How to cast Object Array to Map Array and remove key in the Map?-CodePudding

I am new to Java and I have a Object Array got from API shown as below. Actually, it's just an array of dictionary. I would like to remove one of the key in each dictionary. Below is what I tried but I cannot solve the issue of "remove() is not defined for object". What is the appropriate way to do that? Thanks

Object Array:
List<Object> fruit: [{apple=3, orange=4}, {apple=13, orange=2}, {apple=1, orange=8}]

Code I tried:
List<Object> newList = fruit.stream().filter(x->x.getKey().equals("orange")).collect(Collectors.toList());

And

List<Object> newList = fruit.forEach(x->{
                                     x.remove();
                                     }););

Remark:
API Resp: [{country=US, fields={apple=3, orange=4},{country=CAD, fields={apple=1, orange=4}]
List<Object> fruit= apiResult.stream()
                             .map(x->x.get("fields"))            
                             .collect(Collectors.toList());

CodePudding user response：

This will remove the oranges from the maps:

fruit.forEach(x -> ((Map<?,?>) x).remove("orange"));

(It is more complicated you wanted to create a new list containing new maps with the oranges removed. But that is not what your attempts seemed to be doing.)

One thing you were missing in all of your attempts is that you need to cast the list elements to Map.

But a cleaner solution would be to define List<Object> fruit as List<Map<String, Integer>> fruit.

In the question title you asked:

How to cast Object Array to Map Array and remove key in the Map?

The short answer is that you can't. An array object that has been created as an Object[] cannot be cast to Map<?, ?>[]. But as you can see, you don't need to do that to solve your problem.

CodePudding user response：

How to cast Object Array to Map Array

It seems like you're confusing an array and ArrayList.

An array is a data-container that occupies a contiguous block of memory and has a fixed length.

ArrayList is a built-in implementation of the Dynamic array data structure, which is backed by an array, but contrary to a plain array it's capable to grow is size and provides a lot of useful behavior.

If your background is JavaScript, then ArrayList is closer to arrays in JavaScript.

Regarding casting, as I've said, you shouldn't use the Object at the first place. You list should be of type List<Map<String, Integer>>.

For more information on type conversion see.

I would like to remove one of the key in each dictionary

In your code, you're trying to create a new list in different ways, so it seems like you want to keep the previous version of the list with all maps intact.

If so you can use streams to create a duplicate of every entry (excluding entries with an undesired key "orange") for each map and then collect then into a list.

public static void main(String[] args) {
    List<Object> fruit =
        List.of(Map.of("apple", 3, "orange", 4),
                Map.of("apple", 13, "orange", 2),
                Map.of("apple", 1, "orange", 8));

    List<Map<String, Integer>> newList = fruit.stream()
        .map(map -> ((Map<String, Integer>) map).entrySet().stream()
            .filter(entry -> !entry.getKey().equals("orange")) // will retain entries with key that not equal to "orange"
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)))
        .collect(Collectors.toList());

    System.out.println(newList);
}

Output:

[{apple=3}, {apple=13}, {apple=1}]

But if you don't need the previous version of the list, then you can use a combination of forEach() an remove().