Converting Map<T> to derived class

I have the following class:

class DerivedMap extends Hashmap<String,Object>
{}

Which I use because I want to implement some custom actions for this type of map.

I then pass instances of this class to an external api that wants Map, this naturally works well.

The problem is that I also get Map instances as return values from the api. Since a real object hides behind the interface that the api chose I know I cannot just cast it.

So, how do I convert Map to my custom DerivedMap? Must I ma开发者_运维技巧nually copy all the keys/values?

Thanks.

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You could create a constructor like this:

class DerivedMap extends HashMap<String, Object> {
    public DerivedMap(Map<String, Object> map) {
        super(map);
    }
}

and then create a DerivedMap from a Map like this:

derivedMap = new DerivedMap(otherMap);

The super(map) invokes the HashMap constructor that copies all mappings of the argument map to the newly created map. From the docs:

Constructs a new HashMap with the same mappings as the specified Map.