What is System.in and nextInt() ?

 Let’s break it down simply 👇

🧩 1. System.in

• System is a built-in Java class that gives access to system-level input and output.

• in is a static field inside the System class.

• System.in represents standard input stream — usually the keyboard.

💡 In short:
System.in = the stream that takes input from the keyboard.

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🧮 2. nextInt() (from Scanner class)

• nextInt() is a method of the Scanner class.

• It reads the next integer value from the input stream connected to the scanner.

Example:

import java.util.Scanner;

public class Example {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in); // connect scanner to keyboard input
        System.out.print("Enter a number: ");
        int num = sc.nextInt();              // reads an integer from user
        System.out.println("You entered: " + num);
        sc.close();
    }
}

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🔍 Step-by-step of what happens:

1. new Scanner(System.in) → connects the scanner to keyboard input.

2. sc.nextInt() → waits for the user to type an integer and press Enter.

3. It reads that integer and stores it in a variable (like num).

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🧠 Summary Table

Expression	Type	Meaning
System.in	Input Stream	Takes input from keyboard
new Scanner(System.in)	Scanner Object	Reads input from user
nextInt()	Method	Reads the next integer value entered

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Would you like me to also explain what happens internally (how Java converts the typed number to an integer)?