CODEWITHSUNDEEP
javacasting
ian
ian

Reputation: 303

Java: cast Object to HashMap<String, Object>

I am trying to cast an Object to HashMap<String, Object> in a neat, robust way. So far, every way I tried produces compiler warnings or errors. What is the proper way to do it? I have checked the internet and tried the following:

HashMap<String, Object> map = (HashMap<String, Object>) object;

The code above gives an unchecked conversion warning.

HashMap<String, Object> map = new HashMap<>();
if (object instanceof Map<String, Object>){
    map = (Map<String, Object>) object;
}

The code above gives an error, which says that objects cannot be compared to parameterized collections.

HashMap<String, Object> map = new HashMap<>();
if (object instanceof Map){
    Map genericMap = (Map) object;
    for (Object key : genericMap.keySet()){
        if (key instanceof String){
            map.put((String) key, genericMap.get(key));
        }
        else{
            throw new KeyException();
        }
    }
}

The code above yields a warning that "Map is a raw type. References to generic type Map<K,V> should be parameterized."

So what would be the proper way to do this? Thank you in advance!

Upvotes: 1

Views: 2527

Answers (3)

Matt
Matt

Reputation: 1

Below is the above code written such that you don't need @SuppressWarnings to compile cleanly. You can read up on "Unbounded Wildcards" to understand their proper use better. Java docs state they can be used when:

  • If you are writing a method that can be implemented using functionality provided in the Object class.
  • When the code is using methods in the generic class that don't depend on the type parameter. For example, List.size or List.clear. In fact, Class<?> is so often used because most of the methods in Class do not depend on T.

I think both of these apply to your situation.

Code written with unbounded wildcards "?":

HashMap<String, Object> map = new HashMap<>();
if (object instanceof Map){
    Map<?, ?> genericMap = (Map<?,?>) object;
    for (Object key : genericMap.keySet()){
        if (key instanceof String){
            map.put((String) key, genericMap.get(key));
        }
        else{
            throw new KeyException();
        }
    }
}

Edited for clarity

Upvotes: 0

John Bollinger
John Bollinger

Reputation: 181859

I am trying to cast an Object to HashMap<String, Object> in a neat, robust way. So far, every way I tried produces compiler warnings or errors. What is the proper way to do it?

There is no proper way to do it, supposing that "proper" implies both useful and type safe. Casting is the antithesis of type safety. Other than casts for arithmetic purposes, a safe cast is an unnecessary one.

There is not enough information to determine how to achieve what ultimately you are after, but generally speaking, that sort of thing revolves around writing true generic code instead of using type Object to funnel objects of unrelated type into the same methods, using instanceof to determine what you actually have, or casting.

Upvotes: 5

mmartinez04
mmartinez04

Reputation: 323

Just add @SuppressWarnings("unchecked") to your method

@SuppressWarnings("unchecked")
public void myMethod(){
    ...
}

and you should be able to use

HashMap<String, Object> map = (HashMap<String, Object>) object;

Upvotes: 0

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