Java Basic Input and Output - Scanner, System.in, System.out

Basic Input and Output in Java

When you first write a Java program, it’s natural to want interaction — a two-way conversation between the user and the machine. Java enables this through standard input and output streams.

This tutorial will walk you through how to accept user input and print output using beginner-friendly and practical approaches.

Java Output – Writing to Console

Let’s begin with the most common way to display information in Java: System.out.print() and System.out.println().

Using System.out.print() and println()

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

Hello World!

Explanation:

System.out.print() prints without moving to the next line, whereas System.out.println() adds a new line after printing. It's essential to use the right one depending on whether you want continued or separate output lines.

Java Input – Reading from User

To accept input from the keyboard, Java commonly uses the Scanner class from the java.util package.

Example: Reading User Input

import java.util.Scanner;

public class UserInputExample {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = input.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

Enter your name: Alice
Hello, Alice!

Explanation:

input.nextLine() reads a full line of text. Scanner provides different methods for different types of input:

• nextInt() – for integers
• nextDouble() – for floating-point numbers
• next() – for single word (stops at whitespace)

Reading Different Types of Input

Example: Reading Numbers

import java.util.Scanner;

public class SumCalculator {
    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);
    }
}

Enter first number: 10
Enter second number: 20
Sum: 30

Checks and Validations

While using nextInt(), if the user inputs something non-numeric, your program will throw an InputMismatchException. To handle such cases, use input validation:

if (input.hasNextInt()) {
    int value = input.nextInt();
} else {
    System.out.println("Invalid input. Please enter a number.");
}

Common Input Mistakes and How to Avoid Them

Problem: nextLine() after nextInt()

If you use nextLine() after nextInt(), you might skip input. This happens because nextInt() doesn’t consume the newline character. Use an extra nextLine() to clear the buffer:

input.nextLine(); // consume leftover newline

Key Takeaways

• Use System.out.print() and System.out.println() for output
• Use Scanner for reading user input
• Match method to input type: nextLine(), nextInt(), nextDouble(), etc.
• Validate input to avoid runtime errors
• Handle the newline issue when switching input types