using reflection to get objects from collection class

Question

I have been searching for a few days about the Reflection API in Java. I want to get all the objects from a Collection class variable inside a passed object.

E.g.

public static <S>void getValue(S s)throws Exception
{
    Field[] sourceFields = s.getClass().getDeclaredFields();
    for (Field sf : sourceFields) 
    {
        boolean sa = sf.isAccessible();
        sf.setAccessible(true);

        String targetMethodName = "get" + source.getName().substring(0, 1).toUpperCase()
                + (source.getName().substring(1));
        Method m = s.getClass().getMethod(targetMethodName, null) ;
        Object ret = m.invoke(s, new Object[] {});

        //ret 
        //check whether it is collection
        //if yes
        //get its generic type
        Type type = f.getGenericType();

        //get all the objects inside it

        sf.setAccessible(sa);
    }
}

Answer 1

I think the problem here is that ret could be any type of Collection: List, Set, Map, Array, custom class that implements Collection. A List could be ArrayList, LinkedList or any other type of List implementation. Getting the contents of the List via reflection would not work. What I suggest is that you support certain collection types as follows:

 Object[] containedValues;
 if (ref instanceof Collection)
     containedValues = ((Collection)ref).toArray();
 else if (ref instanceof Map)
     containedValues = ((Map)ref).values().toArray();
 else if (ref instanceof Object[])
     containedValues = (Object[])ref;
 else if (ref instanceof SomeOtherCollectionTypeISupport)
     ...

Then you can work with the elements in the array.

Answer 2

Collection implement the Iterable interface, so you can travel through all items in the collection and get them.

Object ref = // something
if (ref instanceof Collection) {
    Iterator items = ((Collection) ref).iterator();
    while (items != null && items.hasNext()) {
        Object item = items.next();
    // Do what you want
    }
}