You can use the old Stupid Scanner trick oneliner to do that without any additional dependency like guava:

String text = new Scanner(AppropriateClass.class.getResourceAsStream("foo.txt"), "UTF-8").useDelimiter("\\A").next();

Guys, don't use 3rd party stuff unless you really need that. There is a lot of functionality in the JDK already.

For java 7:

new String(Files.readAllBytes(Paths.get(getClass().getResource("foo.txt").toURI())));

For Java 11:

Files.readString(Paths.get(getClass().getResource("foo.txt").toURI()));

Pure and simple, jar-friendly, Java 8+ solution

This simple method below will do just fine if you're using Java 8 or greater:

/**
 * Reads given resource file as a string.
 *
 * @param fileName path to the resource file
 * @return the file's contents
 * @throws IOException if read fails for any reason
 */
static String getResourceFileAsString(String fileName) throws IOException {
    ClassLoader classLoader = ClassLoader.getSystemClassLoader();
    try (InputStream is = classLoader.getResourceAsStream(fileName)) {
        if (is == null) return null;
        try (InputStreamReader isr = new InputStreamReader(is);
             BufferedReader reader = new BufferedReader(isr)) {
            return reader.lines().collect(Collectors.joining(System.lineSeparator()));
        }
    }
}

And it also works with resources in jar files.

About text encoding: InputStreamReader will use the default system charset in case you don't specify one. You may want to specify it yourself to avoid decoding problems, like this:

new InputStreamReader(isr, StandardCharsets.UTF_8);

Avoid unnecessary dependencies

Always prefer not depending on big, fat libraries. Unless you are already using Guava or Apache Commons IO for other tasks, adding those libraries to your project just to be able to read from a file seems a bit too much.