Copilot cannot read UTF-8 from a Java Properties File

I'm speechless: Copilot get this well understood, old problem with a well established, extensively documented platform all wrong. My question was:

"Can you write UTF-8 in your Java .properties file?"

And it said:

Its answer has three problems:

1. The test data can be written with ISO-8859-1 encoding, so it doesn't prove that UTF-8 is working.
2. What's worse, it doesn't work.
3. Lastly, it gives the impression that this used to be a problem, but is no longer, listing different Java versions and so on, adding credibility to its answer. But alas, it doesn't work. You have to recode the string, read the bytes as ISO 8859-1, and then construct a new string as UTF-8.

SMH.

Ok, let's try out what the robot suggested using a recent Java version:

$ java -version
openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment Homebrew (build 25.0.1)
OpenJDK 64-Bit Server VM Homebrew (build 25.0.1, mixed mode, sharing)

The messages.properties file contains one entry with a 4 byte UTF-8 character:

greeting=👻

The properties file was encoded properly as UTF-8:

$ file messages.properties
messages.properties: Unicode text, UTF-8 text

The Copilot Java source code (I added the comment // Copilot solution) copy and pasted into a file called utf.java:

import java.io.*;
import java.util.*;

public class utf {
    public static void main(String... args) throws Exception {
        Properties props = new Properties();
        try (InputStream in = new FileInputStream("messages.properties")) {
            props.load(in); // Loads as UTF-8 from Java 9+
        }
        // Copilot solution
        System.out.println("Copilot reading UTF-8: " + props.getProperty("greeting"));
    }
}

Now, testing it out, shows that the AI code doesn't work at all:

❯ javac utf.java &&  java utf
Copilot reading UTF-8: ð»

Applying a battled tested fix, acquired after some late night debugging back in the day, added as a second print out statement:

        // Human solution, from experience, that works
        System.out.println(
            "Human reading UTF-8: " +
            new String(
                props.getProperty("greeting").getBytes("ISO-8859-1"),
                "UTF-8"));

Now, the ghost displays correctly:

❯ javac utf.java &&  java utf
Copilot reading UTF-8: ð»
Human reading UTF-8: 👻

Of the three shortcomings of Copilot's answer, I think number three is the worst. It's so confident, backing up all its claims with credible sources and proper rationale. If a colleague their case in such a way, you would of course believe her and not second guess it. Here with this AI robot, though, you should verify it. My hunch is, though, most of the time, people don't.

Good grief.

It should be noted, that the above was using the Copilot chat. Giving the same prompts to Copilot CLI, which has access to your machine and file system, so that it can try out things before suggesting them, arrived at the working solution on the first try. My findings still shocks me. Most people use the chat.

Happy encoding!