Question

Short, at-a-glance summary of question (TLDR): how do I ensure a Java Object is of type Map<String, Object>?

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I have a YAML document as a String. I create the YAML using SnakeYAML from a Map<String, Object> but it could be tampered with before I read it back again. I don’t want to resort to PKI signing, it’s enough to see whether the YAML representation is still a Map<String, Object>, even if the Object values may be all over the place. I care about type, not content.

So: how do I ensure an Object is indeed of type Map<String, Object>?

Here’s how I am trying to circumvent the various SuppressWarnings of type erasure:

Object yamlObject = yaml.load(someYamlDocumentAsString);
// I only want to proceed if this YAML object is a Map<String, Object>
if ((yamlObject instanceof Map<?, ?>)) {
  Map<?, ?> propertyMap = (Map<?, ?>) yamlObject;
  for (Entry<?, ?> propertyEntry : propertyMap.entrySet()) {
    // I am checking each key and throwing if it's not a "String"
    Object propertyKey = propertyEntry.getKey();
    if (!(propertyKey instanceof CharSequence)) {
      throw new BananaException("FIXME"); // FIXME
    }
  }
} else {
  throw new CoconutException("FIXME"); // FIXME
}

Can the above be done better?

Incidentally, I don’t suppose Java has something akin to C#’s as keyword (from MSDN: The as operator is like a cast operation. However, if the conversion is not possible, as returns null instead of raising an exception.)?

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After the code above, can I safely suppress warnings for this:

@SuppressWarnings("unchecked")
Map<String, Object> aBeautifulAndCozyMap = (Map<String, Object>) propertyMap;

Answer 1

Because of type erasure, information is not known at runtime, so there is no other way of checking it than instanceof, getClass().getName() or getDeclaredField("...").getGenericType().getActualTypeArguments() things.

In my opinion you should stay with instanceof as it is easily readable and other developer will easily understand what you wanted to do.

PS: I liked the exceptions 😉