Basic Input Output

A school program needs reliable data and clear rules. Input supplies values while a program is running; output reports its results. Scanner can read a number from a text source using the same nextInt operation used with keyboard input.

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.

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.

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.println("Enter your name: ");
        String name = input.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

Enter your name: 
Ram
Hello, Ram!

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)

Example: Reading Numbers

import java.util.Scanner;

public class SumCalculator {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter first number: ");
        int num1 = input.nextInt();
        System.out.println("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.");
}

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

Java · example discussed here

input.nextLine(); // consume leftover newline

• 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

nextInt leaves the line ending behind when reading keyboard text; an immediate nextLine may read the rest of that same line. Validate input before using it.

Before running this Java program, write down each line you expect to see. Identify the value, branch, object or exception responsible for each line. Then explain whether the following assumption is valid: nextInt leaves the line ending behind when reading keyboard text; an immediate nextLine may read the rest of that same line.

Java 17 · Main.java · complete practice program

public class Main  {
  public static void main(String[] args)  {
    java.util.Scanner input = new java.util.Scanner("72");
    int marks = input.nextInt();
    System.out.println(marks);
    input.close();
  }
}

Java 17 · Main.java · source repeated for this answer

public class Main  {
  public static void main(String[] args)  {
    java.util.Scanner input = new java.util.Scanner("72");
    int marks = input.nextInt();
    System.out.println(marks);
    input.close();
  }
}

Expected output

72

Input supplies values while a program is running; output reports its results. Scanner can read a number from a text source using the same nextInt operation used with keyboard input.

nextInt leaves the line ending behind when reading keyboard text; an immediate nextLine may read the rest of that same line. Validate input before using it.

Run with javac Main.java, then java Main. The declaration of Main names the class; main starts the example, and println writes a complete output line.

Input supplies values while a program is running; output reports its results. Scanner can read a number from a text source using the same nextInt operation used with keyboard input. nextInt leaves the line ending behind when reading keyboard text; an immediate nextLine may read the rest of that same line. Validate input before using it.

Related explanations: Java Variables, Java If Statement - Syntax and Examples, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide and Java 17 language specification.