Java User Input Tutorial for Beginners | Scanner & Variables Explained

Learn how to make Java programs interactive using the Scanner class. Step-by-step guide to reading user input, using variables, and building a simple calculator for numbers and decimals. Perfect for beginners!

The Scanner class in Java is a built-in utility class that allows your program to read input from various sources, most commonly from the keyboard (user input). It’s part of the java.util package, so you need to import it before using it:

import java.util.Scanner;

Key Points About Scanner:

1. Purpose: The Scanner class reads data typed by the user, such as text, numbers, or boolean values, and stores it in variables for your program to use.

2. Creating a Scanner Object: Before reading input, you create a Scanner object and connect it to a source. For keyboard input:

   Scanner input = new Scanner(System.in);
   

   • input → the name of the Scanner object
   • System.in → tells Scanner to listen to the standard input (keyboard)

3. Common Methods:

To make your program interactive, we use a built-in Java tool called the Scanner. This allows the program to pause and wait for the user to type something on their keyboard.

The Interactive Greeting Program – Java Scanner Example

This program shows how to make your Java program interactive. Instead of just printing fixed messages, it can ask the user for input and respond dynamically.

Save the code in a file named:

InputExample.java

import java.util.Scanner; // Step 1: Import the Scanner class

public class InputExample {
    public static void main(String[] args) {
        // Step 2: Create a Scanner object to read input
        Scanner reader = new Scanner(System.in);

        System.out.print("Enter your name: ");
        
        // Step 3: Read the input typed by the user
        String name = reader.nextLine();

        System.out.println("Nice to meet you, " + name + "!");
        
        // Step 4: Close the Scanner to free resources
        reader.close();
    }
}

How This Program Works

Think of Scanner as a “telephone line” connecting the keyboard to your code. The program waits for the user to type and then responds.

1️⃣ Import the Scanner Class

import java.util.Scanner;

• Java doesn’t automatically load all tools to keep programs efficient.
• This line tells Java:

“I want to use the Scanner class from the utility library.”

2️⃣ Create the Scanner Object

Scanner reader = new Scanner(System.in);

• reader → the name of your Scanner object
• System.in → tells Scanner to listen to the keyboard
• Now your program can “hear” what the user types.

3️⃣ Read Input From the User

String name = reader.nextLine();

• nextLine() waits for the user to type a full line of text and press Enter.
• The text is stored in the variable name.
• You can now use this variable anywhere in your program.

4️⃣ Display Output

System.out.println("Nice to meet you, " + name + "!");

• Combines the fixed text with the user input using concatenation (+).
• Prints a personalized greeting on the screen.

5️⃣ Close the Scanner

reader.close();

• Always close the Scanner when you are done.
• Frees system resources and prevents memory issues.

Quick Tip for Beginners

⚠️ Common Issue: Mixing nextInt() and nextLine()

If you use nextInt() (or nextDouble()) and then immediately call nextLine(), Java may skip the input. ✅ Fix: Add an extra nextLine() after nextInt() to clear the leftover newline.

int age = reader.nextInt();
reader.nextLine(); // clears the input buffer
String name = reader.nextLine();

💡 For absolute beginners, it’s easiest to use nextLine() for all input at first.

Example Run

Enter your name: Ali
Nice to meet you, Ali!

Common Beginner Mistakes

❌ Forgetting import statement ❌ Wrong file name ❌ Not closing scanner ❌ Using wrong input method

Tasks

Task #1

Modify the program to ask:

• Name
• Age

Then print:

Hello Ali, you are 15 years old!

Task #2

Create a program that asks:

1. Name
2. Favorite subject
3. Favorite number

Then prints a personalized message.

The Simple Calculator

Create a file named Calculator.java and paste this in:

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.println("--- Java Addition Tool ---");

        // 1. Get the first number
        System.out.print("Enter first number: ");
        double num1 = input.nextDouble();

        // 2. Get the second number
        System.out.print("Enter second number: ");
        double num2 = input.nextDouble();

        // 3. Perform the calculation
        double sum = num1 + num2;

        // 4. Output the result
        System.out.println("The total is: " + sum);

        input.close();
    }
}

Key Concepts Used

1. The double Type

We used double instead of int. Why? Because int only handles whole numbers (1, 2, 3). If a user types 5.5, an int would crash the program. A double can handle both.

2. Basic Arithmetic

Java uses standard symbols for math:

• Addition: +
• Subtraction: -
• Multiplication: *
• Division: /

3. Order of Operations

Just like in math class, Java follows PEMDAS (Parentheses, Exponents, Multiplication/Division, Addition/Subtraction). If you want to do a complex calculation, use parentheses: double result = (num1 + num2) * 10;

Common Beginner Mistake: The “Input Bug”

If you try to type a letter when the program asks for a nextDouble(), the program will crash with an InputMismatchException. Java is very strict—if it expects a number, you must give it a number!

📘 Related Topics

• Standard Output
• Data Types and Variables