如何在 java 中将 Object[] 转换为 int[]

Question

After I use the method .toArray() in a Priority Queue of Integers I get an array of Objects like this:

Object[] objects = pq.toArray();

However, I need the array to be int[] . I've tried:

int [] arr = (int[]) objects;

but it says:

Cannot cast from Object[] to int[]

How can I achieve this?

Answer 1

All collections provide an alternative toArray implementation specifying the element type:

Integer[] objects = pq.toArray(Integer[]::new);

Note this requires Java v11 or higher

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Collection.html#toArray(java.util.function.IntFunction)

Note the resultant array is obviously integer wrapper objects in this case, don't know if that's fine for your requirements.

EDIT - quick test:

PriorityQueue<Integer> q = new PriorityQueue<>();
q.add(42);
System.out.println(Arrays.toString(q.toArray(Integer[]::new)));

Answer 2

This is not something you can easily do.

You might know that the Object[] is actually just full of integers, which would mean that it should be possible to cast it, right?

Problem is, the variable "objects" still exists. And in an Object[] you're allowed to put any Object. So after your cast, someone might do something like:

objects[0] = "some string";

So what should happen then? Should your int[] suddenly be invalid since it now contains a String? Should it not be allowed to input things other than Integers into objects which typically allows that?

Casting just means you point to the same thing but look at it differently. What you need to do is to transform it. For that there's already an answer:

java-map-an-array-of-strings-to-an-array-of-integers

So, simple casting is not going to help out here.

Update: @stridecolossus answer is probably just as good as the one I linked to.

Answer 3

Since PriorityQueue implements Collection , you can use a stream to iterate over its elements and map them to int :

PriorityQueue<Integer> pq = new PriorityQueue<>(Arrays.asList(1, 2, 5));

int[] arr = pq.stream().mapToInt(Integer::intValue).toArray();

System.out.println(Arrays.toString(arr)); // [1, 2, 5]