Input/Output

Input/Output (IO for short) refers to how a program communicates with the outside world; specifically, how it receives (input) and sends (output) information. So far, we've mostly talked about how data is stored and manipulated, but a program isn't very useful if it can't interact with a user.

Output

Output is the easier of the two, and you've likely already seen it used throughout this guide. Java provides the built-in System.out object to print information to the console.

Method	Description
System.out.print()	Prints text with no line break afterward
System.out.println()	Prints text followed by a line break

System.out.print("Motor Voltage: ");
System.out.println(6.5);

Output:

Motor Voltage: 6.5

Multiple values can be combined into a single output statement using the + operator, which concatenates (joins together) values into a String.

int numMotors = 4;
System.out.println("Number of motors: " + numMotors);

Output:

Number of motors: 4

Input

To receive input from a user, Java provides the Scanner class, which reads data typed into the console. Unlike System.out, Scanner isn't automatically available; it must be imported and instantiated (created) first.

Importing Scanner

Since Scanner isn't part of the basic Java language, it must be imported at the top of the file, above the class declaration.

import java.util.Scanner;

For more information about imports and packages, see the Object-Oriented Programming section.

Creating a Scanner

Once imported, a Scanner object can be created to read from System.in, which represents input typed into the console.

Scanner scanner = new Scanner(System.in);

For more information about objects and the new keyword, see the Object-Oriented Programming section.

Reading Input

Once a Scanner object exists, it can be used to read different types of input, depending on the method called.

Method	Description
nextInt()	Reads the next int
nextDouble()	Reads the next double
nextBoolean()	Reads the next boolean
next()	Reads the next word (String)
nextLine()	Reads an entire line (String)

Scanner scanner = new Scanner(System.in);

System.out.print("Enter the number of motors: ");
int numMotors = scanner.nextInt();

System.out.println("You have " + numMotors + " motors.");

Mixing nextLine() with Other next Methods

One common pitfall when using Scanner is mixing nextLine() with the other next methods. Methods like nextInt() only consume the number itself, leaving the leftover newline character in the input behind. If nextLine() is called immediately after, it reads that leftover newline instead of waiting for new input, which can seem like it was skipped entirely.

Scanner scanner = new Scanner(System.in);

System.out.print("Enter your age: ");
int age = scanner.nextInt();

System.out.print("Enter your name: ");
String name = scanner.nextLine(); // This gets skipped!

A common fix is to add an extra scanner.nextLine(); call after nextInt() to consume the leftover newline before reading the next line of input.