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Java User Input with Scanner: nextInt, nextLine and Validation

Read keyboard input in Java with the Scanner class: reading numbers, words and whole lines, the nextInt + nextLine trap and how to fix it, checking input with hasNextInt, reading until the input ends, and closing the Scanner.

Beginner9 min readUpdated
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Programs become interactive when they read input. In Java, the easiest way to read what the user types is the Scanner class from java.util.

In the examples below, the output shows exactly what you would see in the terminal, including what the user typed after each prompt.

Creating a Scanner#

Java
import java.util.Scanner;

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

        System.out.print("Your name: ");
        String name = in.next();
        System.out.print("Your age: ");
        int age = in.nextInt();

        System.out.println("Hi " + name + ", next year you will be " + (age + 1));
    }
}
Output
Your name: Asha
Your age: 21
Hi Asha, next year you will be 22
  • new Scanner(System.in) reads from the keyboard (standard input).
  • System.out.print (not println) keeps the cursor on the prompt line.
  • Create one Scanner for System.in and reuse it for every read.

Scanner methods#

MethodReads
next()One word (stops at whitespace)
nextLine()The rest of the current line, including spaces
nextInt()An int
nextDouble()A double
nextBoolean()true or false
hasNextInt(), hasNextLine(), …Whether the next input can be read as that type

Reading whole lines#

next() stops at the first space. Use nextLine() for text that can contain spaces:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Full name: ");
        String fullName = in.nextLine();
        System.out.println("Hello, " + fullName + "!");
    }
}
Output
Full name: Asha Rao
Hello, Asha Rao!

The nextInt + nextLine trap#

After nextInt(), the Enter key the user pressed is still waiting in the input. A following nextLine() reads that empty remainder and returns "" immediately. Fix it by calling nextLine() once to consume the leftover line break:

Java
import java.util.Scanner;

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

        System.out.print("Age: ");
        int age = in.nextInt();
        in.nextLine();                 // discard the rest of the line

        System.out.print("Full name: ");
        String name = in.nextLine();

        System.out.println(name + " is " + age + ".");
    }
}
Output
Age: 21
Full name: Asha Rao
Asha Rao is 21.

A popular alternative is to read every input with nextLine() and convert it yourself with Integer.parseInt(line.trim()).

Validating input#

If the user types abc where nextInt() expects a number, it throws an InputMismatchException. Check first with hasNextInt(), and skip bad input with next():

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int age;

        while (true) {
            System.out.print("Age: ");
            if (in.hasNextInt()) {
                age = in.nextInt();
                if (age > 0 && age < 130) {
                    break;
                }
            } else {
                in.next();   // throw away the invalid word
            }
            System.out.println("Please enter a whole number between 1 and 129.");
        }
        System.out.println("Thank you. You are " + age + ".");
    }
}
Output
Age: abc
Please enter a whole number between 1 and 129.
Age: -5
Please enter a whole number between 1 and 129.
Age: 30
Thank you. You are 30.

Reading until the input ends#

hasNextInt() returns false at the end of the input, which makes it easy to process a list of numbers:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int sum = 0;
        int count = 0;
        while (in.hasNextInt()) {
            sum += in.nextInt();
            count++;
        }
        System.out.println(count + " numbers, sum " + sum);
    }
}
Output
4 8 15 16 23 42
6 numbers, sum 108

In a terminal, end the input with Ctrl + D (macOS/Linux) or Ctrl + Z then Enter (Windows).

Closing the Scanner#

A Scanner wrapping a file should be closed when you are done (see File Handling). A Scanner on System.in is different: closing it also closes System.in, and no later code can read the keyboard again. In small programs, simply leave it open.

Common mistakes#

Practice#

Practice exercise

Ask for the width and height of a rectangle (decimals allowed) and print its area and perimeter. In the example run, the user types 4.5 and 2.

Show solution
Java
import java.util.Scanner;

public class RectangleInput {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Width: ");
        double width = in.nextDouble();
        System.out.print("Height: ");
        double height = in.nextDouble();

        System.out.println("Area: " + width * height);
        System.out.println("Perimeter: " + 2 * (width + height));
    }
}
Output
Width: 4.5
Height: 2
Area: 9.0
Perimeter: 13.0

Key takeaway#

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