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);
    }
}

Why do you want to use reflection? Why simple polymorphism is not enough? — Taky, Sep 23 '13 at 17:19
With your current code, it's not enough to show that S could be a collection. Have a look at http://docs.oracle.com/javase/tutorial/java/generics/wildcards.html to see how to constrain the type of S. — brimble2010, Sep 23 '13 at 17:21
S is just any object i want to fetch every fields including collection classes with their values. — Rahul Thachilath, Sep 23 '13 at 17:25
by "collection" do you mean `Collection` or do you include array and `Map` instances? If `Map` is included, do you want the keys as well as the values? You need to provide clearer requirements. — John B, Sep 23 '13 at 17:28
What is the value of `targetMethodName`? Just using `iterator` or `toArray` might be a better idea. — Bernhard Barker, Sep 23 '13 at 17:30
targetMethodName is just name of the getter of the current field by which i will get the value of the current field. — Rahul Thachilath, Sep 23 '13 at 17:33
What is the **value** of `targetMethodName`? i.e. what's `source` and what motivated the expression you are assigning to `targetMethodName`? — Bernhard Barker, Sep 23 '13 at 17:41
FYI, there is no reason to try to find a getter method when you can just use `Field.get`: http://docs.oracle.com/javase/6/docs/api/java/lang/reflect/Field.html#get(java.lang.Object) — John B, Sep 23 '13 at 17:41

John B · Accepted Answer · 2013-09-23T17:32:56.130

6

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.

edited Sep 23 '13 at 17:32

answered Sep 23 '13 at 17:27

John B

32,493
6
77
98

this code is my last option i was to achieve with minimum amount of hard coding as possible. – Rahul Thachilath Sep 23 '13 at 17:35
It does say `"from a Collection class variable"` (i.e. `implements Collection`), so it's entirely possible that most of that is unnecessary. – Bernhard Barker Sep 23 '13 at 17:36
You might be right, everywhere else the OP uses `collection` instead of `Collection` and I missed the original `C`. – John B Sep 23 '13 at 17:40

score 5 · Answer 2 · answered Sep 23 '13 at 17:53

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
    }
}

using reflection to get objects from collection class

2 Answers2

Linked