Reading Input

Input refers to text written by a user and read by a program. To read input from a user, the Scanner tool from Java is used. This tool can be imported for use in a program by adding the command import java.util.Scanner; before the beginning of the main program's frame (public class ...). The tool itself is created with Scanner scanner = new Scanner(System.in);.

import java.util.Scanner;

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

Below is an example of a program that prompts user input, reads the entered text, and prints it. The comments will guide you through the example for understanding. When finished with receiving input, one should close the scanner with the command scanner.close();.

Close the scanner only after you're done getting input, with scanner.close(). But be cautious, as this also closes System.in, preventing the creation of a new scanner that uses System.in. Close it only when you're sure you won't use System.in again.

// Introduce the scanner tool used for reading user input
import java.util.Scanner;

public class Program {
    public static void main(String[] args) {
        // Create a tool for reading user input and name it scanner
        Scanner scanner = new Scanner(System.in);

        // Print "Write a message: "
        System.out.println("Write a message: ");

        // Read the string written by the user, and assign it to program memory "String message = (string that was given as input)"
        String message = scanner.nextLine();

        // Close the scanner, because we received all input
        scanner.close();

        // Print the message written by the user
        System.out.println(message);
    }
}

Input is read using the scanner tool's nextLine() method. The nextLine() call waits for the user to enter text and press enter. The entered text is then assigned to the String variable message, where String is the variable type used in Java to store text. The program can reference the message variable later, for example to print it. When running the program, enter a message in the console as if you were the user. The nextLine() command reads the input and returns it as a String. To use the returned String in the program, it must be saved to a String variable (e.g. String message = scanner.nextLine();). A value saved in a variable can be reused.

Input String as a Part of Output

To integrate user input with other text as output, we use the addition (+) operator to join two pieces of text (Strings) together. An example of this can be seen in the following program, that takes user input and combines it with another piece of text to print it as final output.

import java.util.Scanner;

public class Program {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Write something: ");
        String message = scanner.nextLine();
        scanner.close();
        System.out.println("You wrote " + message);
    }
}

In examples of the output we indicate in bold what the user has entered in the scanner. When running the program, the following will happen:

Message in the compiler: Write something: will be printed

The user inputs this and presses enter.

The output will be: You wrote this

Program Execution Waits for Input

When the program's execution reaches a statement that attempts to read input from the user (the command reader.nextLine()), the execution stops and waits. The execution continues only after the user has written some input and pressed enter.

In the example below, the program prompts the user for three strings. First, the program prints Write the first string:, and then waits for user input. Once the user has entered a string and pressed enter, the program prints the next prompt and waits for another string. This process repeats for three strings in total. After the program has received all three strings, it prints them. Note that if the scanner is done receiving input, the scanner should be closed.

import java.util.Scanner;

public class Program {

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

        System.out.println("Write the first string:");
        String first = reader.nextLine();
        System.out.println("Write the second string:");
        String second = reader.nextLine();
        System.out.println("Write the third string:");
        String third = reader.nextLine();
        reader.close();

        System.out.println("Last string you wrote was " + third + ", which ");
        System.out.println("was preceded by " + second+ ".");
        System.out.println("The first string was" + first + ".");

        System.out.println("All together: " + first + second + third);
    }
}