Summary  
This chapter covers using single-line (//) and multi-line (/* */) comments to annotate code and temporarily disable portions of it without affecting program execution.

General domain of usage  
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 are notes in the code that are ignored by the computer but help explain what the code does. They make programs easier to understand and maintain, and can also be used to temporarily disable parts of the code for testing or debugging.



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

A streamlined beginner module focused on the essential Java programming concepts: syntax, variables, data types, control flow, loops, arrays, and strings.

Comments

Hiding Code in From the Compiler

Why do We Need Comments?

解答