CS F213 Objected Oriented Programming Labsheet 1

Spring 2024

Write your first JAVA program.

class Hello {
  public static void main(String args[]) {
    // Program execution begins here
    System.out.println("Hello world. I can write Java!");
  }
}

Compilation:

Compile your code on a terminal:

javac <YOUR_PROGRAM_NAME>.java

In this case:

javac Hello.java

Execution:

Execute your code through terminal by writing

java <YOUR_PROGRAM_NAME>

In this case:

java Hello

Output:

Hello world. I can write Java!

After your code is compiled, you will find a file with the same name as the class name declared in the file but with the extension .class. A Java class file is a file containing Java bytecode and having .class extension that can be executed by JVM (Java Virtual Machine). A Java class file is created by a Java compiler from .java files as a result of successful compilation. The compilation will result in as many .class files as there are classes in the file you just compiled. A good practice, but not a requirement, is to restrict to one class per java file.

Java Class and Structure

Class:

The Class is the basic unit of Object Oriented Programming. The Class forms the basis for object oriented programming in Java.

General Syntax of a Class:

class classname
{
  type instance-variable1;
  type instance-variable2;
  ...
  type instance-variableN;

  type methodname1(parameter-list)
  { body }
  ...
  type methodnameN(parameter-list)
  { body }
}

Two components of Java Class:

  1. Attributes - Variables

  2. Methods - Functions

Example:

class Square {
  int value;
  public static void printSquare(int x){
    System.out.println(x*x);
  }
  public static void main(String args[]){
    int local_value = 2;
    printSquare(local_value);
    printSquare(3);
    printSquare(local_value*2);
  }
}

To instantiate an object of Square:

Square mySquare = new Square();

To access variables of an object

mySquare.value = 12;

To access methods of an object

mySquare.printSquare(10);

Syntax Guidelines

  1. Every line of code that runs in Java must be inside a class

  2. A good practice is to use CamelCase with the first character capitalised for class names. Class names are usually nouns

  3. A good practice is to use camelCase with the first character in lower case for methods. Method names are usually verbs

  4. The name of the Java file need not match the class name but it is good practice to do so (see Note below).

  5. The main() method is required and you will see it in every Java program. Any code inside the main() method will be executed.

Note: If we are declaring a public class, then it must be in a file of the same name.

Java Identifiers

All Java variables must be identified with unique names called identifiers.

Declaring (Creating) Variables:

type variableName = value;

Types of Java Literals:

  1. String - Stores text

  2. int - Stores integers (whole numbers)

  3. float - Stores floating point numbers

  4. char - Stores single characters

  5. boolean - Stores true or false

Variable Name Guidelines

  • Variable names are usually in all lowercase but meaningful

  • Names can contain letters, digits, underscores, and dollar signs

  • Names must begin with a letter (_ and $ are allowed as the first character but discouraged)

  • Names should start with a lowercase letter and it cannot contain whitespace

  • Names are case sensitive

  • Reserved words (keywords) cannot be used as names

Java Conditional Statements

  • if

if (condition) {
  // block of code to be executed if the condition is true
}

  • else

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

  • else if

if (condition1) {
  // block of code to be executed if condition1 is true
} else if (condition2) {
  // block of code to be executed if the condition1 is false and condition2 is true
} else {
  // block of code to be executed if the condition1 is false and condition2 is false
}

  • switch

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

Shortcut for if-else

variable = (condition) ? expressionTrue :  expressionFalse;

Java Loops

while loop

while (condition) {
  // code block to be executed
}

do-while loop

do {
  // code block to be executed
}
while (condition);

for loop

for (initialization; condition; iteration) {
  // code block to be executed
}

for-each loop (can be used for iterating)

for (type variableName : arrayName) {
  // code block to be executed
}

Java Data Types

Data Type
Size/Format
Description

byte

8-bit

Byte-length integer

short

16-bit

Short integer

int

32-bit

Integer

long

64-bit

Long integer

float

32-bit

Single-precision floating point

double

64-bit

Double-precision floating point

char

16-bit

Unicode character A single character

boolean

true or false

A boolean value (true or false)

Array

Declaring an array

elementDataType[] arrayName = new elementDataType[arraySize];
elementDataType[] arrayName = {element_1, element_2, element_3, ..., element_N};

Example

int[] firstArray = new int[10];
int[] firstArray = {0,1,2,3,4,5,6,7,8,9};

Declaring a 2D array

elementDataType[][] arrayName = new elementDataType[rowSize][colSize];
elementDataType[][] arrayName = {{element_1_1, ..., element_1_M}, ..., {element_N_1, ..., element_N_M}};

Example

int[][] firstTwoDArray = new int[10][20];
int[][] firstTwoDArray = {{0,1,2},{3,4,5},{6,7,8}};

Taking input from user

The Scanner class is used to get user input, and it is found in the java.util package.

Method
Description

nextBoolean()

Reads a boolean value from the user

nextByte()

Reads a byte value from the user

nextDouble()

Reads a double value from the user

nextFloat()

Reads a float value from the user

nextInt()

Reads a int value from the user

nextLine()

Reads a String value from the user

nextLong()

Reads a long value from the user

nextShort()

Reads a short value from the user

import java.util.Scanner;

class UserInput {
  public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      String str = sc.nextLine();
      System.out.println("String is: " + str);
      int first = sc.nextInt();
      int second = sc.nextInt();
      int sum = first + second;
      System.out.println("Sum is: " + sum);
    }
}

Exercise Problems:

Exercise 1

What happens when we declare multiple classes in the same file and then use javac to compile the file and the file name is any one of the declared classes?

Exercise 2

Write a Java program to compute the final amount a person has to repay for a loan of 10 years with interest rate of 5% per annum. The principal amount is to be taken as input from user and display the final amount. Use double datatype for principal and final amount.

Formula:

FinalAmount = PrincipalAmount(1 + (TimePeriodInYears * InterestRate / 100))

Input

A double representing the principal amount

Output

A double representing the final amount

Sample Test Input

1000.0

Sample Test Output

1500.0

Exercise 3

Write a Java program to print a menu to the user asking to choose between by entering the corresponding option number:

1. Circle
2. Rectangle

If 1 (circle) is chosen, take the input of radius (int) and print the area of the circle (take pi = 3.14), otherwise take the input of length (int) and breadth (int) of rectangle and print the area. Do this exercise with both if-else and switch statements.

Sample Test Input 1

1
10

Sample Test Output 1

314.0

Sample Test Input 2

2
60
20

Sample Test Output 2

1200

Exercise 4

Write a Java program to declare a 2D array of size 4 X 3, then take the input from user such that the array contains following elements:

12   3    4 
4    34   2
65   1   56
76   24   7

After that, using for-each loop find the sum of the whole array and print it.

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