LAB 1 - Java Basics

University of Khartoum
Electrical and Electronic Engineering
Java Lab – 3^rd Year – Lab 1 (Java Basics)

Introduction:

Java is a pure object-oriented programming language. It is designed to be written and compiled once, and run everywhere. This is possible because a Java program is not compiled directly to the machine code; instead it is compiled to bytecode that can run on every Java virtual machine (JVM). The JVM then takes the responsibility of translating bytecode to the machine code and handling other low-level details.

Java programs are organized into classes. They are written and saved in .java files and compiled into .class files. These classes are grouped into packages. Packages in Java acts similarly to namespaces in C++.

·         To compile java file from cmd:                       javac   file_name.java

·         To execute the compiled file from cmd:        java     file_name

Data types:

Variables types in Java are either primitive or reference types. The primitive types are: byte, short, int, long, float, double, char, and boolean. Reference types include arrays and classes.

A floating-point literal is of type float if it ends with the letter F or f; otherwise its type is double and it can optionally end with the letter D or d.

Beside the primitive data there are: Integer class, Float class and Double Class which wraps a value of the primitive type int, float and double respectively in an object, these classes provides several methods for converting the primitive data to a String and a String to primitive data. Example :

int num = Integer.parseInt(“3”);                     // num will be assigned value of 3 as integer.

float fnum =  Float.parseFloat(“3”);               // fnum will be assigned value of 3.0 as float.

Arrays:

An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.

Array declaration and creation:

type[] array_name ;                // type is the data type of the contained elements i.e. int, String

array_name = type[size]         // or in one line: type[] array_name = type[size].

 Each array has a built-in length property (arrayname.length) to determine the size of the array by using.

Operators:

Java operators are almost the same as in C++: (arithmetic, bitwise, relational, logical, … )

Control statements:

In addition to the well-known statements (if-else, switch, for, while, and do-while), Java presents the for-each statement (Enhanced for loop “used mainly for unknown array size”) throughout its execution, it iterates through the items of an iterable object (e.g. an array).

String[] jobs = {“Accountant”, “Manager”, “Engineer”};        // Define array of type String hold 3                                                                                                  //data

for(String job: jobs) {               // In first iterate job will have “Accountant” value, in second //iterate “Manager” value, … 

// do something

}

Java standard I/O

STD output syntax:

 System.out.println();

STD input syntax: 

import java.util.Scanner

Scanner sc = new Scanner(System.in);

int i = sc.nextInt();  // to get the next integer

String str = sc.next();             // to get the next string

Method Overloading

Feature that allows a class to have two or more methods having same name, if their argument lists are different. Argument lists could differ in:

1. Number of parameters.

2. Data type of parameters.

3. Sequence of Data type of parameters.

Method overloading is also known as Static Polymorphism.

String API

Strings are a sequence of characters. In java Strings are treated as objects from String class.

The String class is immutable, so that once it is created a String object cannot be changed.

Important String class constructors:

String str1 = new String();                   //Initializes a newly created String object so that it represents //an empty character sequence.

String str2 = new String(char[] value) //Allocates a new String so that it represents the sequence of //characters currently contained in the character array //argument.

String str3 = new String(String original);        //Initializes a newly created String object so that it represents //the same sequence of characters as the argument; in other //words, the newly created string is a copy of the argument //string.

String str4 = “java lab”;

Important String class methods:
int length = str1.length();        // Returns the length of this –str1- string.

char ch = str2.charAt(int index);         // Returns the char value at the specified index.

int result = str3.compareTo(String anotherString);    //Compares two strings lexicographically.

int indx = str4.indexOf(char ch);         // Returns the index within this string of the first //occurrence of the specified character.

String str5 = str4.toLowerCase();        //Converts all of the characters in this String to lower case.

String str6 = str5.concat(str4);            // Concatenates the specified string to the end of this string (str5), or simply; String str6 = str5 + str4;

Program structure:

The file name should match the main class name. Also, only one class can be public per file.

package package_name;

import imported_package_name;

// …

public Class MainClass {

 Public static void main(String[] args) {

 // Program starts execution from here

}

// Other methods definitions

}

// Other classes and interfaces definitions

TASK 1:

Purpose of this task is to:

1.      Use Java Standard I/O.

2.      Use control statement.

Ø  Create class called CheckVowelsDigits, which prompts the user for a string, then counts the number of vowels (a, e, i, o, u, A, E, I, O, U) and digits (0-9) contained in the string, and prints the counts and the percentages (with 2 decimal digits).  For example,

Enter a String: testing12345

Number of vowels: 2 (16.67%)

Number of digits: 5 (41.67%)

TASK 2:

Purpose of this task is to:

1.      Use method overloading.

Ø  Create class called Summation that overload the main method in order to sum two integer numbers, two float numbers and two double numbers.

TASK 3:

Purposes of this task to:

1.      Understand main function.

2.      Use control statement.

Create a class called ResistorColorCode inside a package called electronics, the user must specify the colors of the resistor at the execution time and the class should calculate the equivalent resistance value according to the table shown below.

-

Helpful method:

Math.pow(double a, double b) : Returns the value of the first argument raised to the power of the second argument.

TASK 4:

Purpose of this task is to:

1.      Work with String API

Ø  Create a class called SeventyThree which do the following:

i)        Convert the number 73 into its binary representation and construct a new String object to hold that binary representation then print it.

ii)      Construct a new String object of the first four bits of the binary representation then print it.

iii)    Construct a new String object of the last four bits of the binary representation then print it.

iv)    Compare the String object in (ii) and (iii) and print the compression result.

v)      Reverse the binary representation in (i) and display it. 

Helpful method:

String str = Integer.toBinaryString(73);          // Returns a string representation of the integer //argument as an unsigned integer in base 2.