Java Programming

 

Java Programming Notes

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1. Introduction to Java Programming

1.1 What is Java?

• Java is a high-level, class-based, object-oriented programming language developed by James Gosling and Mike Sheridan at Sun Microsystems (now owned by Oracle).
• Java is platform-independent, meaning it can run on any device or operating system that has the Java Runtime Environment (JRE) installed.

1.2 Structure of a Java Program

A basic Java program consists of:

1. Class declaration: The class is the blueprint of the object.
2. Main method: The entry point of a Java program, public static void main(String[] args).
3. Statements inside the main method to perform tasks.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

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2. Java Data Types

2.1 Primitive Data Types

Java has eight primitive data types:

• byte: 8-bit integer.
• short: 16-bit integer.
• int: 32-bit integer.
• long: 64-bit integer.
• float: 32-bit floating point.
• double: 64-bit floating point.
• char: 16-bit Unicode character.
• boolean: Represents true or false.

byte b = 100;
short s = 200;
int i = 300;
long l = 100000L;
float f = 3.14f;
double d = 3.14159;
char c = 'A';
boolean flag = true;

2.2 Reference Data Types

• Arrays: A collection of elements of the same data type.
• String: A sequence of characters.
• Objects: Instances of classes.

String name = "John";
int[] arr = {1, 2, 3, 4};

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3. Java Operators

3.1 Arithmetic Operators

These operators perform mathematical operations.

int sum = 5 + 3;
int diff = 5 - 3;
int prod = 5 * 3;
int quotient = 5 / 3;
int remainder = 5 % 3;

3.2 Relational Operators

These operators compare two values.

5 == 3;  // Equal to
5 != 3;  // Not equal to
5 > 3;   // Greater than
5 < 3;   // Less than
5 >= 3;  // Greater than or equal to
5 <= 3;  // Less than or equal to

3.3 Logical Operators

These operators are used for logical operations.

true && false;  // AND
true || false;  // OR
!true;           // NOT

3.4 Assignment Operators

Used to assign values to variables.

int a = 5;
a += 3;  // a = a + 3
a -= 2;  // a = a - 2
a *= 4;  // a = a * 4
a /= 2;  // a = a / 2

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4. Java Control Structures

4.1 If-Else Statement

The if-else statement is used to execute code conditionally.

int a = 5, b = 3;
if (a > b) {
    System.out.println("a is greater than b");
} else {
    System.out.println("a is less than or equal to b");
}

4.2 Switch-Case Statement

The switch-case statement allows you to choose from multiple options.

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Invalid day");
}

4.3 Loops

• For loop: Used when the number of iterations is known.

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

• While loop: Used when the number of iterations is unknown.

int i = 0;
while (i < 5) {
    System.out.println(i);
    i++;
}

• Do-While loop: Executes the loop body at least once before checking the condition.

int i = 0;
do {
    System.out.println(i);
    i++;
} while (i < 5);

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5. Methods in Java

5.1 Method Declaration

A method is a block of code that performs a specific task.

public class Calculator {
    // Method to add two numbers
    public int add(int a, int b) {
        return a + b;
    }
}

5.2 Calling a Method

You can call a method from within another method.

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        int result = calc.add(5, 3);
        System.out.println("Result: " + result);
    }
}

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6. Arrays in Java

6.1 One-Dimensional Arrays

An array is a collection of elements of the same type.

int[] arr = {1, 2, 3, 4, 5};
System.out.println(arr[0]);  // Output: 1

6.2 Multi-Dimensional Arrays

Multi-dimensional arrays are used to represent matrices.

int[][] arr = {{1, 2}, {3, 4}, {5, 6}};
System.out.println(arr[2][1]);  // Output: 6

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7. Object-Oriented Concepts in Java

7.1 Classes and Objects

A class is a blueprint for creating objects. An object is an instance of a class.

public class Person {
    String name;
    int age;

    void greet() {
        System.out.println("Hello, my name is " + name + " and I am " + age + " years old.");
    }
}

public class Main {
    public static void main(String[] args) {
        Person p1 = new Person();
        p1.name = "John";
        p1.age = 30;
        p1.greet();
    }
}

7.2 Constructors

A constructor is a special method used to initialize objects.

class Person {
    String name;
    int age;

    // Constructor to initialize name and age
    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    void greet() {
        System.out.println("Hello, my name is " + name + " and I am " + age + " years old.");
    }
}

public class Main {
    public static void main(String[] args) {
        Person p1 = new Person("John", 30);
        p1.greet();
    }
}

7.3 Inheritance

Inheritance allows a class to inherit properties and behaviors from another class.

class Animal {
    void sound() {
        System.out.println("Animal makes sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.sound();  // Output: Dog barks
    }
}

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8. Exception Handling in Java

8.1 Try-Catch Block

Exception handling in Java is done using try-catch blocks.

try {
    int result = 10 / 0;  // This will cause an ArithmeticException
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
}

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9. File Handling in Java

9.1 Reading and Writing Files

You can use FileReader and FileWriter to read and write to files in Java.

import java.io.*;

public class FileHandling {
    public static void main(String[] args) {
        try {
            // Writing to a file
            FileWriter writer = new FileWriter("output.txt");
            writer.write("Hello, Java File Handling!");
            writer.close();

            // Reading from a file
            FileReader reader = new FileReader("output.txt");
            int data;
            while ((data = reader.read()) != -1) {
                System.out.print((char) data);
            }
            reader.close();
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

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