Fundamentals

Hybrid Language

Java file is firstly compiled using javac compiler.

And .class file is generated.

Then It is executed using interpreter inside JVM, so, no new file is generated at this stage, since Interpreter is used.

Platform Independent

Java language is platform independent, because all the system calls are handled by JVM.

So, after compiling the Java program, we can port the compiled(.class) file to other OS, and JVM for that particular OS can execute the .class file.

But in languages like C++, system calls are directly inside the compiled(.exe) file.

So, we cannot port the compiled file to other OS.

Running a Java program

Firstly, we need to compile the Java program.

javac First.java

A .class file is generated after this. (First.class)

Then, we'll run the Java program.

java First

Java File Name

It is NOT compulsory to have file name same as class name.

It is compulsory only when the class is declared public.

Case 1

// "First.java"

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

We can run the program,

javac First.java
java First

Case 2

// "First.java"

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

We can run the program,

javac First.java
java Second

Here, we need to take care when we are executing java byte code.

This happens because generated .class file name will have the same as the class name. (Second.class)

Case 3

"First.java"

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

Try running this program:

javac First.java

This will generate error, since class Second is declared public.

So to solve the problem, we compulsory has to have same class name as file name.

`main()` in Java

If the main() doesn't contain String[] args as parameter, still the program will be compiled successfully.

But there will be an error when executing the program.

Output in Java

public class Main {
    public static void main(String[] args) {
        System.out.print("Hello World!");       // Prints in the same Line
        System.out.println("Hello World!");     // Prints and then Moves printing pointer to the Next Line
        System.out.print("Hello World!\n");     // It also Prints and then Moves the printing pointer to the Next Line
    }
}

Input in Java

To take input we need a object of Scanner class.

import java.util.Scanner;

Class has many methods, for input of different data types.

Scanner sc = new Scanner(System.in);
String fName = sc.next();   // Word (until a space is encountered)
System.out.println("Hello " + fName) ;
sc.close();

String name = sc.nextLine();  // Multiples Words
System.out.println("Hello " + name);

nextBoolean() Reads a boolean value from the user

nextInt() Reads a int value from the user

nextLong() Reads a long value from the user

nextFloat() Reads a float value from the user

nextDouble() Reads a double value from the user

Last modified: 08 February 2026