page.
4. Indent your work so that content of a class and methods are pushed inside by a tab or at least three spaces.
5. Use comments at the beginning of each program to explain its purpose.  Also include your name and ID number as part of the comment.
6. Use comments to explain each variable whose purpose cannot be determined from its name.