Summary  
This chapter covers the use of comments to hide or disable code from the compiler and to annotate code for clarity, using single-line (`//`) and multi-line (`/* */`) comment syntax.  

General domain of usage  
Debugging and code documentation

# Hiding Code in From the Compiler
You can hide code from the compiler by commenting out the code.

**Code comments** are fragments that the compiler **will not process**. 

You can comment out lines of code using the `//` symbols or enclose a specific fragment of text or code using `/* */`.


### Why do We Need Comments?

- **Comments** let you **write notes** to explain what parts of the code do. These notes are **ignored** by the computer but make the code easier to **understand** for yourself or others;

- **Comments** can temporarily **disable parts** of your code. For instance, if you think a specific line is causing an error, you can comment it out (`/* code fragment */`) to test the program **without** that line;

- **Comments** can be used to **describe** how your code works, making it easier to **understand** later or for others working with your code.


You saw an example of commenting in the previous chapter, where a comment was placed where the code was supposed to go.

Here's another example of how to use comments:


cGFja2FnZSBjb20uZXhhbXBsZTsKCnB1YmxpYyBjbGFzcyBNYWluIHsKICAgIHB1YmxpYyBzdGF0aWMgdm9pZCBtYWluKFN0cmluZ1tdIGFyZ3MpIHsKICAgICAgICBTeXN0ZW0ub3V0LnByaW50bG4oIk1lc3NhZ2UgMSIpOwogICAgICAgIC8vIFN5c3RlbS5vdXQucHJpbnRsbigiTWVzc2FnZSAyIik7CgkgICAgU3lzdGVtLm91dC5wcmludGxuKCJNZXNzYWdlIDMiKTsKICAgIH0KfQ==

Only the first and third messages will be displayed here because the second part is commented out, so the compiler ignores it.

Now, let's take a look at an example of **multi-line** code commenting:


cGFja2FnZSBjb20uZXhhbXBsZTsKCnB1YmxpYyBjbGFzcyBNYWluIHsKICAgIHB1YmxpYyBzdGF0aWMgdm9pZCBtYWluKFN0cmluZ1tdIGFyZ3MpIHsKICAgICAgICAvKiBTeXN0ZW0ub3V0LnByaW50bG4oIk1lc3NhZ2UgMSIpOwogICAgICAgIFN5c3RlbS5vdXQucHJpbnRsbigiTWVzc2FnZSAyIik7ICovCiAgICAgICAgU3lzdGVtLm91dC5wcmludGxuKCJNZXNzYWdlIDMiKTsKICAgIH0KfQ==

As you can see, only the third message is displayed in the console because the first and second ones are commented out.





import static org.junit.jupiter.api.Assertions.assertTrue;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.regex.Pattern;

import com.example.Main;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

public class MainTest {

    @Test
    @DisplayName("The code prints the right message")
    void testPrintsHelloWorldFlexible() {
        ByteArrayOutputStream outContent = new ByteArrayOutputStream();
        PrintStream originalOut = System.out;
        System.setOut(new PrintStream(outContent));

        Main.main(new String[]{});

        System.setOut(originalOut);

        String output = outContent.toString().trim();

        Pattern pattern = Pattern.compile("Hey\\\s+I'm\\\s+working", Pattern.CASE_INSENSITIVE);
        assertTrue(pattern.matcher(output).find(), "Actual output: " + output);
    }

    @Test
    @DisplayName("Comment was used to ignore the line with mistake.")
    public void testContainsCommentToRemoveThirdLine() throws IOException {
        String path = "/app/devResources/java/src/main/java/com/example/Main.java";
        String content = new String(Files.readAllBytes(Paths.get(path)));

        String[] lines = content.split("\\\R");
        String normalizedContent = content.replaceAll("\\\s+", "");

        boolean foundAsComment = normalizedContent.contains("//System.out.println");

        assertTrue(foundAsComment);
    }
}

MainTest.java

Learn the fundamentals of Java and its key features in this course. By the end, you'll be able to solve simple algorithmic tasks and gain a clear understanding of how basic console Java applications operate.

Discover the unique features of Java and understand why it's a recommended programming language to learn. Write your first Java code and explore how to print text to the console.

Learn the basics of programming, dive into primitive data types, and explore operations with them. Write code that performs mathematical tasks and evaluates conditions for seamless execution.

Explore various loops such as for, while, and do-while, which are essential in any programming language. Loops enable efficient iteration over large datasets, saving significant time. Apply them extensively in tasks like sorting, working with arrays, collections, and more.

Learn how to effectively use arrays in Java programming by discovering how to group data into arrays and exploring common methods and properties for working with them. Techniques for manipulating and optimizing arrays in real-world applications will also be examined.

Learn about String, a crucial data type in Java, and discover its many built-in methods and operations. Explore String and StringBuilder, understanding their basic methods and how to use strings effectively in Java.

Comments

Hiding Code in From the Compiler

Why do We Need Comments?

解答