Why does sort take an Object array?

Question

I just stumbled upon the following overloads of sort in java.util.Arrays:

public static void sort(Object[] a)

public static <T> void sort(T[] a, Comparator<? super T> c)

Why is the first overload not generic, but the second one is? Why does the first overload take an Object[] instead of a Comparable[]? The documentation even states:

All elements in the array must implement the Comparable interface.

So what's the point of not verifying that constraint with Java's static type system?

Maybe the first one existed before generics were introduced? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 5, 2013 at 9:08
@Oli It could still have taken a Comparable[] before generics, right? — fredoverflow
– fredoverflow, Commented Jun 5, 2013 at 9:11
Note that TreeSet has the same "bizzareness": you can actually created a TreeSet with a class not implementing Comparable, but then at runtime you'll get ClassCastException. Arrays are another problem though, since A[] is not the same as B[] at runtime, unlike genericized collections. — fge
– fge, Commented Jun 5, 2013 at 9:18
@fge: i think it is for compatibility reasons that TreeSet takes elements that do not implement Comparable if you provide a Comparator — Marco Forberg
– Marco Forberg, Commented Jun 5, 2013 at 9:21

Evgeniy Dorofeev · Accepted Answer · 2013-06-05 10:13:19Z

3

The first method could be better if it were generic, but it would break compatibility with pre 1.5 code:

static <T extends Comparable<T>> void sort(T[] a) {
    ...
}

static void main(String[] args) {
   Object[] a = {"1", "2"};
   sort(a); <- compile time error, need explicit cast sort((Comparable[])a);
}

While the second one compiles with 1.4 code with warning only

public static <T> void sort(T[] a, Comparator<? super T> c) {
    ...
}

public static void main(String[] args) throws Exception {
    Object[] a = { "1", "2" };
    sort(a, new Comparator() {   <-- warning 
        @Override
        public int compare(Object o1, Object o2) {
            return 0;
        }
    });
}

edited Jun 5, 2013 at 10:13

answered Jun 5, 2013 at 9:25

Evgeniy Dorofeev

137k31 gold badges209 silver badges288 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Andremoniy · Accepted Answer · 2013-06-05 09:18:58Z

2

The first one method (void sort(Object[] a)) is not generic because:

it was introduced before generics appear in Java
it actually doesn't need generics

The second one requires generics, I think, only due to this construction Comparator<? super T>, because there isn't another convenient way to declare restrictions under second parameter with Comparator class.

answered Jun 5, 2013 at 9:18

Andremoniy

35.1k25 gold badges143 silver badges258 bronze badges

Collectives™ on Stack Overflow

Why does sort take an Object array?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related