src/main/java/org/fest/assertions/error/MessageFormatter.java
public String format(Description d, String format, Object... args) {
    checkNotNull(format);
    checkNotNull(args);
    return descriptionFormatter.format(d) + String.format(format, format(args));
  }
public String format(Description d, String format, Object... args) {
    checkNotNull(format);
    checkNotNull(args);
    return descriptionFormatter.format(d) + String.format(format, format(args));
  }
Interprets a printf-style format {@code String} for failed assertion messages. It is similar to
<code>{@link String#format(String, Object...)}</code>, except for:
<ol>
<li>the value of the given <code>{@link Description}</code> is used as the first argument referenced in the format
string</li>
<li>each of the arguments in the given array is converted to a {@code String} by invoking
<code>{@link ToString#toStringOf(Object)}</code>.
</ol>

@param d the description of the failed assertion, may be {@code null}.
@param format the format string.
@param args arguments referenced by the format specifiers in the format string.
@throws NullPointerException if the format string is {@code null}.
@return A formatted {@code String}.