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01 - Input

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#java/basics #l0 #concept #must-know #status/known

Reading User Input (Scanner)

What is it?

Input is data the user types into the program while it’s running. Java reads this using the Scanner class - a tool built into Java’s standard library.

Why does it exist?

Without input, programs are static - they do the same thing every time. Input makes programs dynamic and interactive (e.g., asking a user for their name, a number to calculate, etc.).

How does it work?

Step 1 - Import Scanner

Scanner isn’t available by default. You have to import it at the top of your file:

import java.util.Scanner;

This tells Java: “I want to use the Scanner class from the java.util package.”

Step 2 - Create a Scanner object

Scanner scanner = new Scanner(System.in);

Breaking this down:

Part Meaning
Scanner The type (class) we’re using
scanner The variable name we chose (can be anything)
new Scanner(...) Creates a new Scanner instance
System.in The input stream - i.e., what the user types in the keyboard

Note System.in is the keyboard input, just like System.out is the screen output.

Step 3 - Read input with nextLine()

String userInput = scanner.nextLine();
  • scanner.nextLine() pauses the program, waits for the user to type something and press Enter, then returns what they typed as a String
  • You store it in a variable to use it later

Full working example

import java.util.Scanner;

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

        System.out.println("What is your name?");
        String name = scanner.nextLine();

        System.out.println("Hello, " + name + "!");
    }
}

If user types Alice:

What is your name?
Alice
Hello, Alice!

Reading Different Types of Input

nextLine() always returns a String. To read numbers, you need to either use a different method or convert the string:

Method Returns Use when
scanner.nextLine() String Reading text or a whole line
Integer.parseInt(scanner.nextLine()) int Reading a whole number
Double.parseDouble(scanner.nextLine()) double Reading a decimal number 
System.out.println("Enter your age:");
int age = Integer.parseInt(scanner.nextLine());
System.out.println("You are " + age + " years old.");

Tip Prefer scanner.nextLine() + Integer.parseInt() over scanner.nextInt(). The nextInt() method leaves a leftover newline character in the buffer which causes bugs when you later try to read a string.

Gotchas

  • Forgetting import java.util.Scanner; → Scanner won’t be found, compilation error
  • scanner.nextLine() blocks (pauses) the program until Enter is pressed - this is intentional
  • If the user types letters when you expect a number and use Integer.parseInt(), it will throw a NumberFormatException at runtime

Related: Printing, String, Loops

Linked references: