# CS F213 Objected Oriented Programming Labsheet 1 ### Write your first JAVA program. ```java 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 .java ``` In this case: ``` javac Hello.java ``` #### Execution: Execute your code through terminal by writing ``` java ``` 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: ```java 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: ```java 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: ```java Square mySquare = new Square(); ``` To access variables of an object ```java mySquare.value = 12; ``` To access methods of an object ```java 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. {% hint style="info" %} **Note:** If we are declaring a public class, then it must be in a file of the same name. {% endhint %} ### Java Identifiers All Java variables must be identified with unique names called identifiers. #### Declaring (Creating) Variables: ```java 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` ```java if (condition) { // block of code to be executed if the condition is true } ``` * `else` ```java 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` ```java 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` ```java switch(expression) { case x: // code block break; case y: // code block break; default: // code block } ``` #### Shortcut for if-else ```java variable = (condition) ? expressionTrue : expressionFalse; ``` ### Java Loops #### while loop ```java while (condition) { // code block to be executed } ``` #### do-while loop ```java do { // code block to be executed } while (condition); ``` #### for loop ```java for (initialization; condition; iteration) { // code block to be executed } ``` #### for-each loop (can be used for iterating) ```java 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 ```java elementDataType[] arrayName = new elementDataType[arraySize]; ``` ```java elementDataType[] arrayName = {element_1, element_2, element_3, ..., element_N}; ``` Example ```java int[] firstArray = new int[10]; int[] firstArray = {0,1,2,3,4,5,6,7,8,9}; ``` #### Declaring a 2D array ```java elementDataType[][] arrayName = new elementDataType[rowSize][colSize]; ``` ```java elementDataType[][] arrayName = {{element_1_1, ..., element_1_M}, ..., {element_N_1, ..., element_N_M}}; ``` Example ```java 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 | ```java 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. \*\*\* --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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