Reading input

Input refers to text written by the user read by the program. Input is always read as a string. For reading input, we use the Scanner tool that comes with Java. The 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);

        // We can now use the scanner tool.
        // This tool is used to read input.
    }
}

Below is an example of a program which asks for user input, reads the string entered by the user, and then prints it.

// 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();

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

More precisely, input is read with the scanner tool's nextLine() method. The call scanner.nextLine() is left waiting for the user to write something. When user writes something and presses enter, the provided string is assigned to a string variable (in this instance message). The program is then able to reference the variable message later on — in the example above, the variable message is referenced in the print command.

When the program is run, its output can look like the example below. In this example, the user has written the text "Hello world" — user input is marked with red in the sample examples.

The video below shows the process of making a program that reads user input. Watch the video before doing the next programming exercise. Take special notice of how the user input is provided to the Output window located at the bottom of TMC as the program is running.

Next up, let's take a step back, and examine what on earth String message = ... even means.

Fundamentals of Strings

As you might have noticed, in programming we refer to "strings" rather than "text". The term "string" is shorthand for "string of characters" which describes how the computer sees text on a more fundamental level: as a sequence of individual characters.

We've so far used strings in two ways. When practicing the print command, we passed the string to be printed to the print command in quotation marks, and when practicing reading input, we saved the string we read to a variable.

In practice, variables are named containers that contain information of some specified type and have a name. A string variable is declared in a program by stating the type of the variable (String) and its name (myString, for instance). Typically a variable is also assigned a value during its declaration. You can assign a value by following the declaration with an equals sign followed by the value and a semicolon.

A string variable called message that is assigned the value "Hello world!" is declared like this:

String message = "Hello world!";

When a variable is created, a specific container is made available within the program, the contents of which can later be referenced. Variables are referenced by their name. For instance, creating and printing a string variable is done as shown below:

String message = "Hello world!";
System.out.println(message);

A string enclosed in a programming language's quotation marks is called a "string literal", i.e., a string with a specified value. A common programming mistake is trying to put quotation marks around variable names. If there were quotation marks around the string variable message, the program would print the text "message" instead of the "Hello world!" text held by the message variable.

String message = "Hello world!";
System.out.println("message");

Concatenation - Joining Strings Together

The string to be printed can be formed from multiple strings using the + operator. For example, the program below prints "Hello world!" on one line.

public class Program {

    public static void main(String[] args) {
        System.out.println("Hello " + "world!");
    }
}

The same method can be used to join a string literal and the value of a string variable.

public class Program {

    public static void main(String[] args) {
        String message = "Hello world!";

        System.out.println(message + " ... and the universe!");
    }
}

We can do the same with any number of strings.

public class Program {

    public static void main(String[] args) {
        String start = "My name is ";
        String end = ", James Bond";

        System.out.println(start + "Bond" + end);
    }
}

Reading Strings

The reader.nextLine(); command reads the user's input and returns a string. If we then want to use the string in the program, it must be saved to a string variable — String message = scanner.nextLine();. A value saved to a variable can be used repeatedly. In the example below, the user input is printed twice.

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

public class Program {

    public static void main(String[] args) {

        //Create the tool for reading, assign it to variable caller "scanner
        Scanner scanner = new Scanner(System.in);

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

        //Read the user's string input, save it to program memory
        //"String message = (user input)"
        String message = scanner.nextLine();

        // Print the user input twice
        System.out.println(message);
        System.out.println(message);
    }
}

Input String as a Part of Output

We noticed in the "Hi Ada Lovelace!" exercise that string literals and string variables can be joined using the + operator. The example below demonstrates a program that takes user input and prints it concatenated with a string literal.

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();

        System.out.println("You wrote " + message);
    }
}

Program Execution Waits for Input

When the program's execution comes 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. When the user writes some text, the program prints Write the second string: , and then waits for user input again. This continues for a third time, after which the program prints all three strings.

import java.util.Scanner;

public class Program {

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

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

        System.out.println("You wrote:");
        System.out.println(first);
        System.out.println(second);
        System.out.println(third);
    }
}

In the previous example, we saved the user input to three different string variables. This can be done as long as the variables all have different names (in the example, the names are first, second and third)

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();

        System.out.println("You wrote:");
        System.out.println(first);
        System.out.println(second);
        System.out.println(third);
    }
}

We can form more complicated texts whose content changes depending on the user's input by using more strings. In the example below, the user is told a bit more about the texts they wrote — notice that the order in which the strings are printed can be changed. In the example below, the third input string is printed first.

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();

        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);
    }
}

Remember to check your points from the ball on the bottom-right corner of the material!