Generic Collection Type

[Dave Radtke]

I'm trying to create a function that can determine the Component Type of any generic collection. I would have though this would be an easy question, but after many hours of searching I haven't found anything.

I have found this works, but only if the collection contains a value, which I cannot rely on.

public static Class getCollectionGenericType(Collection<?> collection){
     return collection.iterator().next().class();
}

Thanks
Dave

[Campbell Ritchie]

Because of erasure that information is no longer available at runtime.
That sort of question comes up several times a year; if you search you will find suggestions, but there is no straightforward way to retrieve "<T>" and workout what "T" stands for.

[Henry Wong]

I have found this works, but only if the collection contains a value, which I cannot rely on.

Also, it may not be reliable... for example, if someone calls your method with a Collection<Animal> object, that contains Dogs, Cats, Horses, Cows, etc.... With your solution, you can mistakenly get the first element, which may be a Cow, and assume that it is a Collection<Cow> object.

Henry

[Dave Radtke]

Campbell Ritchie wrote:Because of erasure that information is no longer available at runtime.
That sort of question comes up several times a year; if you search you will find suggestions, but there is no straightforward way to retrieve "<T>" and workout what "T" stands for.

Well, I feel better that at least I'm not just missing the obvious.

I have been searching, and the closes I have found was this Getting-generic-type-Collection but that sample appears to only work if you have the collection as a declared field within another object. I already have a method to workout it out from a PropertyDescriptor. But in this situation all I have is the collection itself.


What search terms should I be using?

public static Class getGenericType(PropertyDescriptor descriptor){
    	if (descriptor.getReadMethod()!=null ) {
    		Type returnType = descriptor.getReadMethod().getGenericReturnType();
    		if(returnType instanceof ParameterizedType){
    		    ParameterizedType type = (ParameterizedType) returnType;
    		    Type[] typeArguments = type.getActualTypeArguments();
    		    for(Type typeArgument : typeArguments){
    		        Class typeArgClass = (Class) typeArgument;
    		        return typeArgClass;
    		    }
    		}
		}else if(descriptor.getWriteMethod()!=null ){
			Type[] genericParameterTypes =  descriptor.getWriteMethod().getGenericParameterTypes();
			for(Type genericParameterType : genericParameterTypes){
			    if(genericParameterType instanceof ParameterizedType){
			        ParameterizedType aType = (ParameterizedType) genericParameterType;
			        Type[] parameterArgTypes = aType.getActualTypeArguments();
			        for(Type parameterArgType : parameterArgTypes){
			            Class parameterArgClass = (Class) parameterArgType;
			            return parameterArgClass;
			        }
			    }
			}
		}
    	return null;
    }