first java program

Structure of Java File

In Java, source code is saved with the .java extension. Each .java file represents a class, and the class within it must have the same name as the file. The first alphabet of the class name is conventionally in uppercase, although it is not mandatory. The class with the same name as the file must be declared as a public class, and it should contain a main method, which is the entry point of the program.

Converting .java to .class

The javac compiler is used to convert a .java file to a .class file containing bytecode. The command is as follows:

javac Main.java

Running the Program

Hello World Program

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

Explanation:

1. public: Access modifier allowing access from anywhere.

2. class: Keyword representing a group of properties and functions.

3. Main: Class name, matching the file name.

4. public: Allows the program to use the main function from anywhere.

5. static: Keyword allowing the main method to run without using objects.

6. void: Keyword indicating no return value from the method.

7. main: The name of the method.

8. String[] args: Command line arguments of string type array.

9. System: A final class in the java.lang package.

10. out: A variable of PrintStream type, a public and static member field of the System class.

11. println: A method of PrintStream class, printing arguments and adding a new line.

What is a Package?

• A package in Java is a folder containing Java files.

• It provides organizational rules for programs.

Primitive Data Types

Primitive data types in Java are non-breakable and include int, char, float, double, and long. These represent integer, character, floating-point, double-precision, and long integer values, respectively. Boolean stores only true or false. string is not a primitive data type.

int i = 26;
char c = 'A';
float f = 98.67f; // Note the 'f' suffix for float
double d = 45676.58975;
long l = 15876954832558315L; // Note the 'L' suffix for long
boolean b = false;

Literals and Identifiers

• Literals are fixed values that are used in your program. They are constants that represent specific values. For example:

int number = 42;  // Here, 42 is a literal.
char character = 'A';  // 'A' is a literal character.
double pi = 3.14;  // 3.14 is a literal of type double.

• Identifiers are names given to program elements such as variables, constants, functions, classes, and more. They serve as labels for these elements, allowing you to refer to and manipulate them in your code. Some rules for creating valid identifiers include:

  • They must start with a letter, underscore (_), or dollar sign ($).

  • Subsequent characters can be letters, digits, underscores, or dollar signs.

  • Java is case-sensitive, so variable and Variable would be different identifiers. Examples of identifiers:

  int studentCount;  
  // 'studentCount' is an identifier for a variable.
  final double PI_VALUE = 3.14159; 
  // 'PI_VALUE' is an identifier for a constant.
  void calculateSum() { /* ... */ } 
  // 'calculateSum' is an identifier for a function.
  

Comments in Java

Comments in Java are written in the source code but ignored by the compiler. There are single-line comments (//) and multi-line comments (/* */).

Inputs in Java

The Scanner class in java.util is used to take input. To use it:

1. Import java.util.Scanner.

2. Create a Scanner object.

3. Use the object to take input from the keyboard.

import java.util.Scanner;
public class Main{
    public static void main(String[] args){
        Scanner input = new Scanner(System.in);
    }
}

Input Methods

• nextInt(): Takes input of type int.

• nextFloat(): Takes input of type float.

• next(): Takes one word input until a space occurs.

• nextLine(): Takes the entire string input, including spaces.

Sum of Two Numbers

import java.util.Scanner;
public class Sum {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter first number");
        int num1 = input.nextInt();
        System.out.print("Enter second number");
        int num2 = input.nextInt();
        int sum = num1 + num2;
        System.out.println("Sum = " + sum);
    }
}

Conclusion

These topics cover the basics of Java programming, a widely-used language for developing applications. Java is platform-independent, meaning it can run on any operating system without modifications. Understanding these topics will lay the foundation for your Java programming journey. The examples provided help you grasp concepts like variables, data types, control statements, arrays, classes, objects, and methods. Through hands-on practice, you'll gain confidence in coding and problem-solving with Java. Remember, learning Java is iterative and requires patience and practice. Start now and enjoy your learning experience!