Arrays.sort(object[]) is not throwing classcastexception

Question

Code:

public class CompareTest {

    public static void main(String[] args) {

        ArrayList list =  new ArrayList();
        (list).add(new CompareTest());  


        Arrays.sort(list.toArray()); //Does not throw Exception , why ?
        Collections.sort(list);   //throws ClassCastException
    }

}

As per Java Doc: Arrays#sort

Sorts the specified array of objects into ascending order, according to the natural ordering of its elements. All elements in the array must implement the Comparable interface.

Why does Arrays#sort , doesnt throw ClassCastException as stated by JavaDoc ?

Capo

Answer 1

Because the source code of Arrays.sort() has this shortcut :

    int nRemaining  = hi - lo;
    if (nRemaining < 2)
        return;  // Arrays of size 0 and 1 are always sorted

So it doesn't bother checking if the elements of the array implement Comparable, because it doesn't have to sort an array that has only one element.

Note that the javadoc doesn't guarantee that a ClassCastException is thrown.

Inland

Interesting - the same thing was "fixed" in TreeMap in Java 7, but apparently not in Arrays... – Dibasic

So javadoc can be deceiving. – Floriated

Answer 2

Because it has only one element...And the Array.sort() will end without sorting if there are elements less than 2

Anus

Answer 3

The reason is that list has only one elemnt,the compareTo method never invoked in Arrays.sort,so the element is never cast to Comparable.

but it's invoked anyway in Collections.sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {
Object[] a = list.toArray();
Arrays.sort(a);
ListIterator<T> i = list.listIterator();
for (int j=0; j<a.length; j++) {
    i.next();
    i.set((T)a[j]);
}
}

all elemnt is casting to T which extends from Comparable

Vortumnus