How to use compareTo within a Generic array in java?

Question

I am trying to figure out how to compare two items within a T[] array, here is what I have:

public static <T extends Comparable< ? super T>> T getLargest(T [] a, int low, 
               int high){
    if(low>high)
            throw new IllegalArgumentException();
    T[] arrCopy = (T[]) new Object[high-low];
    for(int i=low;i<high;i++){
        if(a[i].compareTo(a[i-1])>0)
            arrCopy[i]=a[i];
        else
            arrCopy[i]=a[i+1];
    }
    return arrCopy[0];
}

and then I get the error: Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;

Any ideas on how I can resolve this?

Answer 1

You can assign the array like this:

@SuppressWarnings("unchecked")
T[] arrCopy = (T[]) Array.newInstance(a.getClass().getComponentType(), high-low);

Although the unchecked warning is necessary, this should actually be safe.

Btw, if you want to find the largest element in an array, here's a oneliner:

public static <T extends Comparable<T>> T max(final T[] data) {
    return Collections.max(Arrays.asList(data));
}

For the complete problem you can use one of these two (they are equivalent):

public static <T extends Comparable<T>> T maxA(final T[] data,int from, int to) {
    return Collections.max(Arrays.asList(Arrays.copyOfRange(data, from, to)));
}
public static <T extends Comparable<T>> T maxB(final T[] data,int from, int to) {
    return Collections.max(Arrays.asList(data).subList(from, to));
}