如何将 object 列表转换为 Java 中的 integer 列表？

Question

I have a inner list of object into a list and I want to convert it in a list of Integer, because I know that its elements are Integer.

List<List<Object>> valuesModel = FCSMs.get(0).getValues();
            for (List<Object> innerList : valuesModel) {

//CONVERT LIST OF OBJECT TO LIST OF INTEGER

}

How Can I do?

Answer 1

For a start it's a good practice to double check that you are in fact dealing with a list of type Integer. You may know that the only input is of that type, but anyone in the future working with your code will not (because it is not typed with Integer). After that you can simply "cast" it to type Integer. Some pseudo code on how to do that can be found below:

List<List<Integer>> result = new ArrayList<>();
for (List<Object> innerList : valuesModel) {
    List<Integer> integerList = new ArrayList<>();
    for (Object object : innerList) {
        if (object instanceof Integer) {
            integerList.add((Integer) object);
        }
    }
    result.add(integerList);
}

Answer 2

You can do it like this. Since you know that all objects are Integers, no checks are done to avoid ClassCastExceptions.

• Stream the list of lists to a stream of lists.
• Then flatten those lists into a stream of object.
• cast the object to an Integer.
• and collect into a List

List<List<Object>> valuesModel = List.of(List.of(1,2,3,4), List.of(5,6,7,8));

List<Integer> integers = valuesModel.stream()
           .flatMap(Collection::stream)
            .map(ob->(Integer)ob)
             .collect(Collectors.toList());

System.out.println(integers);

Prints

[1, 2, 3, 4, 5, 6, 7, 8]