1. Variables and Constants

Variables and constants are the primary data holders in a Java program.

• Variables: They are used to store data that may change during program execution. For example:
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  int age = 25; // Variable to store age
• Constants: Defined using the final keyword, constants store values that remain unchanged throughout the program. For example:
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  final double PI = 3.14159; // A constant for Pi

2. Data Types

Java is a statically-typed language, meaning every variable must have a defined data type. The main types include:

• Primitive Data Types: These include int, double, char, boolean, etc.
• Reference Data Types: Used for objects like arrays, strings, and user-defined classes.

For example:

3. Classes and Objects

• Classes: The blueprint for creating objects. A class encapsulates data and methods that operate on that data.
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  public class Car { String color; void drive() { System.out.println("Car is driving"); } }
• Objects: Instances of a class. They represent the real-world entities in a program.
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  Car myCar = new Car(); // Creating an object of Car

4. Methods

Methods are blocks of code designed to perform specific tasks. They help in reusing code and improving modularity.

• Syntax:
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  returnType methodName(parameters) { // method body }
• Example:
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  public int addNumbers(int a, int b) { return a + b; }

5. Control Flow Statements

Control flow statements determine the order in which instructions are executed. These include:

• Conditional Statements: if, else, switch
• Loops: for, while, do-while
• Example:
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  if (age >= 18) { System.out.println("You are an adult."); }

6. Arrays

Arrays store multiple values of the same data type in a single variable.

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

7. Packages

Packages organize classes into namespaces, avoiding name conflicts and providing modularity.

• Example:
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  package mypackage; import java.util.Scanner;

8. Exception Handling

Java provides robust exception-handling mechanisms to manage runtime errors using try, catch, and finally blocks.

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

9. Input/Output (I/O)

Java’s I/O system handles data input from users and outputs results. It typically uses classes like Scanner for input and System.out for output.

• Example:
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  Scanner scanner = new Scanner(System.in); System.out.print("Enter your name: "); String name = scanner.nextLine(); System.out.println("Hello, " + name);

10. Comments

Comments are non-executable statements used for documentation and code explanation.

• Single-line comment: Starts with //
• Multi-line comment: Enclosed in /* */
• Javadoc comment: Begins with /** and used for generating documentation

Example: