如何迭代存储为 Object 类型的 Java 集合？

Question

Suppose I have a list of some class A defined as:

Object list = new ArrayList<A>();

I want to iterate over this. Is it possible? If yes then how? Even if I find the type of list using reflections then also I won't get to class A. Also, note that I don't know what class A is. I just have list.

Answer 1

If you instantiate an ArrayList the way that you've shown (you want it to be treated as Object - this class doesn't allow iterating, classes implementing Iterable interface provide this function), and then you need to iterate over its elements, you can cast this list from Object to Iterable , that way iteration becomes available for list :

    for(A a : (Iterable<A>) list) {
        // do stuff
    }

or when you need an Iterator iterating over elements of the list :

    Object list = new ArrayList<A> ();
    Iterator<A> it = ((Iterable<A>)list).iterator();

You might want to have a look at Java API Documentation of to read more about Iterable interface here .

Edit: This answer was edited thanks to helpful suggestions of Federico Peralta Schaffner and luk2302 .

Answer 2

I found a way to do so. I did something like this:

List<?> list1 = (List<?>) list;

now I can iterate as,

for(Object o : list1) {
// code
}