Creating a new generic type array using Array.newInstance()

Question

For practice, I'm trying to implement the merge sort algorithm but got stuck pretty fast when trying to instantiate a generic type array. I'm not entirely sure if my plan will work out, but right now the interesting part (or frustrating, depending on when you ask me) is the second row in the merge() method. The newInstance() method needs to know what class it should initiate as, but even if arr1.getClass() compiles perfectly fine, it won't work during runtime.

public void mergeSort(T[] arr) {

    T[] merged = merge(Arrays.copyOfRange(arr, 0, arr.length/2), Arrays.copyOfRange(arr, arr.length/2+1, arr.length-1));

}

@SuppressWarnings({"unchecked"})
public T[] merge(T[] arr1, T[] arr2) {
    // A new array of type T that will contain a merged version of arr1 and arr2
    T[] merged = (T[]) Array.newInstance(arr1.getClass(), arr1.length+arr2.length);

    int i1 = 0, i2 = 0;
    for (int i = 0; i < arr1.length + arr2.length; i++) {
        if (arr1[i1].compareTo(arr2[i2]) < 0) {
            merged[i] = arr1[i1];
            i1++;
        } else {
            merged[i] = arr2[i2];
            i2++;
        }
    }

    return merged;
}

The error message is:

Exception in thread "main" java.lang.ClassCastException: [[Ljava.lang.String; cannot be cast to [Ljava.lang.Comparable;
    at sort.SortingAndSearching.merge(SortingAndSearching.java:94)
    at sort.SortingAndSearching.mergeSort(SortingAndSearching.java:84)
    at sort.SortingAndSearching.main(SortingAndSearching.java:19)

Alex Coleman · Accepted Answer · 2012-10-08 21:48:47Z

16

Think I see the issue... When you do Array.newInstance(...), it takes in the component type (in your case, you want it to be String). However, you're handing the array class (arr1 is of type String[], and you're doing arr1.getClass()). Instead, do

arr1.getClass().getComponentType()

to get the String class out of String[]

Per javadoc:

 static Object newInstance(Class<?> componentType, int[] dimensions)

componentType - the Class object representing the component type of the new array

edited Oct 8, 2012 at 21:48

answered Oct 8, 2012 at 21:43

Alex Coleman

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Comments

vainolo · Accepted Answer · 2012-10-08 21:51:51Z

1

I copied and pasted the code you gave and it doesn't even compile. After doing some fixes, I came to this:

public class Test {

  public <T extends Comparable> void mergeSort(T[] arr) {
    T[] merged = merge(Arrays.copyOfRange(arr, 0, arr.length / 2), Arrays.copyOfRange(arr, arr.length / 2 + 1, arr.length - 1));
  }

  public <T extends Comparable> T[] merge(T[] arr1, T[] arr2) {
    // A new array of type T that will contain a merged version of arr1 and arr2
    T[] merged = (T[]) Array.newInstance(arr1.getClass(), arr1.length + arr2.length);

    int i1 = 0, i2 = 0;
    for(int i = 0; i < arr1.length + arr2.length; i++) {
      if(arr1[i1].compareTo(arr2[i2]) < 0) {
        merged[i] = arr1[i1];
        i1++;
      } else {
        merged[i] = arr2[i2];
        i2++;
      }
    }
    return merged;
  }
}

Since you are using the compareTo method, you must tell the compiler that T implements this interface.

answered Oct 8, 2012 at 21:51

vainolo

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1 Comment

Jimmy C Over a year ago

Yes, I was using implements Comparable in the class declaration.

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