Java Basics

1. Sample Code in Java

a. What is main

The main method is the entry point of a Java application.

• Syntax:

  public static void main(String[] args) {
      // Code to be executed
  }

• Key Points:

  • public: Accessible from anywhere.
  • static: Belongs to the class, not an instance.
  • void: Returns nothing.
  • String[] args: Command-line arguments.

b. What is static

• Definition: A keyword to denote a method or variable that belongs to the class rather than an instance.

• Example:

  class Example {
      static int count = 0; // Static variable
  
      static void displayCount() { // Static method
          System.out.println("Count: " + count);
      }
  }

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2. Comments

a. Single Line

• Syntax: // This is a single-line comment
• Example:

  // This is a comment
  int x = 10; // Variable declaration

b. Multi-Line

• Syntax:

  /* This is a
     multi-line comment */

• Example:

  /*
     This code demonstrates
     multi-line comments.
  */
  int y = 20;

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3. Data Types

a. byte

• 1 byte (8 bits), range: -128 to 127.
• Example:

  byte b = 100;

b. short

• 2 bytes (16 bits), range: -32,768 to 32,767.
• Example:

  short s = 32000;

c. int

• 4 bytes (32 bits), range: -2^31 to 2^31-1.
• Example:

  int i = 123456;

d. float

• 4 bytes (32 bits), precision: 6-7 decimal digits.
• Example:

  float f = 3.14f;

e. double

• 8 bytes (64 bits), precision: 15-16 decimal digits.
• Example:

  double d = 3.14159;

f. char

• 2 bytes (16 bits), stores a single Unicode character.
• Example:

  char c = 'A';

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4. Operators

a. Arithmetic

int sum = 5 + 3; // Addition
int diff = 5 - 3; // Subtraction
int prod = 5 * 3; // Multiplication
int quo = 5 / 3; // Division
int rem = 5 % 3; // Modulus

b. Unary

int x = 5;
System.out.println(++x); // Pre-increment
System.out.println(x--); // Post-decrement

c. Relational

int a = 10, b = 20;
System.out.println(a > b); // false

d. Logical

boolean res = (a > b) && (b > 0); // AND

e. Assignment

int num = 5;
num += 10; // Equivalent to num = num + 10

f. Bitwise

int result = 5 & 3; // Bitwise AND

g. Ternary

String result = (a > b) ? "Greater" : "Smaller";

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5. String

a. Strings and Immutability

• Strings in Java:

  • A String is a sequence of characters.
  • Strings in Java are objects of the String class.
  • They are defined using double quotes:

    String str = "Hello";

• Immutability:

  • Strings are immutable, meaning their content cannot be changed once created.
  • Any operation that appears to modify a string (e.g., concatenation) creates a new String object instead of modifying the existing one.

• Example:

  String s1 = "Hello";
  String s2 = s1.concat(" World");
  System.out.println(s1); // Output: Hello
  System.out.println(s2); // Output: Hello World

b. Char Array to String

• Strings can be constructed from character arrays.
• Example:

  char[] chars = {'J', 'a', 'v', 'a'};
  String str = new String(chars);
  System.out.println(str); // Output: Java

c. String Methods

1. Concatenate:

• Combines two strings.
• Example:

  String first = "Hello";
  String second = "World";
  String result = first + " " + second; // Using `+` operator
  System.out.println(result); // Output: Hello World

2. Length:

• Returns the number of characters in a string.
• Example:

  String str = "Java";
  System.out.println(str.length()); // Output: 4

3. charAt:

• Returns the character at a specified index.
• Example:

  String str = "Java";
  System.out.println(str.charAt(2)); // Output: v

4. Substring:

• Extracts a part of the string.
• Example:

  String str = "Programming";
  System.out.println(str.substring(3, 8)); // Output: gramm

5. Equals:

• Compares two strings for equality.
• Example:

  String str1 = "Java";
  String str2 = "java";
  System.out.println(str1.equals(str2)); // Output: false
  System.out.println(str1.equalsIgnoreCase(str2)); // Output: true

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6. Input/Output

a. Scanner Class - Implementation

• The Scanner class in java.util is used to take input from the user.

• Key Methods:

  • nextInt(): Reads an integer.
  • nextDouble(): Reads a double.
  • nextLine(): Reads a full line (including spaces).
  • nextBoolean(): Reads a boolean.
  • next(): Reads a single word (until a space is encountered).

• Example Implementation:

  import java.util.Scanner;
  
  public class InputExample {
      public static void main(String[] args) {
          Scanner scanner = new Scanner(System.in);
  
          System.out.print("Enter an integer: ");
          int num = scanner.nextInt();
  
          System.out.print("Enter a double: ");
          double decimal = scanner.nextDouble();
  
          scanner.nextLine(); // Consume the leftover newline
  
          System.out.print("Enter a string: ");
          String text = scanner.nextLine();
  
          System.out.println("Integer: " + num);
          System.out.println("Double: " + decimal);
          System.out.println("String: " + text);
  
          scanner.close();
      }
  }

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b. Common Scanner Methods

1. nextInt():

• Reads an integer value.
• Example:

  int number = scanner.nextInt();

2. nextDouble():

• Reads a double value.
• Example:

  double price = scanner.nextDouble();

3. nextLine():

• Reads the entire line (including spaces).
• Example:

  String sentence = scanner.nextLine();

4. nextBoolean():

• Reads a boolean value (true or false).
• Example:

  boolean isActive = scanner.nextBoolean();

5. next():

• Reads a single word until a space or newline is encountered.
• Example:

  String word = scanner.next();

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c. BufferedReader

• Used for faster input than the Scanner class.

• Requires handling exceptions.

• Example:

  import java.io.BufferedReader;
  import java.io.InputStreamReader;
  import java.io.IOException;
  
  public class BufferedReaderExample {
      public static void main(String[] args) {
          BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
  
          try {
              System.out.print("Enter a string: ");
              String input = reader.readLine();
  
              System.out.println("You entered: " + input);
          } catch (IOException e) {
              System.out.println("An error occurred: " + e.getMessage());
          }
      }
  }

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7. Type Casting

a. Implicit

int x = 10;
double y = x; // Automatic conversion

b. Explicit

double d = 10.5;
int i = (int) d; // Manual conversion

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8. Constants

a. final Keyword

final int MAX = 100;

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9. Arrays

a. Declaration and Access

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

b. For Each Loop

for (int num : arr) {
    System.out.println(num);
}

c. 2D Array

int[][] matrix = {{1, 2}, {3, 4}};

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10. Conditional Statements

a. If-Else

if (x > 0) {
    System.out.println("Positive");
} else {
    System.out.println("Negative");
}

b. Switch

switch (day) {
    case 1: System.out.println("Monday"); break;
    default: System.out.println("Invalid");
}

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11. Loops

a. For Loop

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

b. While Loop

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

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12. Exception Handling

Try-Catch

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
}