Computer Language
Lecture 2

Reading Input and Strings

Josue Obregon
jobregon@seoultech.ac.kr

Seoul National University of Science and Technology
Information Technology Management
Lecture slides index

April 14, 2025

Class contents

  1. Introduction to Computer Programming and Java
  2. Reading Input and Strings
  3. Variables and calculation with numbers
  4. Conditional statements
  5. Repeating functionality
  6. Methods & Debugging
  7. Lists, Arrays and Strings
  8. Midterm
  1. Introduction to Object Oriented Programming
  2. Deeper look into Object Oriented Programming
  3. Inheritance and Interfaces I
  4. Inheritance and Interfaces II
  5. Class diagrams, packages and exceptions
  6. Project Live Code Review and Evaluation
  7. Final examination

Class contents

  1. Introduction to Computer Programming and Java
  2. Reading Input
  3. Variables and calculation with numbers
  4. Conditional statements
  5. Repeating functionality
  6. Methods & Debugging
  7. Lists, Arrays and Strings
  8. Midterm
  1. Introduction to Object Oriented Programming
  2. Deeper look into Object Oriented Programming
  3. Inheritance and Interfaces I
  4. Inheritance and Interfaces II
  5. Class diagrams, packages and exceptions
  6. Project Live Code Review and Evaluation
  7. Final examination

Agenda

  • Reading input from the user
  • Fundamentals of Strings
  • Concatenation - Joining Strings Together
  • Reading Strings
  • Input String as a Part of Output

Objectives

  • Learn to write a program that reads text written by a user.

  • Know what a “string” refers to in programming.

  • Know how to join (i.e., “concatenate”) strings together.

Interacting with the user

  • 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.
    }
}

Using the Scanner tool

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

// 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);
    }
}

Using the Scanner tool (explanation)

  • 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 annotated on the right (user input).

Write a message:
  (user input) Hello world
Hello world

Exercise 1 - Message

  • Write a program that asks the user to write a string.
    • When the user has provided a string (i.e., written some text and pressed the enter key), the program should print the string that was provided by the user.

The exercise template comes with a program template that includes the creation of a Scanner tool.

import java.util.Scanner;

public class Message {

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

        System.out.println("Write a message: ");
        // Write your program here
    }
}

Example output for when the user writes “Bye”.

Write a message:
  (user input) Bye
Bye

Example output for when the user writes “Once upon a time…”.

Write a message:
  (user input) Once upon a time...
Once upon a time...

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 “strings” 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.

Declaring a String 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!";

String literals

  • 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:

Sample code

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

Output{.smaller-font-80

Hello world!
  • A string enclosed in a programming language’s quotation marks is called a “string literal”, i.e., a string with a specified value.

Common mistake

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.

Sample code

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

Output{.smaller-font-80

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!");
    }
}

Concatenation with String variables

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!");
    }
}

Output

Hello world! ... and the universe!

Concatenating multiple Strings

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);
    }
}

Output

My name is Bond, James Bond

Exercise 2 - Hi Ada Lovelace!

The exercise template contains the following program.

public class HiAdaLovelace {

    public static void main(String[] args) {
        String name = "Ada Lovelace";

    }
}

Modify the program so that it prints the contents of the variable name, and the printed text is the following in its full form:

Hi Ada Lovelace!

Note

When using the System.out.println command, do not pass in the string “Ada Lovelace” as a parameter. Instead, use the existing variable name: System.out.println("Hi " + ...)

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.

Reading Strings

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);
    }
}
Write a message:
  (user input) This will be printed twice...
This will be printed twice...
This will be printed twice...

Exercise 3 - Message three times

  • Write a program that asks the user to write a string.
    • When the user has given a string (that is, written some text and pressed enter), the program must print the user’s string three times (you can use the System.out.println command multiple times).

The exercise template already includes the code that creates the Scanner tool.

import java.util.Scanner;

public class Message {

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

        System.out.println("Write a message: ");
        // Write your program here
    }
}

Example output for when the user writes the string “Hi”.

Write a message:
  (user input) Hi
Hi
Hi
Hi

Example output for when the user writes “I am learning Java!”.

Write a message:
  (user input) I am learning Java!
I am learning Java!
I am learning Java!
I am learning Java!

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);
    }
}

Output

Write something:
  (user input) this
You wrote this

Exercise 4 - Greeting

Write a program that prompts the user for their name with the message “What’s your name?”. When the user has written their name, the program has to print “Hi” followed by the user’s name.

The exercise template already includes the code that creates the Scanner tool.

import java.util.Scanner;

public class Greeting {

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

        // Write your program here
    }
}

Example output when user gives the name Ada.


What's your name?
  (user input) Ada
Hi Ada

Example output when user gives the name Richard.

What's your name?
  (user input) Richard
Hi Richard

Program Execution Waits for Input

  • When during 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.

Program Execution Waits for Input (example)

  • In this example, the program prompts the user for three strings.
    1. The program prints Write the first string:, and then waits for user input.
    2. When the user writes some text, the program prints Write the second string:, and then waits for user input again.
    3. 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);
    }
}
Write the first string:
  (user input) The first
Write the second string:
  (user input) second
Write the third string:
  (user input) third
You wrote:
The first
second
third

Exercise 5 - Conversation

Write a program that works as follows:

Sample Output 1

Greetings! How are you doing?
  (user input) Good thank you!
Oh, how interesting. Tell me more!
  (user input) Well, there's really nothing to tell.
Thanks for sharing!

Sample Output 2

Greetings! How are you doing?
  (user input) Nice and dandy like cotton candy!
Oh, how interesting. Tell me more!
  (user input) Just went shopping.
Thanks for sharing!

The exercise template already includes the code that creates the Scanner tool.

import java.util.Scanner;

public class Conversation {

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

        //Write your program here
    }
}

More on input

  • 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);
    }
}

More Strings

  • We can form more complicated texts whose content changes depending on the user’s input by using more strings.
  • In the example on the next tab, 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);
    }
}
Write the first string:
  (user input) One
Write the second string:
  (user input) two
Write the third string:
  (user input) three
Last string you wrote was three, which
was preceded by two.
The first string was one.
All together: onetwothree

Exercise 6 - Story

Note

The example output might align wrong on narrow displays. If you’re using only a limited portion of the browser window, or your display is otherwise very narrow, try to stretch the display horizontally to see if the text aligns differently. The exercise expects the text to align as it does on wider displays.

  • Write a program that asks the user for a character’s name and their job. The program then prints a short story.

  • The output must be as shown in the next tab – note, the name and job depend on the user’s input.

I will tell you a story, but I need some information first.
What is the main character called?
  (user input) Legolas
What is their job?
  (user input) master archer
Here is the story:
Once upon a time there was Legolas, who was a master archer.
On the way to work, Legolas reflected on life.
Perhaps Legolas will not be a master archer forever.

The exercise template already includes the code that creates the Scanner tool.

import java.util.Scanner;

public class Story {

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

        //Write your program here
    }
}
I will tell you a story, but I need some information first.
What is the main character called?
  (user input) Ada
What is their job?
  (user input) a Data scientist
Here is the story:
Once upon a time there was Ada, who was a a Data scientist.
On the way to work, Ada reflected on life.
Perhaps Ada will not be a a Data scientist forever.

Checking our learning objectives

  • Learn to write a program that reads text written by a user.

  • Know what a “string” refers to in programming.

  • Know how to join (i.e., “concatenate”) strings together.

Next week

  • Variables and calculating with numbers

Acknowledgements


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