How do I cast a Java Object into a List of integers?

Question

A method of some poorly documented library is returning me a List of Objects. I know that the list contains ArrayLists of integers. As follows:

List<?> result = lib.get();
Iterator<?> iterator = result.iterator();
while(iterator.hasNext()){
    Object next = iterator.next();
    System.out.println(next.toString());
    System.out.println(next.getClass());
}

Gets me

 [0, 0] 
 class java.util.Arrays$ArrayList 
 [0, 1] 
 class java.util.Arrays$ArrayList 
 [0, 2] 
 class java.util.Arrays$ArrayList 

How can I get to turn those objects into ArrayList so that I can use them properly?

Answer 1

Just cast the value returned by lib.get() to List<List<Integer>>.

@SuppressWarnings("unchecked")
List<List<Integer>> result = (List<List<Integer>>) lib.get();

Proof

// Simulate lib.get() and prove that it is same as question code
List<?> result = Arrays.asList(Arrays.asList(0, 0),
                               Arrays.asList(0, 1),
                               Arrays.asList(0, 2));
for (Object next : result) {
    System.out.println(next);
    System.out.println(next.getClass());
}

// Fix it
@SuppressWarnings("unchecked")
List<List<Integer>> fixedResult = (List<List<Integer>>) result;

// Print the fixed result
for (List<Integer> list : fixedResult) {
    for (int value : list)
        System.out.print(value + " ");
    System.out.println();
}

Output

[0, 0]
class java.util.Arrays$ArrayList
[0, 1]
class java.util.Arrays$ArrayList
[0, 2]
class java.util.Arrays$ArrayList
0 0 
0 1 
0 2 

Answer 2

I think you can only cast the Object next = ... object into the type java.util.Arrays$ArrayList, but because it is a private static class, it is better to cast it to java.util.List class.

Like:

List<?> result = lib.get();
Iterator<?> iterator = result.iterator();
while(iterator.hasNext()){
    Object next = iterator.next();
    // -------------------
    List<Integer> list = (List<Integer>) next;
    System.out.println(list.get(0));
    System.out.println(list.get(1));
    // -------------------
}

Answer 3

tl;dr

You cannot cast the entire thing, as it is a List of List objects that hold Integer objects (apparently). So loop the outer list, and cast each element List object to be a List< Integer > object.

List of lists of integers

You appear to have a list of lists of integers, not a list of integers.

    List < Object > objects = new ArrayList <> ( 3 );
    objects.add ( List.of ( 0 , 0 ) );
    objects.add ( List.of ( 0 , 1 ) );
    objects.add ( List.of ( 0 , 2 ) );

    System.out.println ( objects );

[[0, 0], [0, 1], [0, 2]]

Unfortunately, you cannot simply cast to a concrete type as suggested in the other Answer because we do not know the concrete type. We can presume that we have a List, but not a ArrayList.

See this example. The List.of method generates an object of an unspecified class that implements List. We can see here that whatever concrete class is used, it is not ArrayList.

    List < Object > objects = new ArrayList <> ( 3 );
    objects.add ( List.of ( 0 , 0 ) );
    objects.add ( List.of ( 0 , 1 ) );
    objects.add ( List.of ( 0 , 2 ) );

    System.out.println ( objects );

    Iterator < ? > iterator = objects.iterator ();
    while ( iterator.hasNext () )
    {
        Object next = iterator.next ();
        List < Integer > list = ( ArrayList < Integer > ) next;
        System.out.println ( list.get ( 0 ) );
        System.out.println ( list.get ( 1 ) );
    }

…throws an exception:

Exception in thread "main" java.lang.ClassCastException: class java.util.ImmutableCollections$List12 cannot be cast to class java.util.ArrayList (java.util.ImmutableCollections$List12 and java.util.ArrayList are in module java.base of loader 'bootstrap')

We can cast each element to a List< Integer >.

List < Object > objects = new ArrayList <> ( 3 );
objects.add ( List.of ( 0 , 0 ) );
objects.add ( List.of ( 0 , 1 ) );
objects.add ( List.of ( 0 , 2 ) );

System.out.println ( objects );

Iterator < ? > iterator = objects.iterator ();
while ( iterator.hasNext () )
{
    Object next = iterator.next ();
    List < Integer > list = ( List < Integer > ) next;
    System.out.println ( list.get ( 0 ) );
    System.out.println ( list.get ( 1 ) );
}

When run.

[[0, 0], [0, 1], [0, 2]]
0
0
0
1
0
2

I do not know of a way to directly cast the original list. This next line gives a compiler error of inconvertible types.

List < List < Integer > > listOfListOfIntegers = ( List < List < Integer > > ) objects;

Answer 4

Assuming you have a list of lists of integers, you can simply do

List<List<Integer>> newList = result.stream()
                             .map(e->(List<Integer>)e)
                             .collect(Collectors.toList());