CODEWITHSUNDEEP
javagenericsmergesort
MessitÖzil
MessitÖzil

Reputation: 1378

Java Generics MergeSort, exception in thread "main" java.lang.ClassCastException error

I wrote a java program for MergeSort using java generics. However I get following errors, I don't understand where I have gone wrong. please Help me out

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable; at src.MergeSort.sort(MergeSort.java:9) at src.MergeSort.main(MergeSort.java:57)

package src; import java.util.Arrays;

public class MergeSort {

  // sort array of type T using the Mergesort algorithm
  public static <T extends Comparable<T>> T[] sort(T[] arr) {
  if (arr.length > 1){
      T[] firstHalf =  (T[]) new Object[arr.length/2];
      System.arraycopy(arr, 0, firstHalf, 0, arr.length /2);
      sort(firstHalf);

 int secondHalfLength = arr.length -arr.length/2;
 T[] secondHalf = (T[]) new Object[secondHalfLength];
 System.arraycopy(arr, arr.length/2, secondHalf, 0, secondHalfLength);
 sort(secondHalf);
 merge(firstHalf, secondHalf, arr);

  }


      return arr;
  }

  public static <T extends Comparable<T>> void merge(T[] arr1, T[] arr2, T[] temp) {
  int current1 = 0;
  int current2 = 0;
  int current3 = 0;

  while(current1 < arr1.length && current2 < arr2.length){
      if (arr1[current1].compareTo(arr2[current2]) < 0)
          temp[current3++] = arr1[current1++];
      else
          temp[current3++] = arr2[current2++];

          }

  while (current1 < arr1.length)
  temp[current3++] = arr1[current1++];
  while (current2 < arr2.length)
  temp[current3++] = arr2[current2++];


  }




  public static void main(String[] args) {



// sort list of Characters
Character[] charArr = {'H','e','l','l','o',' ','w','o','r','l','d','!'};

charArr = MergeSort.<Character>sort(charArr);
System.out.println(Arrays.toString(charArr));

// sort list of Integers
Integer[] intArr = {23,4,15,8,42,16};

intArr = MergeSort.<Integer>sort(intArr);
System.out.println(Arrays.toString(intArr));
      }
    }

Upvotes: 1

Views: 411

Answers (1)

awksp
awksp

Reputation: 11877

Long story short, it's because your type parameter is actually being erased to Comparable.

Remember, generics are erased to their bounding type. In most cases, where there is no bounding type, type parameters would be erased to Object, but here, because you're bounding T by Comparable, T is erased to Comparable. Thus, casts like this:

T[] firstHalf =  (T[]) new Object[arr.length/2];

Become

Comparable[] firstHalf = (Comparable[]) new Object[arr.length / 2];

And because Object doesn't implement Comparable, the cast fails at runtime.

The solution? Simply make your arrays Comparable[]. Then your cast won't fail at runtime.

Upvotes: 4

Related Questions