Java: Error Trying to Convert Comparable Array to Integer Array

Question

I am currently implementing a generic version of the longest increasing subsequence problem in Java. The method works as intended, but when I try to use Comparable[] instead of Integer[] (or int[]), the program won't compile. The error given is "Comparable cannot be cast to Integer". I understand the error and what it means, but I don't know how to fix it. Any help would be greatly appreciated :)

I have already tried making the method's return type a generic (>), but the problem is that Java does not allow generic array creation. I've tried just using Integer[] as my return type, and while that compiles and works properly, it's not what I want.

public class LIS {
  public static void main(String[] args) {
    final Integer[] arr = {-1, 2, 4, 2, 33, 4, 7, 8, 10, 7, 5, 4, 5, 5, 1};
    final Integer[] LIS = (Integer[]) lis(arr);
    for (int i : LIS) {
      System.out.print(i + " ");
    }
  }

  public static Comparable[] lis(Comparable[] arr) {
    // We use Comparable[] so we can use interchangably with any Comparable type

    final int N = arr.length;

    // Java conveniently initializes array values to 0:
    int[] lisEndingHere = new int[N];

    for (int i = 0; i < N; i++) {
      lisEndingHere[i] = 1;
      int curMax = 0;
      for (int j = 0; j <= i; j++) {
        if (arr[i].compareTo(arr[j]) <= 0) continue;
        if (lisEndingHere[j] > curMax) {
          curMax = lisEndingHere[j];
        }
      }
      lisEndingHere[i] += curMax;
    }

    // Find and return the longest increasing subsequence:
    int max = 0;
    for (int i = 0; i < N; i++) {
      if (lisEndingHere[i] > max) max = lisEndingHere[i];
    }

    Comparable[] LIS = new Comparable[max];
    for (int i = N-1; i >= 0 && max != 0; i--) {
      if (lisEndingHere[i] == max) {
        LIS[--max] = arr[i];
      }
    }

    return LIS;
  }
}

@GhostCat Thanks for the tip, you're right- I'll change it right away! — Ori Yonay
– Ori Yonay, Commented Aug 10, 2019 at 3:52
And just for the record : you can accept only one answer, but you are free to upvote any answer you received to show your appreciation for people helping you :-) — GhostCat
– GhostCat, Commented Aug 10, 2019 at 5:20

BeyondPerception · Accepted Answer · 2019-08-10 03:55:09Z

1

Just change the line

final Integer[] LIS = (Integer[]) lis(arr);

to

final Comparable[] LIS = lis(arr);

and also update the for loop.

Your method returns a Comparable array, so you cant downcast to an Integer array, but since the implementation of your numbers are Integers, they are still treated as such during runtime.

Setting the result to an Integer array is against the purpose of making a generic method anyways. For something to be passed to your method, it must have a compareTo method, and inherently has a toString method, and that satisfies everything you need the program to do.

answered Aug 10, 2019 at 3:55

BeyondPerception

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Comments

GhostCat · Accepted Answer · 2019-08-10 04:05:01Z

1

There is nothing to fix here. Here:

Integer[] LIS = (Integer[]) lis(...)

Your method lis() returns an array of Comparable objects. A Comparable array isn't an Integer array! Thus that cast can't work conceptually.

Yes, that array contains Integer objects, but the array type isn't "array of integer".

You would have to iterate the resulting array, then you can cast the individual entries. But you can't cast the array type itself into something that it isn't!

Beyond that, you could use generics with lists instead.

edited Aug 10, 2019 at 4:05

answered Aug 10, 2019 at 3:58

GhostCat

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