Java reflection: how to get field value from an object, not knowing its class

Question

Say, I have a method that returns a custom List with some objects. They are returned as Object to me. I need to get value of a certain field from t

Answer 1

Assuming a simple case, where your field is public:

List list; // from your method
for(Object x : list) {
    Class clazz = x.getClass();
    Field field = clazz.getField("fieldName"); //Note, this can throw an exception if the field doesn't exist.
    Object fieldValue = field.get(x);
}

But this is pretty ugly, and I left out all of the try-catches, and makes a number of assumptions (public field, reflection available, nice security manager).

If you can change your method to return a List, this becomes very easy because the iterator then can give you type information:

List list; //From your method
for(Foo foo:list) {
    Object fieldValue = foo.fieldName;
}

Or if you're consuming a Java 1.4 interface where generics aren't available, but you know the type of the objects that should be in the list...

List list;
for(Object x: list) {
   if( x instanceof Foo) {
      Object fieldValue = ((Foo)x).fieldName;
   }
}

No reflection needed :)