How to save data from a Properties object to a File + How to load the properties-formatted file to a Properties object in another Method?

I want to save the data from the properties objecet config to the file configFile in the parameter like following:

@Override
public void saveConfig(Properties config, File configFile) {
	
	try {
		ObjectOutputStream os = new ObjectOutputStream(new FileOutputStream(configFile));
		os.writeObject(config);
		os.close();
	}
	catch (FileNotFoundException e) {
		e.printStackTrace();
	}
	catch (IOException e) {
		e.printStackTrace();
	}
	
}

In the next method I want to load the properties-formatted configFile to a Properties object and return it:

@Override
public Properties loadConfig(File configFile) {
	
	Properties prop = new Properties();
	
	try(InputStream input = new FileInputStream(configFile)){
		prop.load(input);
	}
	catch (FileNotFoundException e) {
		e.printStackTrace();
	}
	catch (IOException e) {
		e.printStackTrace();
	}
	return prop;
}

Somehow the JUnit test is showing me a NullPointerExeption (note: it is an exam)

if (!config.getProperty("testKey").equals("testValue"))
		fail("sample config data doesn't match read config data!");

What am I missing here?

The following examples use the java.nio.file package which should be preferred to java.io.File due to its improved error handling. However, the code will look similarly for java.io.File as well.

Writing Properties

@Override
public void saveConfig(Properties config, Path configFile) throws IOException {
    // Comments to be written at the beginning of the file;
    // `null` for no comments
    String comments = ...

    // try-with-resources to close writer as soon as writing finished
    // java.nio.file.Files.newBufferedWriter​(...) uses UTF-8 by default
    try (Writer writer = Files.newBufferedWriter(configFile)) {
        config.store(writer, comments);
    }
}

Reading Properties

@Override
public Properties loadConfig(Path configFile) throws IOException {
    Properties config = new Properties();

    // try-with-resources to close reader as soon as reading finished
    // java.nio.file.Files.newBufferedReader(...) uses UTF-8 by default
    try (Reader reader = Files.newBufferedReader(configFile)) {
        config.load(reader);
    }

    return config;
}