🖥️ Slides
Typically, when a program prints output to the console (or terminal), the text is plain and without much formatting. However, it is possible to create fairly nice console user interfaces that include formatted, colored text, and use character-based graphics. This lecture explains how to create a text-based console user interface, with an emphasis on how to implement the Chess client user interface.
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When the Java runtime executes your application, it passes any arguments that were specified on the command line in the args parameter of your main function.
public class ArgsExample {
public static void main(String[] args) {
for (var i = 0; i < args.length; i++) {
System.out.printf("%d. %s%n", i+1, args[i]);
}
}
}If you run this code with the following command you will see how the arguments are output.
➜ java ArgsExample.java a b c
1. a
2. b
3. cUsing application arguments is a convenient way to customize how your application operates. For example, with your chess client application you could allow the user to provide the URL of your chess server. The code below specifies a default URL, but allows the user to override it with a provided value.
public class ClientMain {
public static void main(String[] args) {
var serverUrl = "http://localhost:8080";
if (args.length == 1) {
serverUrl = args[0];
}
new Repl(serverUrl).run();
}
}The System.out object is an output stream that is associated with whatever your operating system has decided is the output device. Usually this is the command line console that launched your application. You can use the System.out object to print out text to the command line and communicate with the user of your application. We can combine the use of application arguments and System.out to create a rudimentary calculator that does simple addition.
public class CalculatorExample {
public static void main(String[] args) {
int result = 0;
for (var arg : args) {
result += Integer.parseInt(arg);
}
var equation = String.join(" + ", args);
System.out.printf("%s = %d", equation, result);
}
}➜ java CalculatorExample.java 1 2 3
1 + 2 + 3 = 6Most command line consoles support a common set of escape codes that allow you to change the background and foreground colors of the characters that are output. You control the color changes by outputting a sequence of special escape characters. For example, if you run the following command in your console window you will see that the text red on blue is output with red text and a blue background.
echo -e "\u001b[31;44;1m red on blue "The color command begins the escape sequence \u001b[. This tells the console that you are using unicode characters. If you only plan on using ANSI characters then you can use the code \033[. You then supply sequence commands separated with a semicolon (;). 31 specifies that we are setting the foreground color to red. 44 sets the background color to blue. 1 turns on bold. The sequence is terminated with m.
| Color | Foreground | Background |
|---|---|---|
| Black | 30 | 40 |
| Red | 31 | 41 |
| Green | 32 | 42 |
| Orange | 33 | 43 |
| Blue | 34 | 44 |
| Magenta | 35 | 45 |
| Cyan | 36 | 46 |
| Light gray | 37 | 47 |
| Fallback default | 39 | 49 |
| Dark gray | 100 | |
| Light red | 101 | |
| Light green | 102 | |
| Yellow | 103 | |
| Light blue | 104 | |
| Light purple | 105 | |
| Teal | 106 | |
| White | 107 |
| Modification | Code |
|---|---|
| none | 0 |
| bold | 1 |
| underscore | 4 |
| blink | 5 |
| reverse | 7 |
| concealed | 8 |
We can enhance our calculator to use different colors by printing out the correct control colors before we print the text. We do this by providing the escape sequences for the different foreground and background colors we want to use to System.out before we print any text.
public class FancyCalculatorExample {
public static void main(String[] args) {
int result = 0;
for (var arg : args) {
result += Integer.parseInt(arg);
}
System.out.print("\u001b[35;100m");
System.out.printf(" %s = ", String.join(" + ", args));
System.out.print("\u001b[107m");
System.out.printf(" %d ", result);
}
}Now when we run our calculator, we see the color changes.
In order to create a complete console based application you need to also receive input from your user. When your java program executes, the operating system usually streams the buttons pressed on a keyboard to the System.in object.
To demonstrate reading from the keyboard, we can modify our calculator to read our numbers from what the user is typing instead of reading it from the application arguments. To do this we wrap System.in with the Scanner class so that we can read from the user's input a line at a time. Once the user presses Enter we can parse out the numbers and output the result.
import java.util.Scanner;
public class InteractiveCalculatorExample {
public static void main(String[] args) throws Exception {
while (true) {
System.out.printf("Type your numbers%n>>> ");
Scanner scanner = new Scanner(System.in);
String line = scanner.nextLine();
var numbers = line.split(" ");
int result = 0;
for (var number : numbers) {
result += Integer.parseInt(number);
}
var equation = String.join(" + ", numbers);
System.out.printf("%s = %d%n", equation, result);
}
}
}➜ java InteractiveCalculatorExample.java
Input your numbers
>>> 1 2 3
1 + 2 + 3 = 6- How to use command line arguments
- How to write to the standard out
- How to read from standard in
- How to use terminal control codes to clear the terminal, set background and text colors, and text attributes such as bold, faint, italic, underline, and blinking
- How to use Unicode characters to draw the chess board and pieces for the Chess project
- 🎥 Console User Interfaces (3:02) - [transcript]
- 🎥 Terminal Control Codes (10:53) - [transcript]
- 🎥 Drawing the Chess Board (3:54) - [transcript]
The following demonstration code uses escape sequences to draw a tic tac toe board. This is similar to the concepts you will need in order to implement the rendering of the chess board. The EscapeSequences.java file contains many standard codes that you might find useful in your programming.
